Some refactoring of mounts

- Separate FileMount into separate FileMount and WritableFileMount
   classes. This separates the (relatively simple) read-only code from
   the (soon to be even more complex) read/write code.

   It also allows you to create read-only mounts which don't bother with
   filesystem accounting, which is nice.

 - Make openForWrite/openForAppend always return a SeekableFileHandle.
   Appendable files still cannot be seeked within, but that check is now
   done on the FS side.

 - Refactor the various mount tests to live in test contract interfaces,
   allowing us to reuse them between mounts.

 - Clean up our error handling a little better. (Most) file-specific code
   has been moved to FileMount, and ArchiveMount-derived classes now
   throw correct path-localised exceptions.

projects/common-api/src/main/java/dan200/computercraft/api/ComputerCraftAPI.java

@@ -61,22 +61,25 @@ public final class ComputerCraftAPI {
     /**
      * Creates a file system mount that maps to a subfolder of the save directory for a given world, and returns it.
      * <p>
-     * Use in conjunction with IComputerAccess.mount() or IComputerAccess.mountWritable() to mount a folder from the
+     * Use in conjunction with Use {@link IComputerAccess#mount(String, Mount)} or {@link IComputerAccess#mountWritable(String, WritableMount)}
-     * users save directory onto a computers file system.
+     * to mount this on a computer's file system.
+     * <p>
+     * If the same folder may be mounted on multiple computers at once (for instance, if you provide a network file share),
+     * the same mount instance should be used for all computers. You should NOT have multiple mount instances for the
+     * same folder.
      *
      * @param server   The server which the save dir can be found.
-     * @param subPath  The folder path within the save directory that the mount should map to. eg: "computer/disk/42".
+     * @param subPath  The folder path within the save directory that the mount should map to. eg: "disk/42".
-     *                 Use createUniqueNumberedSaveDir() to create a new numbered folder to use.
+     *                 Use {@link #createUniqueNumberedSaveDir(MinecraftServer, String)} to create a new numbered folder
+     *                 to use.
      * @param capacity The amount of data that can be stored in the directory before it fills up, in bytes.
-     * @return The mount, or null if it could be created for some reason. Use IComputerAccess.mount() or IComputerAccess.mountWritable()
+     * @return The newly created mount.
-     * to mount this on a Computers' file system.
      * @see #createUniqueNumberedSaveDir(MinecraftServer, String)
      * @see IComputerAccess#mount(String, Mount)
      * @see IComputerAccess#mountWritable(String, WritableMount)
      * @see Mount
      * @see WritableMount
      */
-    @Nullable
     public static WritableMount createSaveDirMount(MinecraftServer server, String subPath, long capacity) {
         return getInstance().createSaveDirMount(server, subPath, capacity);
     }

projects/common-api/src/main/java/dan200/computercraft/impl/ComputerCraftAPIService.java

@@ -43,7 +43,6 @@ public interface ComputerCraftAPIService {
 
     int createUniqueNumberedSaveDir(MinecraftServer server, String parentSubPath);
 
-    @Nullable
     WritableMount createSaveDirMount(MinecraftServer server, String subPath, long capacity);
 
     @Nullable

projects/common/build.gradle.kts

@@ -24,6 +24,7 @@ dependencies {
 
     compileOnly(libs.mixin)
     annotationProcessorEverywhere(libs.autoService)
+    testFixturesAnnotationProcessor(libs.autoService)
 
     testImplementation(testFixtures(project(":core")))
     testImplementation(libs.bundles.test)

projects/common/src/main/java/dan200/computercraft/impl/AbstractComputerCraftAPI.java

@@ -7,6 +7,7 @@ package dan200.computercraft.impl;
 
 import dan200.computercraft.api.detail.BlockReference;
 import dan200.computercraft.api.detail.DetailRegistry;
+import dan200.computercraft.api.filesystem.Mount;
 import dan200.computercraft.api.filesystem.WritableMount;
 import dan200.computercraft.api.lua.GenericSource;
 import dan200.computercraft.api.lua.ILuaAPIFactory;
@@ -18,9 +19,10 @@ import dan200.computercraft.api.redstone.BundledRedstoneProvider;
 import dan200.computercraft.api.turtle.TurtleRefuelHandler;
 import dan200.computercraft.core.apis.ApiFactories;
 import dan200.computercraft.core.asm.GenericMethod;
-import dan200.computercraft.core.filesystem.FileMount;
+import dan200.computercraft.core.filesystem.WritableFileMount;
 import dan200.computercraft.impl.detail.DetailRegistryImpl;
 import dan200.computercraft.impl.network.wired.WiredNodeImpl;
+import dan200.computercraft.shared.computer.core.ResourceMount;
 import dan200.computercraft.shared.computer.core.ServerContext;
 import dan200.computercraft.shared.details.BlockDetails;
 import dan200.computercraft.shared.details.ItemDetails;
@@ -57,13 +59,15 @@ public abstract class AbstractComputerCraftAPI implements ComputerCraftAPIServic
     }
 
     @Override
-    public final @Nullable WritableMount createSaveDirMount(MinecraftServer server, String subPath, long capacity) {
+    public final WritableMount createSaveDirMount(MinecraftServer server, String subPath, long capacity) {
         var root = ServerContext.get(server).storageDir().toFile();
-        try {
+        return new WritableFileMount(new File(root, subPath), capacity);
-            return new FileMount(new File(root, subPath), capacity);
+    }
-        } catch (Exception e) {
+
-            return null;
+    @Override
-        }
+    public final @Nullable Mount createResourceMount(MinecraftServer server, String domain, String subPath) {
+        var mount = ResourceMount.get(domain, subPath, server.getResourceManager());
+        return mount.exists("") ? mount : null;
     }
 
     @Override

projects/common/src/main/java/dan200/computercraft/shared/computer/core/ResourceMount.java

@@ -5,6 +5,8 @@
  */
 package dan200.computercraft.shared.computer.core;
 
+import com.google.common.annotations.VisibleForTesting;
+import dan200.computercraft.api.filesystem.FileOperationException;
 import dan200.computercraft.core.filesystem.ArchiveMount;
 import dan200.computercraft.core.filesystem.FileSystem;
 import net.minecraft.ResourceLocationException;
@@ -16,7 +18,6 @@ import net.minecraft.util.profiling.ProfilerFiller;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
-import java.io.FileNotFoundException;
 import java.io.IOException;
 import java.util.HashMap;
 import java.util.Map;
@@ -49,7 +50,8 @@ public final class ResourceMount extends ArchiveMount<ResourceMount.FileEntry> {
         }
     }
 
-    private ResourceMount(String namespace, String subPath, ResourceManager manager) {
+    @VisibleForTesting
+    ResourceMount(String namespace, String subPath, ResourceManager manager) {
         this.namespace = namespace;
         this.subPath = subPath;
         load(manager);
@@ -59,7 +61,7 @@ public final class ResourceMount extends ArchiveMount<ResourceMount.FileEntry> {
         var hasAny = false;
         String existingNamespace = null;
 
-        var newRoot = new FileEntry(new ResourceLocation(namespace, subPath));
+        var newRoot = new FileEntry("", new ResourceLocation(namespace, subPath));
         for (var file : manager.listResources(subPath, s -> true).keySet()) {
             existingNamespace = file.getNamespace();
 
@@ -100,7 +102,7 @@ public final class ResourceMount extends ArchiveMount<ResourceMount.FileEntry> {
                     LOG.warn("Cannot create resource location for {} ({})", part, e.getMessage());
                     return;
                 }
-                lastEntry.children.put(part, nextEntry = new FileEntry(childPath));
+                lastEntry.children.put(part, nextEntry = new FileEntry(path, childPath));
             }
 
             lastEntry = nextEntry;
@@ -129,7 +131,7 @@ public final class ResourceMount extends ArchiveMount<ResourceMount.FileEntry> {
     @Override
     public byte[] getContents(FileEntry file) throws IOException {
         var resource = manager.getResource(file.identifier).orElse(null);
-        if (resource == null) throw new FileNotFoundException(NO_SUCH_FILE);
+        if (resource == null) throw new FileOperationException(file.path, NO_SUCH_FILE);
 
         try (var stream = resource.open()) {
             return stream.readAllBytes();
@@ -139,7 +141,8 @@ public final class ResourceMount extends ArchiveMount<ResourceMount.FileEntry> {
     protected static class FileEntry extends ArchiveMount.FileEntry<FileEntry> {
         final ResourceLocation identifier;
 
-        FileEntry(ResourceLocation identifier) {
+        FileEntry(String path, ResourceLocation identifier) {
+            super(path);
             this.identifier = identifier;
         }
     }

projects/common/src/main/java/dan200/computercraft/shared/network/server/UploadFileMessage.java

@@ -25,6 +25,7 @@ import static dan200.computercraft.core.util.Nullability.assertNonNull;
 public class UploadFileMessage extends ComputerServerMessage {
     public static final int MAX_SIZE = 512 * 1024;
     static final int MAX_PACKET_SIZE = 30 * 1024; // Max packet size is 32767.
+    private static final int HEADER_SIZE = 16 + 1; // 16 bytes for the UUID, 4 for the flag.
 
     public static final int MAX_FILES = 32;
     public static final int MAX_FILE_NAME = 128;
@@ -120,8 +121,8 @@ public class UploadFileMessage extends ComputerServerMessage {
     public static void send(AbstractContainerMenu container, List<FileUpload> files, Consumer<UploadFileMessage> send) {
         var uuid = UUID.randomUUID();
 
-        var remaining = MAX_PACKET_SIZE;
+        var remaining = MAX_PACKET_SIZE - HEADER_SIZE;
-        for (var file : files) remaining -= file.getName().length() * 4 + FileUpload.CHECKSUM_LENGTH;
+        for (var file : files) remaining -= 4 + file.getName().length() * 4 + FileUpload.CHECKSUM_LENGTH;
 
         var first = true;
         List<FileSlice> slices = new ArrayList<>(files.size());
@@ -137,7 +138,7 @@ public class UploadFileMessage extends ComputerServerMessage {
                         ? new UploadFileMessage(container, uuid, FLAG_FIRST, files, new ArrayList<>(slices))
                         : new UploadFileMessage(container, uuid, 0, null, new ArrayList<>(slices)));
                     slices.clear();
-                    remaining = MAX_PACKET_SIZE;
+                    remaining = MAX_PACKET_SIZE - HEADER_SIZE;
                     first = false;
                 }
 

projects/common/src/main/java/dan200/computercraft/shared/peripheral/diskdrive/DiskDriveBlockEntity.java

@@ -202,6 +202,7 @@ public final class DiskDriveBlockEntity extends AbstractContainerBlockEntity {
         ejectQueued.set(true);
     }
 
+    @GuardedBy("this")
     private void mountDisk(IComputerAccess computer, MountInfo info, MediaStack disk) {
         var mount = disk.getMount((ServerLevel) getLevel());
         if (mount != null) {

projects/common/src/test/java/dan200/computercraft/TestPlatformHelper.java

@@ -8,7 +8,6 @@ package dan200.computercraft;
 import com.google.auto.service.AutoService;
 import com.mojang.authlib.GameProfile;
 import com.mojang.brigadier.arguments.ArgumentType;
-import dan200.computercraft.api.filesystem.Mount;
 import dan200.computercraft.api.network.wired.WiredElement;
 import dan200.computercraft.api.peripheral.IPeripheral;
 import dan200.computercraft.impl.AbstractComputerCraftAPI;
@@ -221,12 +220,6 @@ public class TestPlatformHelper extends AbstractComputerCraftAPI implements Plat
         return "1.0";
     }
 
-    @Nullable
-    @Override
-    public Mount createResourceMount(MinecraftServer server, String domain, String subPath) {
-        throw new UnsupportedOperationException("Cannot create resource mount");
-    }
-
     @Nullable
     @Override
     public <T> T tryGetRegistryObject(ResourceKey<Registry<T>> registry, ResourceLocation id) {

projects/common/src/test/java/dan200/computercraft/shared/computer/core/ResourceMountTest.java

@@ -5,73 +5,59 @@
  */
 package dan200.computercraft.shared.computer.core;
 
+import com.google.common.io.MoreFiles;
+import com.google.common.io.RecursiveDeleteOption;
 import dan200.computercraft.api.filesystem.Mount;
+import dan200.computercraft.test.core.CloseScope;
+import dan200.computercraft.test.core.filesystem.MountContract;
 import net.minecraft.Util;
 import net.minecraft.server.packs.PackType;
 import net.minecraft.server.packs.PathPackResources;
 import net.minecraft.server.packs.resources.ReloadableResourceManager;
 import net.minecraft.util.Unit;
-import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.AfterEach;
-import org.junit.jupiter.api.Test;
 
 import java.io.IOException;
-import java.nio.file.Path;
+import java.nio.file.Files;
-import java.util.ArrayList;
-import java.util.Arrays;
-import java.util.Comparator;
 import java.util.List;
 import java.util.concurrent.CompletableFuture;
+import java.util.concurrent.ExecutionException;
 
-import static org.junit.jupiter.api.Assertions.*;
+public class ResourceMountTest implements MountContract {
+    private final CloseScope toClose = new CloseScope();
 
-public class ResourceMountTest {
+    @Override
-    private Mount mount;
+    public Mount createSkeleton() throws IOException {
+        var path = Files.createTempDirectory("cctweaked-test");
+        toClose.add(() -> MoreFiles.deleteRecursively(path, RecursiveDeleteOption.ALLOW_INSECURE));
+
+        Files.createDirectories(path.resolve("data/computercraft/rom/dir"));
+        try (var writer = Files.newBufferedWriter(path.resolve("data/computercraft/rom/dir/file.lua"))) {
+            writer.write("print('testing')");
+        }
+        Files.newBufferedWriter(path.resolve("data/computercraft/rom/f.lua")).close();
 
-    @BeforeEach
-    public void before() {
         var manager = new ReloadableResourceManager(PackType.SERVER_DATA);
-        var done = new CompletableFuture<Unit>();
+        var reload = manager.createReload(Util.backgroundExecutor(), Util.backgroundExecutor(), CompletableFuture.completedFuture(Unit.INSTANCE), List.of(
-        manager.createReload(Util.backgroundExecutor(), Util.backgroundExecutor(), done, List.of(
+            new PathPackResources("resources", path, false)
-            new PathPackResources("resources", Path.of("../core/src/main/resources"), false)
         ));
 
-        mount = ResourceMount.get("computercraft", "lua/rom", manager);
+        try {
+            reload.done().get();
+        } catch (InterruptedException | ExecutionException e) {
+            throw new RuntimeException("Failed to load resources", e);
+        }
+
+        return new ResourceMount("computercraft", "rom", manager);
     }
 
-    @Test
+    @Override
-    public void testList() throws IOException {
+    public boolean hasFileTimes() {
-        List<String> files = new ArrayList<>();
+        return false;
-        mount.list("", files);
-        files.sort(Comparator.naturalOrder());
-
-        assertEquals(
-            Arrays.asList("apis", "autorun", "help", "modules", "motd.txt", "programs", "startup.lua"),
-            files
-        );
     }
 
-    @Test
+    @AfterEach
-    public void testExists() throws IOException {
+    public void after() throws Exception {
-        assertTrue(mount.exists(""));
+        toClose.close();
-        assertTrue(mount.exists("startup.lua"));
-        assertTrue(mount.exists("programs/fun/advanced/paint.lua"));
-
-        assertFalse(mount.exists("programs/fun/advance/paint.lua"));
-        assertFalse(mount.exists("programs/fun/advanced/paint.lu"));
-    }
-
-    @Test
-    public void testIsDir() throws IOException {
-        assertTrue(mount.isDirectory(""));
-    }
-
-    @Test
-    public void testIsFile() throws IOException {
-        assertFalse(mount.isDirectory("startup.lua"));
-    }
-
-    @Test
-    public void testSize() throws IOException {
-        assertNotEquals(mount.getSize("startup.lua"), 0);
     }
 }

projects/core-api/src/main/java/dan200/computercraft/api/filesystem/WritableMount.java

@@ -9,7 +9,7 @@ import dan200.computercraft.api.peripheral.IComputerAccess;
 
 import java.io.IOException;
 import java.io.OutputStream;
-import java.nio.channels.WritableByteChannel;
+import java.nio.channels.SeekableByteChannel;
 
 /**
  * Represents a part of a virtual filesystem that can be mounted onto a computer using {@link IComputerAccess#mount(String, Mount)}
@@ -54,21 +54,19 @@ public interface WritableMount extends Mount {
      * Opens a file with a given path, and returns an {@link OutputStream} for writing to it.
      *
      * @param path A file path in normalised format, relative to the mount location. ie: "programs/myprogram".
-     * @return A stream for writing to. If the channel implements {@link java.nio.channels.SeekableByteChannel}, one
+     * @return A stream for writing to.
-     * will be able to seek to arbitrary positions when using binary mode.
      * @throws IOException If the file could not be opened for writing.
      */
-    WritableByteChannel openForWrite(String path) throws IOException;
+    SeekableByteChannel openForWrite(String path) throws IOException;
 
     /**
      * Opens a file with a given path, and returns an {@link OutputStream} for appending to it.
      *
      * @param path A file path in normalised format, relative to the mount location. ie: "programs/myprogram".
-     * @return A stream for writing to. If the channel implements {@link java.nio.channels.SeekableByteChannel}, one
+     * @return A stream for writing to.
-     * will be able to seek to arbitrary positions when using binary mode.
      * @throws IOException If the file could not be opened for writing.
      */
-    WritableByteChannel openForAppend(String path) throws IOException;
+    SeekableByteChannel openForAppend(String path) throws IOException;
 
     /**
      * Get the amount of free space on the mount, in bytes. You should decrease this value as the user writes to the

projects/core/src/main/java/dan200/computercraft/core/apis/FSAPI.java

@@ -391,12 +391,12 @@ public class FSAPI implements ILuaAPI {
                 case "wb" -> {
                     // Open the file for binary writing, then create a wrapper around the writer
                     var writer = getFileSystem().openForWrite(path, false, Function.identity());
-                    return new Object[]{ BinaryWritableHandle.of(writer.get(), writer) };
+                    return new Object[]{ BinaryWritableHandle.of(writer.get(), writer, true) };
                 }
                 case "ab" -> {
                     // Open the file for binary appending, then create a wrapper around the reader
                     var writer = getFileSystem().openForWrite(path, true, Function.identity());
-                    return new Object[]{ BinaryWritableHandle.of(writer.get(), writer) };
+                    return new Object[]{ BinaryWritableHandle.of(writer.get(), writer, false) };
                 }
                 default -> throw new LuaException("Unsupported mode");
             }

projects/core/src/main/java/dan200/computercraft/core/apis/handles/BinaryWritableHandle.java

@@ -10,14 +10,12 @@ import dan200.computercraft.api.lua.LuaException;
 import dan200.computercraft.api.lua.LuaFunction;
 import dan200.computercraft.api.lua.LuaValues;
 import dan200.computercraft.core.filesystem.TrackingCloseable;
-import dan200.computercraft.core.util.Nullability;
 
 import javax.annotation.Nullable;
 import java.io.IOException;
 import java.nio.ByteBuffer;
 import java.nio.channels.FileChannel;
 import java.nio.channels.SeekableByteChannel;
-import java.nio.channels.WritableByteChannel;
 import java.util.Optional;
 
 /**
@@ -27,23 +25,16 @@ import java.util.Optional;
  * @cc.module fs.BinaryWriteHandle
  */
 public class BinaryWritableHandle extends HandleGeneric {
-    private final WritableByteChannel writer;
+    final SeekableByteChannel channel;
-    final @Nullable SeekableByteChannel seekable;
     private final ByteBuffer single = ByteBuffer.allocate(1);
 
-    protected BinaryWritableHandle(WritableByteChannel writer, @Nullable SeekableByteChannel seekable, TrackingCloseable closeable) {
+    protected BinaryWritableHandle(SeekableByteChannel channel, TrackingCloseable closeable) {
         super(closeable);
-        this.writer = writer;
+        this.channel = channel;
-        this.seekable = seekable;
     }
 
-    public static BinaryWritableHandle of(WritableByteChannel channel, TrackingCloseable closeable) {
+    public static BinaryWritableHandle of(SeekableByteChannel channel, TrackingCloseable closeable, boolean canSeek) {
-        var seekable = asSeekable(channel);
+        return canSeek ? new Seekable(channel, closeable) : new BinaryWritableHandle(channel, closeable);
-        return seekable == null ? new BinaryWritableHandle(channel, null, closeable) : new Seekable(seekable, closeable);
-    }
-
-    public static BinaryWritableHandle of(WritableByteChannel channel) {
-        return of(channel, new TrackingCloseable.Impl(channel));
     }
 
     /**
@@ -66,9 +57,9 @@ public class BinaryWritableHandle extends HandleGeneric {
                 single.put((byte) number);
                 single.flip();
 
-                writer.write(single);
+                channel.write(single);
             } else if (arg instanceof String) {
-                writer.write(arguments.getBytes(0));
+                channel.write(arguments.getBytes(0));
             } else {
                 throw LuaValues.badArgumentOf(0, "string or number", arg);
             }
@@ -87,15 +78,15 @@ public class BinaryWritableHandle extends HandleGeneric {
         checkOpen();
         try {
             // Technically this is not needed
-            if (writer instanceof FileChannel channel) channel.force(false);
+            if (channel instanceof FileChannel channel) channel.force(false);
         } catch (IOException e) {
             throw new LuaException(e.getMessage());
         }
     }
 
     public static class Seekable extends BinaryWritableHandle {
-        public Seekable(SeekableByteChannel seekable, TrackingCloseable closeable) {
+        public Seekable(SeekableByteChannel channel, TrackingCloseable closeable) {
-            super(seekable, seekable, closeable);
+            super(channel, closeable);
         }
 
         /**
@@ -121,7 +112,7 @@ public class BinaryWritableHandle extends HandleGeneric {
         @LuaFunction
         public final Object[] seek(Optional<String> whence, Optional<Long> offset) throws LuaException {
             checkOpen();
-            return handleSeek(Nullability.assertNonNull(seekable), whence, offset);
+            return handleSeek(channel, whence, offset);
         }
     }
 }

projects/core/src/main/java/dan200/computercraft/core/apis/handles/HandleGeneric.java

@@ -12,7 +12,6 @@ import dan200.computercraft.core.util.IoUtil;
 
 import javax.annotation.Nullable;
 import java.io.IOException;
-import java.nio.channels.Channel;
 import java.nio.channels.SeekableByteChannel;
 import java.util.Optional;
 
@@ -75,16 +74,4 @@ public abstract class HandleGeneric {
             return null;
         }
     }
-
-    @Nullable
-    protected static SeekableByteChannel asSeekable(Channel channel) {
-        if (!(channel instanceof SeekableByteChannel seekable)) return null;
-
-        try {
-            seekable.position(seekable.position());
-            return seekable;
-        } catch (IOException | UnsupportedOperationException e) {
-            return null;
-        }
-    }
 }

projects/core/src/main/java/dan200/computercraft/core/computer/ComputerEnvironment.java

@@ -6,7 +6,7 @@
 package dan200.computercraft.core.computer;
 
 import dan200.computercraft.api.filesystem.WritableMount;
-import dan200.computercraft.core.filesystem.FileMount;
+import dan200.computercraft.core.filesystem.WritableFileMount;
 import dan200.computercraft.core.metrics.MetricsObserver;
 
 import javax.annotation.Nullable;
@@ -38,7 +38,7 @@ public interface ComputerEnvironment {
      * Construct the mount for this computer's user-writable data.
      *
      * @return The constructed mount or {@code null} if the mount could not be created.
-     * @see FileMount
+     * @see WritableFileMount
      */
     @Nullable
     WritableMount createRootMount();

projects/core/src/main/java/dan200/computercraft/core/filesystem/ArchiveMount.java

@@ -27,6 +27,7 @@ import java.util.concurrent.TimeUnit;
  */
 public abstract class ArchiveMount<T extends ArchiveMount.FileEntry<T>> implements Mount {
     protected static final String NO_SUCH_FILE = "No such file";
+
     /**
      * Limit the entire cache to 64MiB.
      */
@@ -103,7 +104,7 @@ public abstract class ArchiveMount<T extends ArchiveMount.FileEntry<T>> implemen
      * <p>
      * This should only be called once per file, as the result is cached in {@link #getSize(String)}.
      *
-     * @param file The file to compute the size of.
+     * @param file The file to compute the size of. This will not be a directory.
      * @return The size of the file.
      * @throws IOException If the size could not be read.
      */
@@ -125,7 +126,7 @@ public abstract class ArchiveMount<T extends ArchiveMount.FileEntry<T>> implemen
     /**
      * Read the entirety of a file into memory.
      *
-     * @param file The file to read into memory.
+     * @param file The file to read into memory. This will not be a directory.
      * @return The contents of the file.
      */
     protected abstract byte[] getContents(T file) throws IOException;
@@ -141,7 +142,7 @@ public abstract class ArchiveMount<T extends ArchiveMount.FileEntry<T>> implemen
     /**
      * Get all attributes of the file.
      *
-     * @param file The file to compute attributes for.
+     * @param file The file to compute attributes for. This will not be a directory.
      * @return The file's attributes.
      * @throws IOException If the attributes could not be read.
      */
@@ -150,11 +151,16 @@ public abstract class ArchiveMount<T extends ArchiveMount.FileEntry<T>> implemen
     }
 
     protected static class FileEntry<T extends ArchiveMount.FileEntry<T>> {
+        public final String path;
         @Nullable
         public Map<String, T> children;
 
         long size = -1;
 
+        protected FileEntry(String path) {
+            this.path = path;
+        }
+
         protected boolean isDirectory() {
             return children != null;
         }

projects/core/src/main/java/dan200/computercraft/core/filesystem/FileMount.java

@@ -5,370 +5,141 @@
  */
 package dan200.computercraft.core.filesystem;
 
-import com.google.common.collect.Sets;
+import dan200.computercraft.api.filesystem.FileAttributes;
 import dan200.computercraft.api.filesystem.FileOperationException;
-import dan200.computercraft.api.filesystem.WritableMount;
+import dan200.computercraft.api.filesystem.Mount;
-import org.slf4j.Logger;
-import org.slf4j.LoggerFactory;
 
-import java.io.File;
 import java.io.IOException;
-import java.nio.ByteBuffer;
-import java.nio.channels.ClosedChannelException;
-import java.nio.channels.NonReadableChannelException;
 import java.nio.channels.SeekableByteChannel;
-import java.nio.channels.WritableByteChannel;
+import java.nio.file.FileSystemException;
 import java.nio.file.*;
 import java.nio.file.attribute.BasicFileAttributes;
 import java.util.Collections;
 import java.util.List;
 import java.util.Set;
 
-public class FileMount implements WritableMount {
+/**
-    private static final Logger LOG = LoggerFactory.getLogger(FileMount.class);
+ * A {@link Mount} implementation which provides read-only access to a directory.
-    private static final long MINIMUM_FILE_SIZE = 500;
+ */
+public class FileMount implements Mount {
     private static final Set<OpenOption> READ_OPTIONS = Collections.singleton(StandardOpenOption.READ);
-    private static final Set<OpenOption> WRITE_OPTIONS = Sets.newHashSet(StandardOpenOption.WRITE, StandardOpenOption.CREATE, StandardOpenOption.TRUNCATE_EXISTING);
-    private static final Set<OpenOption> APPEND_OPTIONS = Sets.newHashSet(StandardOpenOption.WRITE, StandardOpenOption.CREATE, StandardOpenOption.APPEND);
 
-    private class WritableCountingChannel implements WritableByteChannel {
+    protected final Path root;
 
-        private final WritableByteChannel inner;
+    public FileMount(Path root) {
-        long ignoredBytesLeft;
+        this.root = root;
-
-        WritableCountingChannel(WritableByteChannel inner, long bytesToIgnore) {
-            this.inner = inner;
-            ignoredBytesLeft = bytesToIgnore;
-        }
-
-        @Override
-        public int write(ByteBuffer b) throws IOException {
-            count(b.remaining());
-            return inner.write(b);
-        }
-
-        void count(long n) throws IOException {
-            ignoredBytesLeft -= n;
-            if (ignoredBytesLeft < 0) {
-                var newBytes = -ignoredBytesLeft;
-                ignoredBytesLeft = 0;
-
-                var bytesLeft = capacity - usedSpace;
-                if (newBytes > bytesLeft) throw new IOException("Out of space");
-                usedSpace += newBytes;
-            }
-        }
-
-        @Override
-        public boolean isOpen() {
-            return inner.isOpen();
-        }
-
-        @Override
-        public void close() throws IOException {
-            inner.close();
-        }
     }
 
-    private class SeekableCountingChannel extends WritableCountingChannel implements SeekableByteChannel {
+    /**
-        private final SeekableByteChannel inner;
+     * Resolve a mount-relative path to one on the file system.
-
+     *
-        SeekableCountingChannel(SeekableByteChannel inner, long bytesToIgnore) {
+     * @param path The path to resolve.
-            super(inner, bytesToIgnore);
+     * @return The resolved path.
-            this.inner = inner;
+     */
-        }
+    protected Path resolvePath(String path) {
-
+        return root.resolve(path);
-        @Override
-        public SeekableByteChannel position(long newPosition) throws IOException {
-            if (!isOpen()) throw new ClosedChannelException();
-            if (newPosition < 0) {
-                throw new IllegalArgumentException("Cannot seek before the beginning of the stream");
-            }
-
-            var delta = newPosition - inner.position();
-            if (delta < 0) {
-                ignoredBytesLeft -= delta;
-            } else {
-                count(delta);
-            }
-
-            return inner.position(newPosition);
-        }
-
-        @Override
-        public SeekableByteChannel truncate(long size) throws IOException {
-            throw new IOException("Not yet implemented");
-        }
-
-        @Override
-        public int read(ByteBuffer dst) throws ClosedChannelException {
-            if (!inner.isOpen()) throw new ClosedChannelException();
-            throw new NonReadableChannelException();
-        }
-
-        @Override
-        public long position() throws IOException {
-            return inner.position();
-        }
-
-        @Override
-        public long size() throws IOException {
-            return inner.size();
-        }
     }
 
-    private final File rootPath;
+    protected boolean created() {
-    private final long capacity;
+        return Files.exists(root);
-    private long usedSpace;
-
-    public FileMount(File rootPath, long capacity) {
-        this.rootPath = rootPath;
-        this.capacity = capacity + MINIMUM_FILE_SIZE;
-        usedSpace = created() ? measureUsedSpace(this.rootPath) : MINIMUM_FILE_SIZE;
     }
 
-    // IMount implementation
-
     @Override
     public boolean exists(String path) {
-        if (!created()) return path.isEmpty();
+        return path.isEmpty() || Files.exists(resolvePath(path));
-
-        var file = getRealPath(path);
-        return file.exists();
     }
 
     @Override
     public boolean isDirectory(String path) {
-        if (!created()) return path.isEmpty();
+        return path.isEmpty() || Files.isDirectory(resolvePath(path));
-
-        var file = getRealPath(path);
-        return file.exists() && file.isDirectory();
     }
 
     @Override
-    public boolean isReadOnly(String path) throws IOException {
+    public void list(String path, List<String> contents) throws FileOperationException {
-        var file = getRealPath(path);
+        if (path.isEmpty() && !created()) return;
-        while (true) {
+
-            if (file.exists()) return !file.canWrite();
+        try (var stream = Files.newDirectoryStream(resolvePath(path))) {
-            if (file.equals(rootPath)) return false;
+            stream.forEach(x -> contents.add(x.getFileName().toString()));
-            file = file.getParentFile();
+        } catch (IOException e) {
+            throw remapException(path, e);
         }
     }
 
     @Override
-    public void list(String path, List<String> contents) throws IOException {
+    public long getSize(String path) throws FileOperationException {
-        if (!created()) {
+        var attributes = getAttributes(path);
-            if (!path.isEmpty()) throw new FileOperationException(path, "Not a directory");
+        return attributes.isDirectory() ? 0 : attributes.size();
-            return;
-        }
-
-        var file = getRealPath(path);
-        if (!file.exists() || !file.isDirectory()) throw new FileOperationException(path, "Not a directory");
-
-        var paths = file.list();
-        for (var subPath : paths) {
-            if (new File(file, subPath).exists()) contents.add(subPath);
-        }
     }
 
     @Override
-    public long getSize(String path) throws IOException {
+    public BasicFileAttributes getAttributes(String path) throws FileOperationException {
-        if (!created()) {
+        if (path.isEmpty() && !created()) return new FileAttributes(true, 0);
-            if (path.isEmpty()) return 0;
-        } else {
-            var file = getRealPath(path);
-            if (file.exists()) return file.isDirectory() ? 0 : file.length();
-        }
-
-        throw new FileOperationException(path, "No such file");
-    }
-
-    @Override
-    public SeekableByteChannel openForRead(String path) throws IOException {
-        if (created()) {
-            var file = getRealPath(path);
-            if (file.exists() && !file.isDirectory()) return Files.newByteChannel(file.toPath(), READ_OPTIONS);
-        }
-
-        throw new FileOperationException(path, "No such file");
-    }
-
-    @Override
-    public BasicFileAttributes getAttributes(String path) throws IOException {
-        if (created()) {
-            var file = getRealPath(path);
-            if (file.exists()) return Files.readAttributes(file.toPath(), BasicFileAttributes.class);
-        }
-
-        throw new FileOperationException(path, "No such file");
-    }
-
-    // IWritableMount implementation
-
-    @Override
-    public void makeDirectory(String path) throws IOException {
-        create();
-        var file = getRealPath(path);
-        if (file.exists()) {
-            if (!file.isDirectory()) throw new FileOperationException(path, "File exists");
-            return;
-        }
-
-        var dirsToCreate = 1;
-        var parent = file.getParentFile();
-        while (!parent.exists()) {
-            ++dirsToCreate;
-            parent = parent.getParentFile();
-        }
-
-        if (getRemainingSpace() < dirsToCreate * MINIMUM_FILE_SIZE) {
-            throw new FileOperationException(path, "Out of space");
-        }
-
-        if (file.mkdirs()) {
-            usedSpace += dirsToCreate * MINIMUM_FILE_SIZE;
-        } else {
-            throw new FileOperationException(path, "Access denied");
-        }
-    }
-
-    @Override
-    public void delete(String path) throws IOException {
-        if (path.isEmpty()) throw new FileOperationException(path, "Access denied");
-
-        if (created()) {
-            var file = getRealPath(path);
-            if (file.exists()) deleteRecursively(file);
-        }
-    }
-
-    private void deleteRecursively(File file) throws IOException {
-        // Empty directories first
-        if (file.isDirectory()) {
-            var children = file.list();
-            for (var aChildren : children) {
-                deleteRecursively(new File(file, aChildren));
-            }
-        }
-
-        // Then delete
-        var fileSize = file.isDirectory() ? 0 : file.length();
-        var success = file.delete();
-        if (success) {
-            usedSpace -= Math.max(MINIMUM_FILE_SIZE, fileSize);
-        } else {
-            throw new IOException("Access denied");
-        }
-    }
-
-    @Override
-    public void rename(String source, String dest) throws IOException {
-        var sourceFile = getRealPath(source);
-        var destFile = getRealPath(dest);
-        if (!sourceFile.exists()) throw new FileOperationException(source, "No such file");
-        if (destFile.exists()) throw new FileOperationException(dest, "File exists");
-
-        var sourcePath = sourceFile.toPath();
-        var destPath = destFile.toPath();
-        if (destPath.startsWith(sourcePath)) {
-            throw new FileOperationException(source, "Cannot move a directory inside itself");
-        }
-
-        Files.move(sourcePath, destPath);
-    }
-
-    @Override
-    public WritableByteChannel openForWrite(String path) throws IOException {
-        create();
-        var file = getRealPath(path);
-        if (file.exists() && file.isDirectory()) throw new FileOperationException(path, "Cannot write to directory");
-
-        if (file.exists()) {
-            usedSpace -= Math.max(file.length(), MINIMUM_FILE_SIZE);
-        } else if (getRemainingSpace() < MINIMUM_FILE_SIZE) {
-            throw new FileOperationException(path, "Out of space");
-        }
-        usedSpace += MINIMUM_FILE_SIZE;
-
-        return new SeekableCountingChannel(Files.newByteChannel(file.toPath(), WRITE_OPTIONS), MINIMUM_FILE_SIZE);
-    }
-
-    @Override
-    public WritableByteChannel openForAppend(String path) throws IOException {
-        if (!created()) {
-            throw new FileOperationException(path, "No such file");
-        }
-
-        var file = getRealPath(path);
-        if (!file.exists()) throw new FileOperationException(path, "No such file");
-        if (file.isDirectory()) throw new FileOperationException(path, "Cannot write to directory");
-
-        // Allowing seeking when appending is not recommended, so we use a separate channel.
-        return new WritableCountingChannel(
-            Files.newByteChannel(file.toPath(), APPEND_OPTIONS),
-            Math.max(MINIMUM_FILE_SIZE - file.length(), 0)
-        );
-    }
-
-    @Override
-    public long getRemainingSpace() {
-        return Math.max(capacity - usedSpace, 0);
-    }
-
-    @Override
-    public long getCapacity() {
-        return capacity - MINIMUM_FILE_SIZE;
-    }
-
-    private File getRealPath(String path) {
-        return new File(rootPath, path);
-    }
-
-    private boolean created() {
-        return rootPath.exists();
-    }
-
-    private void create() throws IOException {
-        if (!rootPath.exists()) {
-            var success = rootPath.mkdirs();
-            if (!success) {
-                throw new IOException("Access denied");
-            }
-        }
-    }
-
-    private static class Visitor extends SimpleFileVisitor<Path> {
-        long size;
-
-        @Override
-        public FileVisitResult preVisitDirectory(Path dir, BasicFileAttributes attrs) {
-            size += MINIMUM_FILE_SIZE;
-            return FileVisitResult.CONTINUE;
-        }
-
-        @Override
-        public FileVisitResult visitFile(Path file, BasicFileAttributes attrs) {
-            size += Math.max(attrs.size(), MINIMUM_FILE_SIZE);
-            return FileVisitResult.CONTINUE;
-        }
-
-        @Override
-        public FileVisitResult visitFileFailed(Path file, IOException exc) {
-            LOG.error("Error computing file size for {}", file, exc);
-            return FileVisitResult.CONTINUE;
-        }
-    }
-
-    private static long measureUsedSpace(File file) {
-        if (!file.exists()) return 0;
 
         try {
-            var visitor = new Visitor();
+            return Files.readAttributes(resolvePath(path), BasicFileAttributes.class);
-            Files.walkFileTree(file.toPath(), visitor);
-            return visitor.size;
         } catch (IOException e) {
-            LOG.error("Error computing file size for {}", file, e);
+            throw remapException(path, e);
-            return 0;
         }
     }
+
+    @Override
+    public SeekableByteChannel openForRead(String path) throws FileOperationException {
+        var file = resolvePath(path);
+        if (!Files.isRegularFile(file)) throw new FileOperationException(path, "No such file");
+
+        try {
+            return Files.newByteChannel(file, READ_OPTIONS);
+        } catch (IOException e) {
+            throw remapException(path, e);
+        }
+    }
+
+    /**
+     * Remap a {@link IOException} to a friendlier {@link FileOperationException}.
+     *
+     * @param fallbackPath The path currently being operated on. This is used in errors when we cannot determine a more accurate path.
+     * @param exn          The exception that occurred.
+     * @return The wrapped exception.
+     */
+    protected FileOperationException remapException(String fallbackPath, IOException exn) {
+        return exn instanceof FileSystemException fsExn
+            ? remapException(fallbackPath, fsExn)
+            : new FileOperationException(fallbackPath, exn.getMessage() == null ? "Operation failed" : exn.getMessage());
+    }
+
+    /**
+     * Remap a {@link FileSystemException} to a friendlier {@link FileOperationException}, attempting to remap the path
+     * provided.
+     *
+     * @param fallbackPath The path currently being operated on. This is used in errors when we cannot determine a more accurate path.
+     * @param exn          The exception that occurred.
+     * @return The wrapped exception.
+     */
+    protected FileOperationException remapException(String fallbackPath, FileSystemException exn) {
+        var reason = getReason(exn);
+
+        var failedFile = exn.getFile();
+        if (failedFile == null) return new FileOperationException(fallbackPath, reason);
+
+        var failedPath = Path.of(failedFile);
+        return failedPath.startsWith(root)
+            ? new FileOperationException(root.relativize(failedPath).toString(), reason)
+            : new FileOperationException(fallbackPath, reason);
+    }
+
+    /**
+     * Get the user-friendly reason for a {@link FileSystemException}.
+     *
+     * @param exn The exception that occurred.
+     * @return The friendly reason for this exception.
+     */
+    protected String getReason(FileSystemException exn) {
+        if (exn instanceof FileAlreadyExistsException) return "File exists";
+        if (exn instanceof NoSuchFileException) return "No such file";
+        if (exn instanceof NotDirectoryException) return "Not a directory";
+        if (exn instanceof AccessDeniedException) return "Access denied";
+
+        var reason = exn.getReason();
+        return reason != null ? reason.trim() : "Operation failed";
+    }
 }

projects/core/src/main/java/dan200/computercraft/core/filesystem/FileSystem.java

@@ -19,7 +19,6 @@ import java.lang.ref.ReferenceQueue;
 import java.lang.ref.WeakReference;
 import java.nio.channels.Channel;
 import java.nio.channels.SeekableByteChannel;
-import java.nio.channels.WritableByteChannel;
 import java.nio.file.AccessDeniedException;
 import java.nio.file.attribute.BasicFileAttributes;
 import java.util.*;
@@ -354,7 +353,7 @@ public class FileSystem {
         return openFile(mount, channel, open.apply(channel));
     }
 
-    public synchronized <T extends Closeable> FileSystemWrapper<T> openForWrite(String path, boolean append, Function<WritableByteChannel, T> open) throws FileSystemException {
+    public synchronized <T extends Closeable> FileSystemWrapper<T> openForWrite(String path, boolean append, Function<SeekableByteChannel, T> open) throws FileSystemException {
         cleanup();
 
         path = sanitizePath(path);

projects/core/src/main/java/dan200/computercraft/core/filesystem/JarMount.java

@@ -5,6 +5,7 @@
  */
 package dan200.computercraft.core.filesystem;
 
+import dan200.computercraft.api.filesystem.FileOperationException;
 import dan200.computercraft.core.util.Nullability;
 
 import javax.annotation.Nullable;
@@ -42,7 +43,7 @@ public class JarMount extends ArchiveMount<JarMount.FileEntry> implements Closea
         }
 
         // Read in all the entries
-        root = new FileEntry();
+        root = new FileEntry("");
         var zipEntries = zip.entries();
         while (zipEntries.hasMoreElements()) {
             var entry = zipEntries.nextElement();
@@ -68,7 +69,7 @@ public class JarMount extends ArchiveMount<JarMount.FileEntry> implements Closea
 
             var nextEntry = lastEntry.children.get(part);
             if (nextEntry == null || !nextEntry.isDirectory()) {
-                lastEntry.children.put(part, nextEntry = new FileEntry());
+                lastEntry.children.put(part, nextEntry = new FileEntry(localPath.substring(0, nextIndex)));
             }
 
             lastEntry = nextEntry;
@@ -79,26 +80,26 @@ public class JarMount extends ArchiveMount<JarMount.FileEntry> implements Closea
     }
 
     @Override
-    protected long getSize(FileEntry file) throws IOException {
+    protected long getSize(FileEntry file) throws FileOperationException {
-        if (file.zipEntry == null) throw new FileNotFoundException(NO_SUCH_FILE);
+        if (file.zipEntry == null) throw new FileOperationException(file.path, NO_SUCH_FILE);
         return file.zipEntry.getSize();
     }
 
     @Override
-    protected byte[] getContents(FileEntry file) throws IOException {
+    protected byte[] getContents(FileEntry file) throws FileOperationException {
-        if (file.zipEntry == null) throw new FileNotFoundException(NO_SUCH_FILE);
+        if (file.zipEntry == null) throw new FileOperationException(file.path, NO_SUCH_FILE);
 
         try (var stream = zip.getInputStream(file.zipEntry)) {
             return stream.readAllBytes();
         } catch (IOException e) {
             // Mask other IO exceptions as a non-existent file.
-            throw new FileNotFoundException(NO_SUCH_FILE);
+            throw new FileOperationException(file.path, NO_SUCH_FILE);
         }
     }
 
     @Override
-    public BasicFileAttributes getAttributes(FileEntry file) throws IOException {
+    public BasicFileAttributes getAttributes(FileEntry file) throws FileOperationException {
-        if (file.zipEntry == null) throw new FileNotFoundException(NO_SUCH_FILE);
+        if (file.zipEntry == null) throw new FileOperationException(file.path, NO_SUCH_FILE);
         return new ZipEntryAttributes(file.zipEntry);
     }
 
@@ -111,6 +112,10 @@ public class JarMount extends ArchiveMount<JarMount.FileEntry> implements Closea
         @Nullable
         ZipEntry zipEntry;
 
+        protected FileEntry(String path) {
+            super(path);
+        }
+
         void setup(ZipEntry entry) {
             zipEntry = entry;
             size = entry.getSize();

projects/core/src/main/java/dan200/computercraft/core/filesystem/MountWrapper.java

@@ -12,10 +12,6 @@ import dan200.computercraft.api.filesystem.WritableMount;
 import javax.annotation.Nullable;
 import java.io.IOException;
 import java.nio.channels.SeekableByteChannel;
-import java.nio.channels.WritableByteChannel;
-import java.nio.file.AccessDeniedException;
-import java.nio.file.FileAlreadyExistsException;
-import java.nio.file.NoSuchFileException;
 import java.nio.file.attribute.BasicFileAttributes;
 import java.util.List;
 import java.util.OptionalLong;
@@ -158,8 +154,6 @@ class MountWrapper {
             if (mount.exists(path)) {
                 writableMount.delete(path);
             }
-        } catch (AccessDeniedException e) {
-            throw new FileSystemException("Access denied");
         } catch (IOException e) {
             throw localExceptionOf(path, e);
         }
@@ -177,14 +171,12 @@ class MountWrapper {
             }
 
             writableMount.rename(source, dest);
-        } catch (AccessDeniedException e) {
-            throw new FileSystemException("Access denied");
         } catch (IOException e) {
             throw localExceptionOf(source, e);
         }
     }
 
-    public WritableByteChannel openForWrite(String path) throws FileSystemException {
+    public SeekableByteChannel openForWrite(String path) throws FileSystemException {
         if (writableMount == null) throw exceptionOf(path, "Access denied");
 
         path = toLocal(path);
@@ -200,14 +192,12 @@ class MountWrapper {
                 }
                 return writableMount.openForWrite(path);
             }
-        } catch (AccessDeniedException e) {
-            throw new FileSystemException("Access denied");
         } catch (IOException e) {
             throw localExceptionOf(path, e);
         }
     }
 
-    public WritableByteChannel openForAppend(String path) throws FileSystemException {
+    public SeekableByteChannel openForAppend(String path) throws FileSystemException {
         if (writableMount == null) throw exceptionOf(path, "Access denied");
 
         path = toLocal(path);
@@ -225,8 +215,6 @@ class MountWrapper {
             } else {
                 return writableMount.openForAppend(path);
             }
-        } catch (AccessDeniedException e) {
-            throw new FileSystemException("Access denied");
         } catch (IOException e) {
             throw localExceptionOf(path, e);
         }
@@ -244,7 +232,8 @@ class MountWrapper {
         if (e instanceof java.nio.file.FileSystemException ex) {
             // This error will contain the absolute path, leaking information about where MC is installed. We drop that,
             // just taking the reason. We assume that the error refers to the input path.
-            var message = getReason(ex);
+            var message = ex.getReason();
+            if (message == null) message = "Failed";
             return localPath == null ? new FileSystemException(message) : localExceptionOf(localPath, message);
         }
 
@@ -259,15 +248,4 @@ class MountWrapper {
     private static FileSystemException exceptionOf(String path, String message) {
         return new FileSystemException("/" + path + ": " + message);
     }
-
-    private static String getReason(java.nio.file.FileSystemException e) {
-        var reason = e.getReason();
-        if (reason != null) return reason.trim();
-
-        if (e instanceof FileAlreadyExistsException) return "File exists";
-        if (e instanceof NoSuchFileException) return "No such file";
-        if (e instanceof AccessDeniedException) return "Access denied";
-
-        return "Operation failed";
-    }
 }

projects/core/src/main/java/dan200/computercraft/core/filesystem/WritableFileMount.java

@@ -0,0 +1,322 @@
+/*
+ * This file is part of ComputerCraft - http://www.computercraft.info
+ * Copyright Daniel Ratcliffe, 2011-2022. Do not distribute without permission.
+ * Send enquiries to dratcliffe@gmail.com
+ */
+package dan200.computercraft.core.filesystem;
+
+import com.google.common.collect.Sets;
+import dan200.computercraft.api.filesystem.FileOperationException;
+import dan200.computercraft.api.filesystem.WritableMount;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import javax.annotation.Nullable;
+import java.io.File;
+import java.io.IOException;
+import java.nio.ByteBuffer;
+import java.nio.channels.ClosedChannelException;
+import java.nio.channels.NonReadableChannelException;
+import java.nio.channels.SeekableByteChannel;
+import java.nio.file.*;
+import java.nio.file.attribute.BasicFileAttributes;
+import java.util.Set;
+
+/**
+ * A {@link WritableFileMount} implementation which provides read-write access to a directory.
+ */
+public class WritableFileMount extends FileMount implements WritableMount {
+    private static final Logger LOG = LoggerFactory.getLogger(WritableFileMount.class);
+
+    private static final long MINIMUM_FILE_SIZE = 500;
+    private static final Set<OpenOption> WRITE_OPTIONS = Sets.newHashSet(StandardOpenOption.WRITE, StandardOpenOption.CREATE, StandardOpenOption.TRUNCATE_EXISTING);
+    private static final Set<OpenOption> APPEND_OPTIONS = Sets.newHashSet(StandardOpenOption.WRITE, StandardOpenOption.CREATE, StandardOpenOption.APPEND);
+
+    protected final File rootFile;
+    private final long capacity;
+    private long usedSpace;
+
+    public WritableFileMount(File rootFile, long capacity) {
+        super(rootFile.toPath());
+        this.rootFile = rootFile;
+        this.capacity = capacity + MINIMUM_FILE_SIZE;
+        usedSpace = created() ? measureUsedSpace(root) : MINIMUM_FILE_SIZE;
+    }
+
+    protected File resolveFile(String path) {
+        return new File(rootFile, path);
+    }
+
+    private void create() throws FileOperationException {
+        try {
+            Files.createDirectories(root);
+        } catch (IOException e) {
+            throw new FileOperationException("Access denied");
+        }
+    }
+
+    @Override
+    public long getRemainingSpace() {
+        return Math.max(capacity - usedSpace, 0);
+    }
+
+    @Override
+    public long getCapacity() {
+        return capacity - MINIMUM_FILE_SIZE;
+    }
+
+    @Override
+    public boolean isReadOnly(String path) {
+        var file = resolveFile(path);
+        while (true) {
+            if (file.exists()) return !file.canWrite();
+            if (file.equals(rootFile)) return false;
+            file = file.getParentFile();
+        }
+    }
+
+    @Override
+    public void makeDirectory(String path) throws IOException {
+        create();
+        var file = resolveFile(path);
+        if (file.exists()) {
+            if (!file.isDirectory()) throw new FileOperationException(path, "File exists");
+            return;
+        }
+
+        var dirsToCreate = 1;
+        var parent = file.getParentFile();
+        while (!parent.exists()) {
+            ++dirsToCreate;
+            parent = parent.getParentFile();
+        }
+
+        if (getRemainingSpace() < dirsToCreate * MINIMUM_FILE_SIZE) {
+            throw new FileOperationException(path, "Out of space");
+        }
+
+        if (file.mkdirs()) {
+            usedSpace += dirsToCreate * MINIMUM_FILE_SIZE;
+        } else {
+            throw new FileOperationException(path, "Access denied");
+        }
+    }
+
+    @Override
+    public void delete(String path) throws IOException {
+        if (path.isEmpty()) throw new FileOperationException(path, "Access denied");
+
+        if (created()) {
+            var file = resolveFile(path);
+            if (file.exists()) deleteRecursively(file);
+        }
+    }
+
+    private void deleteRecursively(File file) throws IOException {
+        // Empty directories first
+        if (file.isDirectory()) {
+            var children = file.list();
+            for (var aChildren : children) {
+                deleteRecursively(new File(file, aChildren));
+            }
+        }
+
+        // Then delete
+        var fileSize = file.isDirectory() ? 0 : file.length();
+        var success = file.delete();
+        if (success) {
+            usedSpace -= Math.max(MINIMUM_FILE_SIZE, fileSize);
+        } else {
+            throw new IOException("Access denied");
+        }
+    }
+
+    @Override
+    public void rename(String source, String dest) throws FileOperationException {
+        var sourceFile = resolvePath(source);
+        var destFile = resolvePath(dest);
+        if (!Files.exists(sourceFile)) throw new FileOperationException(source, "No such file");
+        if (Files.exists(destFile)) throw new FileOperationException(dest, "File exists");
+
+        if (destFile.startsWith(sourceFile)) {
+            throw new FileOperationException(source, "Cannot move a directory inside itself");
+        }
+
+        try {
+            Files.move(sourceFile, destFile);
+        } catch (IOException e) {
+            throw remapException(source, e);
+        }
+    }
+
+    private @Nullable BasicFileAttributes tryGetAttributes(String path, Path resolved) throws FileOperationException {
+        try {
+            return Files.readAttributes(resolved, BasicFileAttributes.class);
+        } catch (NoSuchFileException ignored) {
+            return null;
+        } catch (IOException e) {
+            throw remapException(path, e);
+        }
+    }
+
+    @Override
+    public SeekableByteChannel openForWrite(String path) throws FileOperationException {
+        create();
+
+        var file = resolvePath(path);
+        var attributes = tryGetAttributes(path, file);
+        if (attributes == null) {
+            if (getRemainingSpace() < MINIMUM_FILE_SIZE) throw new FileOperationException(path, "Out of space");
+        } else if (attributes.isDirectory()) {
+            throw new FileOperationException(path, "Cannot write to directory");
+        } else {
+            usedSpace -= Math.max(attributes.size(), MINIMUM_FILE_SIZE);
+        }
+
+        usedSpace += MINIMUM_FILE_SIZE;
+
+        try {
+            return new CountingChannel(Files.newByteChannel(file, WRITE_OPTIONS), MINIMUM_FILE_SIZE, true);
+        } catch (IOException e) {
+            throw remapException(path, e);
+        }
+    }
+
+    @Override
+    public SeekableByteChannel openForAppend(String path) throws IOException {
+        create();
+
+        var file = resolvePath(path);
+        var attributes = tryGetAttributes(path, file);
+        if (attributes == null) {
+            if (getRemainingSpace() < MINIMUM_FILE_SIZE) throw new FileOperationException(path, "Out of space");
+        } else if (attributes.isDirectory()) {
+            throw new FileOperationException(path, "Cannot write to directory");
+        }
+
+        // Allowing seeking when appending is not recommended, so we use a separate channel.
+        try {
+            return new CountingChannel(
+                Files.newByteChannel(file, APPEND_OPTIONS),
+                Math.max(MINIMUM_FILE_SIZE - (attributes == null ? 0 : attributes.size()), 0),
+                false
+            );
+        } catch (IOException e) {
+            throw remapException(path, e);
+        }
+    }
+
+    private class CountingChannel implements SeekableByteChannel {
+        private final SeekableByteChannel channel;
+        private long ignoredBytesLeft;
+        private final boolean canSeek;
+
+        CountingChannel(SeekableByteChannel channel, long bytesToIgnore, boolean canSeek) {
+            this.channel = channel;
+            ignoredBytesLeft = bytesToIgnore;
+            this.canSeek = canSeek;
+        }
+
+        @Override
+        public int write(ByteBuffer b) throws IOException {
+            count(b.remaining());
+            return channel.write(b);
+        }
+
+        void count(long n) throws IOException {
+            ignoredBytesLeft -= n;
+            if (ignoredBytesLeft < 0) {
+                var newBytes = -ignoredBytesLeft;
+                ignoredBytesLeft = 0;
+
+                var bytesLeft = capacity - usedSpace;
+                if (newBytes > bytesLeft) throw new IOException("Out of space");
+                usedSpace += newBytes;
+            }
+        }
+
+        @Override
+        public boolean isOpen() {
+            return channel.isOpen();
+        }
+
+        @Override
+        public void close() throws IOException {
+            channel.close();
+        }
+
+        @Override
+        public SeekableByteChannel position(long newPosition) throws IOException {
+            if (!isOpen()) throw new ClosedChannelException();
+            if (!canSeek) throw new UnsupportedOperationException("File does not support seeking");
+            if (newPosition < 0) {
+                throw new IllegalArgumentException("Cannot seek before the beginning of the stream");
+            }
+
+            var delta = newPosition - channel.position();
+            if (delta < 0) {
+                ignoredBytesLeft -= delta;
+            } else {
+                count(delta);
+            }
+
+            return channel.position(newPosition);
+        }
+
+        @Override
+        public SeekableByteChannel truncate(long size) throws IOException {
+            throw new UnsupportedOperationException("File cannot be truncated");
+        }
+
+        @Override
+        public int read(ByteBuffer dst) throws ClosedChannelException {
+            if (!channel.isOpen()) throw new ClosedChannelException();
+            throw new NonReadableChannelException();
+        }
+
+        @Override
+        public long position() throws IOException {
+            return channel.position();
+        }
+
+        @Override
+        public long size() throws IOException {
+            return channel.size();
+        }
+    }
+
+    private static long measureUsedSpace(Path path) {
+        if (!Files.exists(path)) return 0;
+
+        class CountingVisitor extends SimpleFileVisitor<Path> {
+            long size;
+
+            @Override
+            public FileVisitResult preVisitDirectory(Path dir, BasicFileAttributes attrs) {
+                size += MINIMUM_FILE_SIZE;
+                return FileVisitResult.CONTINUE;
+            }
+
+            @Override
+            public FileVisitResult visitFile(Path file, BasicFileAttributes attrs) {
+                size += Math.max(attrs.size(), MINIMUM_FILE_SIZE);
+                return FileVisitResult.CONTINUE;
+            }
+
+            @Override
+            public FileVisitResult visitFileFailed(Path file, IOException exc) {
+                LOG.error("Error computing file size for {}", file, exc);
+                return FileVisitResult.CONTINUE;
+            }
+        }
+
+        try {
+            var visitor = new CountingVisitor();
+            Files.walkFileTree(path, visitor);
+            return visitor.size;
+        } catch (IOException e) {
+            LOG.error("Error computing file size for {}", path, e);
+            return 0;
+        }
+    }
+}

projects/core/src/test/java/dan200/computercraft/core/ComputerTestDelegate.java

@@ -13,8 +13,8 @@ import dan200.computercraft.api.peripheral.IPeripheral;
 import dan200.computercraft.core.computer.Computer;
 import dan200.computercraft.core.computer.ComputerSide;
 import dan200.computercraft.core.computer.mainthread.NoWorkMainThreadScheduler;
-import dan200.computercraft.core.filesystem.FileMount;
 import dan200.computercraft.core.filesystem.FileSystemException;
+import dan200.computercraft.core.filesystem.WritableFileMount;
 import dan200.computercraft.core.terminal.Terminal;
 import dan200.computercraft.test.core.computer.BasicEnvironment;
 import org.junit.jupiter.api.*;
@@ -85,7 +85,7 @@ public class ComputerTestDelegate {
         if (Files.deleteIfExists(REPORT_PATH)) LOG.info("Deleted previous coverage report.");
 
         var term = new Terminal(80, 100, true);
-        WritableMount mount = new FileMount(TestFiles.get("mount").toFile(), 10_000_000);
+        WritableMount mount = new WritableFileMount(TestFiles.get("mount").toFile(), 10_000_000);
 
         // Remove any existing files
         List<String> children = new ArrayList<>();

projects/core/src/test/java/dan200/computercraft/core/computer/ComputerBootstrap.java

@@ -16,6 +16,7 @@ import dan200.computercraft.core.computer.mainthread.MainThread;
 import dan200.computercraft.core.terminal.Terminal;
 import dan200.computercraft.test.core.computer.BasicEnvironment;
 import dan200.computercraft.test.core.filesystem.MemoryMount;
+import dan200.computercraft.test.core.filesystem.ReadOnlyWritableMount;
 import org.junit.jupiter.api.Assertions;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
@@ -37,7 +38,7 @@ public class ComputerBootstrap {
             .addFile("test.lua", program)
             .addFile("startup.lua", "assertion.assert(pcall(loadfile('test.lua', nil, _ENV))) os.shutdown()");
 
-        run(mount, setup, maxTimes);
+        run(new ReadOnlyWritableMount(mount), setup, maxTimes);
     }
 
     public static void run(String program, int maxTimes) {

projects/core/src/test/java/dan200/computercraft/core/filesystem/FileMountTest.java

@@ -7,9 +7,10 @@ package dan200.computercraft.core.filesystem;
 
 import com.google.common.io.MoreFiles;
 import com.google.common.io.RecursiveDeleteOption;
-import dan200.computercraft.api.filesystem.WritableMount;
+import dan200.computercraft.api.filesystem.Mount;
+import dan200.computercraft.test.core.filesystem.MountContract;
 import org.junit.jupiter.api.AfterEach;
-import org.junit.jupiter.api.Assumptions;
+import org.junit.jupiter.api.Test;
 
 import java.io.IOException;
 import java.nio.file.Files;
@@ -17,50 +18,60 @@ import java.nio.file.Path;
 import java.util.ArrayList;
 import java.util.List;
 
-public class FileMountTest implements WritableMountContract {
+import static org.junit.jupiter.api.Assertions.*;
+
+public class FileMountTest implements MountContract {
     private final List<Path> cleanup = new ArrayList<>();
 
+    @Override
+    public Mount createSkeleton() throws IOException {
+        var path = Files.createTempDirectory("cctweaked-test");
+        cleanup.add(path);
+
+        Files.createDirectories(path.resolve("dir"));
+        try (var writer = Files.newBufferedWriter(path.resolve("dir/file.lua"))) {
+            writer.write("print('testing')");
+        }
+        Files.setLastModifiedTime(path.resolve("dir/file.lua"), MODIFY_TIME);
+        Files.newBufferedWriter(path.resolve("f.lua")).close();
+
+        return new FileMount(path);
+    }
+
+    private Mount createEmpty() throws IOException {
+        var path = Files.createTempDirectory("cctweaked-test");
+        cleanup.add(path);
+        return new FileMount(path.resolve("empty"));
+    }
+
     @AfterEach
     public void cleanup() throws IOException {
         for (var mount : cleanup) MoreFiles.deleteRecursively(mount, RecursiveDeleteOption.ALLOW_INSECURE);
     }
 
-    @Override
+    @Test
-    public MountAccess createMount(long capacity) throws IOException {
+    public void testRootExistsWhenEmpty() throws IOException {
-        var path = Files.createTempDirectory("cctweaked-test");
+        var mount = createEmpty();
-        cleanup.add(path);
+        assertTrue(mount.exists(""), "Root always exists");
-        return new MountAccessImpl(path.resolve("mount"), capacity);
+        assertTrue(mount.isDirectory(""), "Root always is a directory");
     }
 
-    private static final class MountAccessImpl implements MountAccess {
+    @Test
-        private final Path root;
+    public void testListWhenEmpty() throws IOException {
-        private final long capacity;
+        var mount = createEmpty();
-        private final WritableMount mount;
 
-        private MountAccessImpl(Path root, long capacity) {
+        List<String> list = new ArrayList<>();
-            this.root = root;
+        mount.list("", list);
-            this.capacity = capacity;
+        assertEquals(List.of(), list, "Root has no children");
-            mount = new FileMount(root.toFile(), capacity);
-        }
 
-        @Override
+        assertThrows(IOException.class, () -> mount.list("elsewhere", list));
-        public WritableMount mount() {
+    }
-            return mount;
-        }
 
-        @Override
+    @Test
-        public void makeReadOnly(String path) {
+    public void testAttributesWhenEmpty() throws IOException {
-            Assumptions.assumeTrue(root.resolve(path).toFile().setReadOnly(), "Change file to read-only");
+        var mount = createEmpty();
-        }
 
-        @Override
+        assertEquals(0, mount.getSize(""));
-        public void ensuresExist() throws IOException {
+        assertNotNull(mount.getAttributes(""));
-            Files.createDirectories(root);
-        }
-
-        @Override
-        public long computeRemainingSpace() {
-            return new FileMount(root.toFile(), capacity).getRemainingSpace();
-        }
     }
 }

projects/core/src/test/java/dan200/computercraft/core/filesystem/FileSystemTest.java

@@ -27,7 +27,7 @@ public class FileSystemTest {
     private static final long CAPACITY = 1000000;
 
     private static FileSystem mkFs() throws FileSystemException {
-        WritableMount writableMount = new FileMount(ROOT, CAPACITY);
+        WritableMount writableMount = new WritableFileMount(ROOT, CAPACITY);
         return new FileSystem("hdd", writableMount);
 
     }
@@ -65,7 +65,7 @@ public class FileSystemTest {
     @Test
     public void testUnmountCloses() throws FileSystemException {
         var fs = mkFs();
-        WritableMount mount = new FileMount(new File(ROOT, "child"), CAPACITY);
+        WritableMount mount = new WritableFileMount(new File(ROOT, "child"), CAPACITY);
         fs.mountWritable("disk", "disk", mount);
 
         var writer = fs.openForWrite("disk/out.txt", false, EncodedWritableHandle::openUtf8);

projects/core/src/test/java/dan200/computercraft/core/filesystem/JarMountTest.java

@@ -8,6 +8,9 @@ package dan200.computercraft.core.filesystem;
 import com.google.common.io.ByteStreams;
 import dan200.computercraft.api.filesystem.Mount;
 import dan200.computercraft.core.TestFiles;
+import dan200.computercraft.test.core.CloseScope;
+import dan200.computercraft.test.core.filesystem.MountContract;
+import org.junit.jupiter.api.AfterEach;
 import org.junit.jupiter.api.BeforeAll;
 import org.junit.jupiter.api.Test;
 
@@ -16,50 +19,67 @@ import java.io.FileOutputStream;
 import java.io.IOException;
 import java.nio.channels.Channels;
 import java.nio.charset.StandardCharsets;
-import java.nio.file.attribute.FileTime;
-import java.time.Instant;
-import java.time.temporal.ChronoUnit;
 import java.util.zip.ZipEntry;
 import java.util.zip.ZipOutputStream;
 
 import static org.junit.jupiter.api.Assertions.*;
 
-public class JarMountTest {
+public class JarMountTest implements MountContract {
     private static final File ZIP_FILE = TestFiles.get("jar-mount.zip").toFile();
 
-    private static final FileTime MODIFY_TIME = FileTime.from(Instant.EPOCH.plus(2, ChronoUnit.DAYS));
+    private final CloseScope toClose = new CloseScope();
 
     @BeforeAll
     public static void before() throws IOException {
         ZIP_FILE.getParentFile().mkdirs();
 
         try (var stream = new ZipOutputStream(new FileOutputStream(ZIP_FILE))) {
-            stream.putNextEntry(new ZipEntry("dir/"));
+            stream.putNextEntry(new ZipEntry("root/"));
             stream.closeEntry();
 
-            stream.putNextEntry(new ZipEntry("dir/file.lua").setLastModifiedTime(MODIFY_TIME));
+            stream.putNextEntry(new ZipEntry("root/dir/"));
+            stream.closeEntry();
+
+            stream.putNextEntry(new ZipEntry("root/dir/file.lua").setLastModifiedTime(MODIFY_TIME));
             stream.write("print('testing')".getBytes(StandardCharsets.UTF_8));
             stream.closeEntry();
+
+            stream.putNextEntry(new ZipEntry("root/f.lua"));
+            stream.closeEntry();
         }
     }
 
+    @AfterEach
+    public void after() throws Exception {
+        toClose.close();
+    }
+
+    @Override
+    public Mount createSkeleton() throws IOException {
+        return toClose.add(new JarMount(ZIP_FILE, "root"));
+    }
+
+    private Mount createMount(String path) throws IOException {
+        return toClose.add(new JarMount(ZIP_FILE, "root/" + path));
+    }
+
     @Test
     public void mountsDir() throws IOException {
-        Mount mount = new JarMount(ZIP_FILE, "dir");
+        var mount = createMount("dir");
         assertTrue(mount.isDirectory(""), "Root should be directory");
         assertTrue(mount.exists("file.lua"), "File should exist");
     }
 
     @Test
     public void mountsFile() throws IOException {
-        Mount mount = new JarMount(ZIP_FILE, "dir/file.lua");
+        var mount = createMount("dir/file.lua");
         assertTrue(mount.exists(""), "Root should exist");
         assertFalse(mount.isDirectory(""), "Root should be a file");
     }
 
     @Test
     public void opensFileFromFile() throws IOException {
-        Mount mount = new JarMount(ZIP_FILE, "dir/file.lua");
+        var mount = createMount("dir/file.lua");
         byte[] contents;
         try (var stream = mount.openForRead("")) {
             contents = ByteStreams.toByteArray(Channels.newInputStream(stream));
@@ -70,7 +90,7 @@ public class JarMountTest {
 
     @Test
     public void opensFileFromDir() throws IOException {
-        Mount mount = new JarMount(ZIP_FILE, "dir");
+        var mount = createMount("dir");
         byte[] contents;
         try (var stream = mount.openForRead("file.lua")) {
             contents = ByteStreams.toByteArray(Channels.newInputStream(stream));
@@ -78,19 +98,4 @@ public class JarMountTest {
 
         assertEquals(new String(contents, StandardCharsets.UTF_8), "print('testing')");
     }
-
-    @Test
-    public void fileAttributes() throws IOException {
-        var attributes = new JarMount(ZIP_FILE, "dir").getAttributes("file.lua");
-        assertFalse(attributes.isDirectory());
-        assertEquals("print('testing')".length(), attributes.size());
-        assertEquals(MODIFY_TIME, attributes.lastModifiedTime());
-    }
-
-    @Test
-    public void directoryAttributes() throws IOException {
-        var attributes = new JarMount(ZIP_FILE, "dir").getAttributes("");
-        assertTrue(attributes.isDirectory());
-        assertEquals(0, attributes.size());
-    }
 }

projects/core/src/test/java/dan200/computercraft/core/filesystem/WritableFileMountTest.java

@@ -0,0 +1,67 @@
+/*
+ * This file is part of ComputerCraft - http://www.computercraft.info
+ * Copyright Daniel Ratcliffe, 2011-2022. Do not distribute without permission.
+ * Send enquiries to dratcliffe@gmail.com
+ */
+package dan200.computercraft.core.filesystem;
+
+import com.google.common.io.MoreFiles;
+import com.google.common.io.RecursiveDeleteOption;
+import dan200.computercraft.api.filesystem.WritableMount;
+import dan200.computercraft.test.core.filesystem.WritableMountContract;
+import org.junit.jupiter.api.AfterEach;
+import org.junit.jupiter.api.Assumptions;
+
+import java.io.IOException;
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.util.ArrayList;
+import java.util.List;
+
+public class WritableFileMountTest implements WritableMountContract {
+    private final List<Path> cleanup = new ArrayList<>();
+
+    @Override
+    public MountAccess createMount(long capacity) throws IOException {
+        var path = Files.createTempDirectory("cctweaked-test");
+        cleanup.add(path);
+        return new MountAccessImpl(path.resolve("mount"), capacity);
+    }
+
+    @AfterEach
+    public void cleanup() throws IOException {
+        for (var mount : cleanup) MoreFiles.deleteRecursively(mount, RecursiveDeleteOption.ALLOW_INSECURE);
+    }
+
+    private static final class MountAccessImpl implements MountAccess {
+        private final Path root;
+        private final long capacity;
+        private final WritableMount mount;
+
+        private MountAccessImpl(Path root, long capacity) {
+            this.root = root;
+            this.capacity = capacity;
+            mount = new WritableFileMount(root.toFile(), capacity);
+        }
+
+        @Override
+        public WritableMount mount() {
+            return mount;
+        }
+
+        @Override
+        public void makeReadOnly(String path) {
+            Assumptions.assumeTrue(root.resolve(path).toFile().setReadOnly(), "Change file to read-only");
+        }
+
+        @Override
+        public void ensuresExist() throws IOException {
+            Files.createDirectories(root);
+        }
+
+        @Override
+        public long computeRemainingSpace() {
+            return new WritableFileMount(root.toFile(), capacity).getRemainingSpace();
+        }
+    }
+}

projects/core/src/testFixtures/java/dan200/computercraft/test/core/CloseScope.java

@@ -0,0 +1,61 @@
+/*
+ * This file is part of ComputerCraft - http://www.computercraft.info
+ * Copyright Daniel Ratcliffe, 2011-2022. Do not distribute without permission.
+ * Send enquiries to dratcliffe@gmail.com
+ */
+package dan200.computercraft.test.core;
+
+import javax.annotation.Nullable;
+import java.util.ArrayDeque;
+import java.util.Deque;
+import java.util.Objects;
+
+/**
+ * An {@link AutoCloseable} implementation which can be used to combine other [AutoCloseable] instances.
+ * <p>
+ * Values which implement {@link AutoCloseable} can be dynamically registered with [CloseScope.add]. When the scope is
+ * closed, each value is closed in the opposite order.
+ * <p>
+ * This is largely intended for cases where it's not appropriate to nest try-with-resources blocks, for instance when
+ * nested would be too deep or when objects are dynamically created.
+ */
+public class CloseScope implements AutoCloseable {
+    private final Deque<AutoCloseable> toClose = new ArrayDeque<>();
+
+    /**
+     * Add a value to be closed when this scope exists.
+     *
+     * @param value The value to be closed.
+     * @param <T>   The type of the provided value.
+     * @return The provided value.
+     */
+    public <T extends AutoCloseable> T add(T value) {
+        Objects.requireNonNull(value, "Value cannot be null");
+        toClose.add(value);
+        return value;
+    }
+
+    @Override
+    public void close() throws Exception {
+        close(null);
+    }
+
+    public void close(@Nullable Exception baseException) throws Exception {
+        while (true) {
+            var close = toClose.pollLast();
+            if (close == null) break;
+
+            try {
+                close.close();
+            } catch (Exception e) {
+                if (baseException == null) {
+                    baseException = e;
+                } else {
+                    baseException.addSuppressed(e);
+                }
+            }
+        }
+
+        if (baseException != null) throw baseException;
+    }
+}

projects/core/src/testFixtures/java/dan200/computercraft/test/core/computer/BasicEnvironment.java

@@ -15,6 +15,7 @@ import dan200.computercraft.core.filesystem.JarMount;
 import dan200.computercraft.core.metrics.Metric;
 import dan200.computercraft.core.metrics.MetricsObserver;
 import dan200.computercraft.test.core.filesystem.MemoryMount;
+import dan200.computercraft.test.core.filesystem.ReadOnlyWritableMount;
 
 import java.io.File;
 import java.io.IOException;
@@ -32,7 +33,7 @@ public class BasicEnvironment implements ComputerEnvironment, GlobalEnvironment,
     private final WritableMount mount;
 
     public BasicEnvironment() {
-        this(new MemoryMount());
+        this(new ReadOnlyWritableMount(new MemoryMount()));
     }
 
     public BasicEnvironment(WritableMount mount) {
@@ -100,7 +101,7 @@ public class BasicEnvironment implements ComputerEnvironment, GlobalEnvironment,
 
             if (!wholeFile.exists()) throw new IllegalStateException("Cannot find ROM mount at " + file);
 
-            return new FileMount(wholeFile, 0);
+            return new FileMount(wholeFile.toPath());
         }
     }
 

projects/core/src/testFixtures/java/dan200/computercraft/test/core/filesystem/MemoryMount.java

@@ -6,22 +6,17 @@
 package dan200.computercraft.test.core.filesystem;
 
 import dan200.computercraft.api.filesystem.FileOperationException;
-import dan200.computercraft.api.filesystem.WritableMount;
+import dan200.computercraft.api.filesystem.Mount;
 import dan200.computercraft.core.apis.handles.ArrayByteChannel;
 
-import java.io.ByteArrayOutputStream;
-import java.io.File;
-import java.io.IOException;
-import java.nio.channels.Channels;
 import java.nio.channels.SeekableByteChannel;
-import java.nio.channels.WritableByteChannel;
 import java.nio.charset.StandardCharsets;
 import java.util.*;
 
 /**
- * In-memory file mounts.
+ * A read-only mount {@link Mount} which provides a list of in-memory set of files.
  */
-public class MemoryMount implements WritableMount {
+public class MemoryMount implements Mount {
     private final Map<String, byte[]> files = new HashMap<>();
     private final Set<String> directories = new HashSet<>();
 
@@ -29,69 +24,6 @@ public class MemoryMount implements WritableMount {
         directories.add("");
     }
 
-    @Override
-    public void makeDirectory(String path) {
-        var file = new File(path);
-        while (file != null) {
-            directories.add(file.getPath());
-            file = file.getParentFile();
-        }
-    }
-
-    @Override
-    public void delete(String path) {
-        if (files.containsKey(path)) {
-            files.remove(path);
-        } else {
-            directories.remove(path);
-            for (var file : files.keySet().toArray(new String[0])) {
-                if (file.startsWith(path)) {
-                    files.remove(file);
-                }
-            }
-
-            var parent = new File(path).getParentFile();
-            if (parent != null) delete(parent.getPath());
-        }
-    }
-
-    @Override
-    public void rename(String source, String dest) throws IOException {
-        throw new IOException("Not supported");
-    }
-
-    @Override
-    public WritableByteChannel openForWrite(final String path) {
-        return Channels.newChannel(new ByteArrayOutputStream() {
-            @Override
-            public void close() throws IOException {
-                super.close();
-                files.put(path, toByteArray());
-            }
-        });
-    }
-
-    @Override
-    public WritableByteChannel openForAppend(final String path) throws IOException {
-        var stream = new ByteArrayOutputStream() {
-            @Override
-            public void close() throws IOException {
-                super.close();
-                files.put(path, toByteArray());
-            }
-        };
-
-        var current = files.get(path);
-        if (current != null) stream.write(current);
-
-        return Channels.newChannel(stream);
-    }
-
-    @Override
-    public long getRemainingSpace() {
-        return 1000000L;
-    }
-
     @Override
     public boolean exists(String path) {
         return files.containsKey(path) || directories.contains(path);
@@ -114,11 +46,6 @@ public class MemoryMount implements WritableMount {
         throw new RuntimeException("Not implemented");
     }
 
-    @Override
-    public long getCapacity() {
-        return Long.MAX_VALUE;
-    }
-
     @Override
     public SeekableByteChannel openForRead(String path) throws FileOperationException {
         var file = files.get(path);

projects/core/src/testFixtures/java/dan200/computercraft/test/core/filesystem/MountContract.java

@@ -0,0 +1,128 @@
+/*
+ * This file is part of ComputerCraft - http://www.computercraft.info
+ * Copyright Daniel Ratcliffe, 2011-2022. Do not distribute without permission.
+ * Send enquiries to dratcliffe@gmail.com
+ */
+package dan200.computercraft.test.core.filesystem;
+
+import dan200.computercraft.api.filesystem.FileOperationException;
+import dan200.computercraft.api.filesystem.Mount;
+import org.junit.jupiter.api.Test;
+
+import java.io.IOException;
+import java.nio.channels.Channels;
+import java.nio.charset.StandardCharsets;
+import java.nio.file.attribute.BasicFileAttributes;
+import java.nio.file.attribute.FileTime;
+import java.time.Instant;
+import java.time.temporal.ChronoUnit;
+import java.util.ArrayList;
+import java.util.Comparator;
+import java.util.List;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+/**
+ * The contract that all {@link Mount} implementations must fulfill.
+ *
+ * @see WritableMountContract
+ */
+public interface MountContract {
+    FileTime EPOCH = FileTime.from(Instant.EPOCH);
+    FileTime MODIFY_TIME = FileTime.from(Instant.EPOCH.plus(2, ChronoUnit.DAYS));
+
+    /**
+     * Create a skeleton mount. This should contain the following files:
+     *
+     * <ul>
+     *     <li>{@code dir/file.lua}, containing {@code print('testing')}.</li>
+     *     <li>{@code f.lua}, containing nothing.</li>
+     * </ul>
+     *
+     * @return The skeleton mount.
+     */
+    Mount createSkeleton() throws IOException;
+
+    /**
+     * Determine if this attributes provided by this mount support file times.
+     *
+     * @return Whether this mount supports {@link BasicFileAttributes#lastModifiedTime()}.
+     */
+    default boolean hasFileTimes() {
+        return true;
+    }
+
+    @Test
+    default void testIsDirectory() throws IOException {
+        var mount = createSkeleton();
+
+        assertTrue(mount.isDirectory(""), "Root should be directory");
+        assertTrue(mount.isDirectory("dir"), "dir/ should be directory");
+        assertFalse(mount.isDirectory("dir/file.lua"), "dir/file.lua should not be a directory");
+        assertFalse(mount.isDirectory("doesnt/exist"), "doesnt/exist should not be a directory");
+    }
+
+    @Test
+    default void testExists() throws IOException {
+        var mount = createSkeleton();
+
+        assertTrue(mount.exists(""), "Root should exist");
+        assertTrue(mount.exists("dir"), "dir/ should exist");
+        assertFalse(mount.isDirectory("doesnt/exist"), "doesnt/exist should not exist");
+    }
+
+    @Test
+    default void testList() throws IOException {
+        var mount = createSkeleton();
+
+        List<String> list = new ArrayList<>();
+        mount.list("", list);
+        list.sort(Comparator.naturalOrder());
+
+        assertEquals(List.of("dir", "f.lua"), list);
+    }
+
+    @Test
+    default void testOpenFile() throws IOException {
+        var mount = createSkeleton();
+
+        byte[] contents;
+        try (var stream = mount.openForRead("dir/file.lua")) {
+            contents = Channels.newInputStream(stream).readAllBytes();
+        }
+
+        assertEquals(new String(contents, StandardCharsets.UTF_8), "print('testing')");
+    }
+
+    @Test
+    default void testOpenFileFails() throws IOException {
+        var mount = createSkeleton();
+
+        var exn = assertThrows(FileOperationException.class, () -> mount.openForRead("doesnt/exist"));
+        assertEquals("doesnt/exist", exn.getFilename());
+        assertEquals("No such file", exn.getMessage());
+    }
+
+    @Test
+    default void testSize() throws IOException {
+        var mount = createSkeleton();
+
+        assertEquals(0, mount.getSize("f.lua"), "Empty file has 0 size");
+        assertEquals("print('testing')".length(), mount.getSize("dir/file.lua"));
+        assertEquals(0, mount.getSize("dir"), "Directory has 0 size");
+    }
+
+    @Test
+    default void testFileAttributes() throws IOException {
+        var attributes = createSkeleton().getAttributes("dir/file.lua");
+        assertFalse(attributes.isDirectory());
+        assertEquals("print('testing')".length(), attributes.size());
+        assertEquals(hasFileTimes() ? MODIFY_TIME : EPOCH, attributes.lastModifiedTime());
+    }
+
+    @Test
+    default void testDirectoryAttributes() throws IOException {
+        var attributes = createSkeleton().getAttributes("");
+        assertTrue(attributes.isDirectory());
+    }
+}

projects/core/src/testFixtures/java/dan200/computercraft/test/core/filesystem/ReadOnlyWritableMount.java

@@ -0,0 +1,92 @@
+/*
+ * This file is part of ComputerCraft - http://www.computercraft.info
+ * Copyright Daniel Ratcliffe, 2011-2022. Do not distribute without permission.
+ * Send enquiries to dratcliffe@gmail.com
+ */
+package dan200.computercraft.test.core.filesystem;
+
+import dan200.computercraft.api.filesystem.FileOperationException;
+import dan200.computercraft.api.filesystem.Mount;
+import dan200.computercraft.api.filesystem.WritableMount;
+
+import java.io.IOException;
+import java.nio.channels.SeekableByteChannel;
+import java.nio.file.attribute.BasicFileAttributes;
+import java.util.List;
+
+/**
+ * Wraps a {@link Mount} into a read-only {@link WritableMount}.
+ *
+ * @param mount The original read-only mount we're wrapping.
+ */
+public record ReadOnlyWritableMount(Mount mount) implements WritableMount {
+    @Override
+    public boolean exists(String path) throws IOException {
+        return mount.exists(path);
+    }
+
+    @Override
+    public boolean isDirectory(String path) throws IOException {
+        return mount.isDirectory(path);
+    }
+
+    @Override
+    public void list(String path, List<String> contents) throws IOException {
+        mount.list(path, contents);
+    }
+
+    @Override
+    public long getSize(String path) throws IOException {
+        return mount.getSize(path);
+    }
+
+    @Override
+    public SeekableByteChannel openForRead(String path) throws IOException {
+        return mount.openForRead(path);
+    }
+
+    @Override
+    public BasicFileAttributes getAttributes(String path) throws IOException {
+        return mount.getAttributes(path);
+    }
+
+    @Override
+    public void makeDirectory(String path) throws IOException {
+        throw new FileOperationException(path, "Access denied");
+    }
+
+    @Override
+    public void delete(String path) throws IOException {
+        throw new FileOperationException(path, "Access denied");
+    }
+
+    @Override
+    public void rename(String source, String dest) throws IOException {
+        throw new FileOperationException(source, "Access denied");
+    }
+
+    @Override
+    public SeekableByteChannel openForWrite(String path) throws IOException {
+        throw new FileOperationException(path, "Access denied");
+    }
+
+    @Override
+    public SeekableByteChannel openForAppend(String path) throws IOException {
+        throw new FileOperationException(path, "Access denied");
+    }
+
+    @Override
+    public long getRemainingSpace() {
+        return Integer.MAX_VALUE;
+    }
+
+    @Override
+    public long getCapacity() {
+        return Integer.MAX_VALUE;
+    }
+
+    @Override
+    public boolean isReadOnly(String path) {
+        return true;
+    }
+}

projects/core/src/testFixtures/java/dan200/computercraft/test/core/filesystem/WritableMountContract.java

@@ -3,7 +3,7 @@
  * Copyright Daniel Ratcliffe, 2011-2022. Do not distribute without permission.
  * Send enquiries to dratcliffe@gmail.com
  */
-package dan200.computercraft.core.filesystem;
+package dan200.computercraft.test.core.filesystem;
 
 import dan200.computercraft.api.filesystem.WritableMount;
 import org.junit.jupiter.api.Test;
@@ -14,7 +14,9 @@ import java.io.IOException;
 import static org.junit.jupiter.api.Assertions.*;
 
 /**
- * The contract that all {@link WritableMount}s must fulfill.
+ * The contract that all {@link WritableMount} implementations must fulfill.
+ *
+ * @see MountContract
  */
 public interface WritableMountContract {
     long CAPACITY = 1_000_000;

projects/fabric/src/main/java/dan200/computercraft/impl/ComputerCraftAPIImpl.java

@@ -8,14 +8,11 @@ package dan200.computercraft.impl;
 import com.google.auto.service.AutoService;
 import dan200.computercraft.api.ComputerCraftAPI;
 import dan200.computercraft.api.detail.DetailRegistry;
-import dan200.computercraft.api.filesystem.Mount;
 import dan200.computercraft.impl.detail.DetailRegistryImpl;
-import dan200.computercraft.shared.computer.core.ResourceMount;
 import dan200.computercraft.shared.details.FluidDetails;
 import net.fabricmc.fabric.api.transfer.v1.fluid.FluidVariant;
 import net.fabricmc.fabric.api.transfer.v1.storage.StorageView;
 import net.fabricmc.loader.api.FabricLoader;
-import net.minecraft.server.MinecraftServer;
 
 import javax.annotation.Nullable;
 
@@ -33,13 +30,6 @@ public final class ComputerCraftAPIImpl extends AbstractComputerCraftAPI impleme
             .orElse("unknown");
     }
 
-    @Nullable
-    @Override
-    public Mount createResourceMount(MinecraftServer server, String domain, String subPath) {
-        var mount = ResourceMount.get(domain, subPath, server.getResourceManager());
-        return mount.exists("") ? mount : null;
-    }
-
     @Override
     public DetailRegistry<StorageView<FluidVariant>> getFluidDetailRegistry() {
         return fluidDetails;

projects/forge/src/main/java/dan200/computercraft/impl/ComputerCraftAPIImpl.java

@@ -8,22 +8,18 @@ package dan200.computercraft.impl;
 import com.google.auto.service.AutoService;
 import dan200.computercraft.api.ComputerCraftAPI;
 import dan200.computercraft.api.detail.DetailRegistry;
-import dan200.computercraft.api.filesystem.Mount;
 import dan200.computercraft.api.network.wired.WiredElement;
 import dan200.computercraft.api.peripheral.IPeripheralProvider;
 import dan200.computercraft.impl.detail.DetailRegistryImpl;
-import dan200.computercraft.shared.computer.core.ResourceMount;
 import dan200.computercraft.shared.details.FluidData;
 import dan200.computercraft.shared.peripheral.generic.GenericPeripheralProvider;
 import net.minecraft.core.BlockPos;
 import net.minecraft.core.Direction;
-import net.minecraft.server.MinecraftServer;
 import net.minecraft.world.level.BlockGetter;
 import net.minecraftforge.common.capabilities.Capability;
 import net.minecraftforge.common.util.LazyOptional;
 import net.minecraftforge.fluids.FluidStack;
 import net.minecraftforge.fml.ModList;
-import net.minecraftforge.server.ServerLifecycleHooks;
 
 import javax.annotation.Nullable;
 
@@ -43,13 +39,6 @@ public final class ComputerCraftAPIImpl extends AbstractComputerCraftAPI impleme
             .orElse("unknown");
     }
 
-    @Override
-    public @Nullable Mount createResourceMount(MinecraftServer server, String domain, String subPath) {
-        var manager = ServerLifecycleHooks.getCurrentServer().getResourceManager();
-        var mount = ResourceMount.get(domain, subPath, manager);
-        return mount.exists("") ? mount : null;
-    }
-
     @Override
     public void registerPeripheralProvider(IPeripheralProvider provider) {
         Peripherals.register(provider);

tools/parse-reports.py

@@ -98,7 +98,7 @@ def _parse_junit_file(path: pathlib.Path):
 
             print("::group::Full error message")
             print(full_message)
-            print("::endgroup")
+            print("::endgroup::")
 
 
 def parse_junit() -> None: