Flesh out MemoryMount into a writable mount

This moves MemoryMount to the main core module, and converts it to be a
"proper" WritableMount. It's still naively implemented - definitely
would be good to flesh out our tests in the future - but enough for what
we need it for.

We also do the following:
 - Remove the FileEntry.path variable, and instead pass the path around
   as a variable.
 - Clean up BinaryReadableHandle to use ByteBuffers in a more idiomatic
   way.
 - Add a couple more tests to our FS tests. These are in a bit of an odd
   place, where we want both Lua tests (for emulator compliance) and
   Java tests (for testing different implementations) - something to
   think about in the future.

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

@@ -58,7 +58,7 @@ private void load(ResourceManager manager) {
         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();
 
@@ -86,24 +86,23 @@ private void load(ResourceManager manager) {
     }
 
     private FileEntry createEntry(String path) {
-        return new FileEntry(path, new ResourceLocation(namespace, subPath + "/" + path));
+        return new FileEntry(new ResourceLocation(namespace, subPath + "/" + path));
     }
 
     @Override
-    public byte[] getFileContents(FileEntry file) throws IOException {
+    protected byte[] getFileContents(String path, FileEntry file) throws IOException {
         var resource = manager.getResource(file.identifier).orElse(null);
-        if (resource == null) throw new FileOperationException(file.path, NO_SUCH_FILE);
+        if (resource == null) throw new FileOperationException(path, NO_SUCH_FILE);
 
         try (var stream = resource.open()) {
             return stream.readAllBytes();
         }
     }
 
-    protected static class FileEntry extends ArchiveMount.FileEntry<FileEntry> {
+    protected static final class FileEntry extends ArchiveMount.FileEntry<FileEntry> {
         final ResourceLocation identifier;
 
-        FileEntry(String path, ResourceLocation identifier) {
-            super(path);
+        FileEntry(ResourceLocation identifier) {
             this.identifier = identifier;
         }
     }

projects/common/src/main/java/dan200/computercraft/shared/computer/menu/ServerInputState.java

@@ -4,6 +4,7 @@
 
 package dan200.computercraft.shared.computer.menu;
 
+import dan200.computercraft.core.apis.handles.ByteBufferChannel;
 import dan200.computercraft.core.apis.transfer.TransferredFile;
 import dan200.computercraft.core.apis.transfer.TransferredFiles;
 import dan200.computercraft.shared.computer.upload.FileSlice;
@@ -22,7 +23,6 @@
 import javax.annotation.Nullable;
 import java.util.List;
 import java.util.UUID;
-import java.util.stream.Collectors;
 
 /**
  * The default concrete implementation of {@link ServerInputHandler}.
@@ -156,7 +156,7 @@ private UploadResultMessage finishUpload(ServerPlayer player) {
 
         computer.queueEvent(TransferredFiles.EVENT, new Object[]{
             new TransferredFiles(
-                toUpload.stream().map(x -> new TransferredFile(x.getName(), x.getBytes())).collect(Collectors.toList()),
+                toUpload.stream().map(x -> new TransferredFile(x.getName(), new ByteBufferChannel(x.getBytes()))).toList(),
                 () -> {
                     if (player.isAlive() && player.containerMenu == owner) {
                         PlatformHelper.get().sendToPlayer(UploadResultMessage.consumed(owner), player);

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

@@ -77,12 +77,10 @@ public final Object[] read(Optional<Integer> countArg) throws LuaException {
                     var buffer = ByteBuffer.allocate(BUFFER_SIZE);
                     var read = channel.read(buffer);
                     if (read < 0) return null;
+                    buffer.flip();
 
                     // If we failed to read "enough" here, let's just abort
-                    if (read >= count || read < BUFFER_SIZE) {
-                        buffer.flip();
-                        return new Object[]{ buffer };
-                    }
+                    if (read >= count || read < BUFFER_SIZE) return new Object[]{ buffer };
 
                     // Build up an array of ByteBuffers. Hopefully this means we can perform less allocation
                     // than doubling up the buffer each time.
@@ -90,11 +88,13 @@ public final Object[] read(Optional<Integer> countArg) throws LuaException {
                     List<ByteBuffer> parts = new ArrayList<>(4);
                     parts.add(buffer);
                     while (read >= BUFFER_SIZE && totalRead < count) {
-                        buffer = ByteBuffer.allocate(Math.min(BUFFER_SIZE, count - totalRead));
+                        buffer = ByteBuffer.allocateDirect(Math.min(BUFFER_SIZE, count - totalRead));
                         read = channel.read(buffer);
                         if (read < 0) break;
+                        buffer.flip();
 
                         totalRead += read;
+                        assert read == buffer.remaining();
                         parts.add(buffer);
                     }
 
@@ -102,9 +102,11 @@ public final Object[] read(Optional<Integer> countArg) throws LuaException {
                     var bytes = new byte[totalRead];
                     var pos = 0;
                     for (var part : parts) {
-                        System.arraycopy(part.array(), 0, bytes, pos, part.position());
-                        pos += part.position();
+                        int length = part.remaining();
+                        part.get(bytes, pos, length);
+                        pos += length;
                     }
+                    assert pos == totalRead;
                     return new Object[]{ bytes };
                 }
             } else {

projects/core/src/main/java/dan200/computercraft/core/apis/transfer/TransferredFile.java

@@ -6,10 +6,9 @@
 
 import dan200.computercraft.api.lua.LuaFunction;
 import dan200.computercraft.core.apis.handles.BinaryReadableHandle;
-import dan200.computercraft.core.apis.handles.ByteBufferChannel;
 import dan200.computercraft.core.methods.ObjectSource;
 
-import java.nio.ByteBuffer;
+import java.nio.channels.SeekableByteChannel;
 import java.util.Collections;
 import java.util.Optional;
 
@@ -26,9 +25,9 @@ public class TransferredFile implements ObjectSource {
     private final String name;
     private final BinaryReadableHandle handle;
 
-    public TransferredFile(String name, ByteBuffer contents) {
+    public TransferredFile(String name, SeekableByteChannel contents) {
         this.name = name;
-        handle = BinaryReadableHandle.of(new ByteBufferChannel(contents));
+        handle = BinaryReadableHandle.of(contents);
     }
 
     /**

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

@@ -28,7 +28,7 @@ public abstract class AbstractInMemoryMount<T extends AbstractInMemoryMount.File
     @Nullable
     protected T root;
 
-    private @Nullable T get(String path) {
+    protected final @Nullable T get(String path) {
         var lastEntry = root;
         var lastIndex = 0;
 
@@ -57,58 +57,61 @@ public final boolean isDirectory(String path) {
     @Override
     public final void list(String path, List<String> contents) throws IOException {
         var file = get(path);
-        if (file == null || !file.isDirectory()) throw new FileOperationException(path, "Not a directory");
+        if (file == null || file.children == null) throw new FileOperationException(path, "Not a directory");
 
-        file.list(contents);
+        contents.addAll(file.children.keySet());
     }
 
     @Override
     public final long getSize(String path) throws IOException {
         var file = get(path);
         if (file == null) throw new FileOperationException(path, NO_SUCH_FILE);
-        return getSize(file);
+        return getSize(path, file);
     }
 
     /**
      * Get the size of a file.
      *
+     * @param path The file path, for error messages.
      * @param file The file to get the size of.
      * @return The size of the file. This should be 0 for directories, and equal to {@code openForRead(_).size()} for files.
      * @throws IOException If the size could not be read.
      */
-    protected abstract long getSize(T file) throws IOException;
+    protected abstract long getSize(String path, T file) throws IOException;
 
     @Override
     public final SeekableByteChannel openForRead(String path) throws IOException {
         var file = get(path);
         if (file == null || file.isDirectory()) throw new FileOperationException(path, NO_SUCH_FILE);
-        return openForRead(file);
+        return openForRead(path, file);
     }
 
     /**
      * Open a file for reading.
      *
+     * @param path The file path, for error messages.
      * @param file The file to read. This will not be a directory.
      * @return The channel for this file.
      */
-    protected abstract SeekableByteChannel openForRead(T file) throws IOException;
+    protected abstract SeekableByteChannel openForRead(String path, T file) throws IOException;
 
     @Override
     public final BasicFileAttributes getAttributes(String path) throws IOException {
         var file = get(path);
         if (file == null) throw new FileOperationException(path, NO_SUCH_FILE);
-        return getAttributes(file);
+        return getAttributes(path, file);
     }
 
     /**
      * Get all attributes of the file.
      *
+     * @param path The file path, for error messages.
      * @param file The file to compute attributes for.
      * @return The file's attributes.
      * @throws IOException If the attributes could not be read.
      */
-    protected BasicFileAttributes getAttributes(T file) throws IOException {
-        return new FileAttributes(file.isDirectory(), getSize(file));
+    protected BasicFileAttributes getAttributes(String path, T file) throws IOException {
+        return new FileAttributes(file.isDirectory(), getSize(path, file));
     }
 
     protected T getOrCreateChild(T lastEntry, String localPath, Function<String, T> factory) {
@@ -133,20 +136,11 @@ protected T getOrCreateChild(T lastEntry, String localPath, Function<String, T>
     }
 
     protected static class FileEntry<T extends FileEntry<T>> {
-        public final String path;
         @Nullable
         public Map<String, T> children;
 
-        protected FileEntry(String path) {
-            this.path = path;
-        }
-
         public boolean isDirectory() {
             return children != null;
         }
-
-        protected void list(List<String> contents) {
-            if (children != null) contents.addAll(children.keySet());
-        }
     }
 }

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

@@ -41,35 +41,36 @@ public abstract class ArchiveMount<T extends ArchiveMount.FileEntry<T>> extends
         .build();
 
     @Override
-    protected final long getSize(T file) throws IOException {
+    protected final long getSize(String path, T file) throws IOException {
         if (file.size != -1) return file.size;
         if (file.isDirectory()) return file.size = 0;
 
         var contents = CONTENTS_CACHE.getIfPresent(file);
-        return file.size = contents != null ? contents.length : getFileSize(file);
+        return file.size = contents != null ? contents.length : getFileSize(path, file);
     }
 
     /**
      * Get the size of the file by reading it (or its metadata) from disk.
      *
+     * @param path The file path, for error messages.
      * @param file The file to get the size of.
      * @return The file's size.
      * @throws IOException If the size could not be computed.
      */
-    protected long getFileSize(T file) throws IOException {
-        return getContents(file).length;
+    protected long getFileSize(String path, T file) throws IOException {
+        return getContents(path, file).length;
     }
 
     @Override
-    protected final SeekableByteChannel openForRead(T file) throws IOException {
-        return new ArrayByteChannel(getContents(file));
+    protected final SeekableByteChannel openForRead(String path, T file) throws IOException {
+        return new ArrayByteChannel(getContents(path, file));
     }
 
-    private byte[] getContents(T file) throws IOException {
+    private byte[] getContents(String path, T file) throws IOException {
         var cachedContents = CONTENTS_CACHE.getIfPresent(file);
         if (cachedContents != null) return cachedContents;
 
-        var contents = getFileContents(file);
+        var contents = getFileContents(path, file);
         CONTENTS_CACHE.put(file, contents);
         return contents;
     }
@@ -77,16 +78,13 @@ private byte[] getContents(T file) throws IOException {
     /**
      * Read the entirety of a file into memory.
      *
+     * @param path The file path, for error messages.
      * @param file The file to read into memory. This will not be a directory.
      * @return The contents of the file.
      */
-    protected abstract byte[] getFileContents(T file) throws IOException;
+    protected abstract byte[] getFileContents(String path, T file) throws IOException;
 
     protected static class FileEntry<T extends FileEntry<T>> extends AbstractInMemoryMount.FileEntry<T> {
         long size = -1;
-
-        protected FileEntry(String path) {
-            super(path);
-        }
     }
 }

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

@@ -22,7 +22,7 @@
 /**
  * A mount which reads zip/jar files.
  */
-public class JarMount extends ArchiveMount<JarMount.FileEntry> implements Closeable {
+public final class JarMount extends ArchiveMount<JarMount.FileEntry> implements Closeable {
     private final ZipFile zip;
 
     public JarMount(File jarFile, String subPath) throws IOException {
@@ -42,7 +42,7 @@ public JarMount(File jarFile, String subPath) throws IOException {
         }
 
         // Read in all the entries
-        var root = this.root = new FileEntry("");
+        var root = this.root = new FileEntry();
         var zipEntries = zip.entries();
         while (zipEntries.hasMoreElements()) {
             var entry = zipEntries.nextElement();
@@ -51,32 +51,32 @@ public JarMount(File jarFile, String subPath) throws IOException {
             if (!entryPath.startsWith(subPath)) continue;
 
             var localPath = FileSystem.toLocal(entryPath, subPath);
-            getOrCreateChild(root, localPath, FileEntry::new).setup(entry);
+            getOrCreateChild(root, localPath, x -> new FileEntry()).setup(entry);
         }
     }
 
     @Override
-    protected long getFileSize(FileEntry file) throws FileOperationException {
-        if (file.zipEntry == null) throw new FileOperationException(file.path, NO_SUCH_FILE);
+    protected long getFileSize(String path, FileEntry file) throws FileOperationException {
+        if (file.zipEntry == null) throw new FileOperationException(path, NO_SUCH_FILE);
         return file.zipEntry.getSize();
     }
 
     @Override
-    protected byte[] getFileContents(FileEntry file) throws FileOperationException {
-        if (file.zipEntry == null) throw new FileOperationException(file.path, NO_SUCH_FILE);
+    protected byte[] getFileContents(String path, FileEntry file) throws FileOperationException {
+        if (file.zipEntry == null) throw new FileOperationException(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 FileOperationException(file.path, NO_SUCH_FILE);
+            throw new FileOperationException(path, NO_SUCH_FILE);
         }
     }
 
     @Override
-    protected BasicFileAttributes getAttributes(FileEntry file) throws IOException {
-        return file.zipEntry == null ? super.getAttributes(file) : new FileAttributes(
-            file.isDirectory(), getSize(file), orEpoch(file.zipEntry.getCreationTime()), orEpoch(file.zipEntry.getLastModifiedTime())
+    protected BasicFileAttributes getAttributes(String path, FileEntry file) throws IOException {
+        return file.zipEntry == null ? super.getAttributes(path, file) : new FileAttributes(
+            file.isDirectory(), getSize(path, file), orEpoch(file.zipEntry.getCreationTime()), orEpoch(file.zipEntry.getLastModifiedTime())
         );
     }
 
@@ -85,14 +85,10 @@ public void close() throws IOException {
         zip.close();
     }
 
-    protected static class FileEntry extends ArchiveMount.FileEntry<FileEntry> {
+    protected static final class FileEntry extends ArchiveMount.FileEntry<FileEntry> {
         @Nullable
         ZipEntry zipEntry;
 
-        protected FileEntry(String path) {
-            super(path);
-        }
-
         void setup(ZipEntry entry) {
             zipEntry = entry;
             if (children == null && entry.isDirectory()) children = new HashMap<>(0);

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

@@ -0,0 +1,334 @@
+// SPDX-FileCopyrightText: 2019 The CC: Tweaked Developers
+//
+// SPDX-License-Identifier: MPL-2.0
+
+package dan200.computercraft.core.filesystem;
+
+import dan200.computercraft.api.filesystem.FileAttributes;
+import dan200.computercraft.api.filesystem.FileOperationException;
+import dan200.computercraft.api.filesystem.Mount;
+import dan200.computercraft.api.filesystem.WritableMount;
+import dan200.computercraft.core.util.Nullability;
+
+import javax.annotation.Nullable;
+import java.io.IOException;
+import java.nio.ByteBuffer;
+import java.nio.channels.ClosedChannelException;
+import java.nio.channels.SeekableByteChannel;
+import java.nio.charset.StandardCharsets;
+import java.nio.file.AccessDeniedException;
+import java.nio.file.attribute.BasicFileAttributes;
+import java.nio.file.attribute.FileTime;
+import java.time.Instant;
+import java.util.*;
+
+import static dan200.computercraft.core.filesystem.WritableFileMount.MINIMUM_FILE_SIZE;
+
+/**
+ * A basic {@link Mount} which stores files and directories in-memory.
+ */
+public final class MemoryMount extends AbstractInMemoryMount<MemoryMount.FileEntry> implements WritableMount {
+    private static final byte[] EMPTY = new byte[0];
+    private static final FileTime EPOCH = FileTime.from(Instant.EPOCH);
+
+    private final long capacity;
+
+    /**
+     * Create a memory mount with a 1GB capacity.
+     */
+    public MemoryMount() {
+        this(1000_000_000);
+    }
+
+    /**
+     * Create a memory mount with a custom capacity. Note, this is only used in calculations for {@link #getCapacity()}
+     * and {@link #getRemainingSpace()}, it is not checked when creating or writing files.
+     *
+     * @param capacity The capacity of this mount.
+     */
+    public MemoryMount(long capacity) {
+        this.capacity = capacity;
+        root = new FileEntry();
+        root.children = new HashMap<>();
+    }
+
+    public MemoryMount addFile(String file, byte[] contents, FileTime created, FileTime modified) {
+        var entry = getOrCreateChild(Nullability.assertNonNull(root), file, x -> new FileEntry());
+        entry.contents = contents;
+        entry.length = contents.length;
+        entry.created = created;
+        entry.modified = modified;
+        return this;
+    }
+
+    public MemoryMount addFile(String file, String contents, FileTime created, FileTime modified) {
+        return addFile(file, contents.getBytes(StandardCharsets.UTF_8), created, modified);
+    }
+
+    public MemoryMount addFile(String file, byte[] contents) {
+        return addFile(file, contents, EPOCH, EPOCH);
+    }
+
+    public MemoryMount addFile(String file, String contents) {
+        return addFile(file, contents, EPOCH, EPOCH);
+    }
+
+    @Override
+    protected long getSize(String path, FileEntry file) {
+        return file.length;
+    }
+
+    @Override
+    protected SeekableByteChannel openForRead(String path, FileEntry file) throws IOException {
+        if (file.contents == null) throw new FileOperationException(path, "File is a directory");
+        return new EntryChannel(file, 0);
+    }
+
+    @Override
+    protected BasicFileAttributes getAttributes(String path, FileEntry file) throws IOException {
+        return new FileAttributes(file.isDirectory(), file.length, file.created, file.modified);
+    }
+
+    private @Nullable ParentAndName getParentAndName(String path) {
+        if (path.isEmpty()) throw new IllegalArgumentException("Path is empty");
+        var index = path.lastIndexOf('/');
+        if (index == -1) {
+            return new ParentAndName(Nullability.assertNonNull(Nullability.assertNonNull(root).children), path);
+        }
+
+        var entry = get(path.substring(0, index));
+        return entry == null || entry.children == null
+            ? null
+            : new ParentAndName(entry.children, path.substring(index + 1));
+    }
+
+    @Override
+    public void makeDirectory(String path) throws IOException {
+        if (path.isEmpty()) return;
+
+        var lastEntry = Nullability.assertNonNull(root);
+        var lastIndex = 0;
+        while (lastIndex < path.length()) {
+            if (lastEntry.children == null) throw new NullPointerException("children is null");
+
+            var nextIndex = path.indexOf('/', lastIndex);
+            if (nextIndex < 0) nextIndex = path.length();
+
+            var part = path.substring(lastIndex, nextIndex);
+            var nextEntry = lastEntry.children.get(part);
+            if (nextEntry == null) {
+                lastEntry.children.put(part, nextEntry = FileEntry.newDir());
+            } else if (nextEntry.children == null) {
+                throw new FileOperationException(path, "File exists");
+            }
+
+            lastEntry = nextEntry;
+            lastIndex = nextIndex + 1;
+        }
+    }
+
+    @Override
+    public void delete(String path) throws IOException {
+        if (path.isEmpty()) throw new AccessDeniedException("Access denied");
+        var node = getParentAndName(path);
+        if (node != null) node.parent().remove(node.name());
+    }
+
+    @Override
+    public void rename(String source, String dest) throws IOException {
+        if (dest.startsWith(source)) throw new FileOperationException(source, "Cannot move a directory inside itself");
+
+        var sourceParent = getParentAndName(source);
+        if (sourceParent == null || !sourceParent.exists()) throw new FileOperationException(source, "No such file");
+
+        var destParent = getParentAndName(dest);
+        if (destParent == null) throw new FileOperationException(dest, "Parent directory does not exist");
+        if (destParent.exists()) throw new FileOperationException(dest, "File exists");
+
+        destParent.put(sourceParent.parent().remove(sourceParent.name()));
+    }
+
+    private FileEntry getForWrite(String path) throws FileOperationException {
+        if (path.isEmpty()) throw new FileOperationException(path, "Cannot write to directory");
+
+        var parent = getParentAndName(path);
+        if (parent == null) throw new FileOperationException(path, "Parent directory does not exist");
+
+        var file = parent.get();
+        if (file != null && file.isDirectory()) throw new FileOperationException(path, "Cannot write to directory");
+        if (file == null) parent.put(file = FileEntry.newFile());
+
+        return file;
+    }
+
+    @Override
+    public SeekableByteChannel openForWrite(String path) throws IOException {
+        var file = getForWrite(path);
+
+        // Truncate the file.
+        file.contents = EMPTY;
+        file.length = 0;
+        return new EntryChannel(file, 0);
+    }
+
+    @Override
+    public SeekableByteChannel openForAppend(String path) throws IOException {
+        var file = getForWrite(path);
+        return new EntryChannel(file, file.length);
+    }
+
+    @Override
+    public long getRemainingSpace() {
+        return capacity - computeUsedSpace();
+    }
+
+    @Override
+    public long getCapacity() {
+        return capacity;
+    }
+
+    private long computeUsedSpace() {
+        Queue<FileEntry> queue = new ArrayDeque<>();
+        queue.add(root);
+
+        long size = 0;
+
+        FileEntry entry;
+        while ((entry = queue.poll()) != null) {
+            if (entry.children == null) {
+                size += Math.max(MINIMUM_FILE_SIZE, Nullability.assertNonNull(entry.contents).length);
+            } else {
+                size += MINIMUM_FILE_SIZE;
+                queue.addAll(entry.children.values());
+            }
+        }
+
+        return size - MINIMUM_FILE_SIZE; // Subtract one file for the root.
+    }
+
+    protected static final class FileEntry extends AbstractInMemoryMount.FileEntry<FileEntry> {
+        FileTime created = EPOCH;
+        FileTime modified = EPOCH;
+        @Nullable
+        byte[] contents;
+
+        int length;
+
+        static FileEntry newFile() {
+            var entry = new FileEntry();
+            entry.contents = EMPTY;
+            entry.created = entry.modified = FileTime.from(Instant.now());
+            return entry;
+        }
+
+        static FileEntry newDir() {
+            var entry = new FileEntry();
+            entry.children = new HashMap<>();
+            entry.created = entry.modified = FileTime.from(Instant.now());
+            return entry;
+        }
+    }
+
+    private record ParentAndName(Map<String, FileEntry> parent, String name) {
+        boolean exists() {
+            return parent.containsKey(name);
+        }
+
+        @Nullable
+        FileEntry get() {
+            return parent.get(name);
+        }
+
+        void put(FileEntry entry) {
+            assert !parent.containsKey(name);
+            parent.put(name, entry);
+        }
+    }
+
+    private static final class EntryChannel implements SeekableByteChannel {
+        private final FileEntry entry;
+        private long position;
+        private boolean isOpen = true;
+
+        private void checkClosed() throws ClosedChannelException {
+            if (!isOpen()) throw new ClosedChannelException();
+        }
+
+        private EntryChannel(FileEntry entry, int position) {
+            this.entry = entry;
+            this.position = position;
+        }
+
+        @Override
+        public int read(ByteBuffer destination) throws IOException {
+            checkClosed();
+
+            var backing = Nullability.assertNonNull(entry.contents);
+            if (position >= backing.length) return -1;
+
+            var remaining = Math.min(backing.length - (int) position, destination.remaining());
+            destination.put(backing, (int) position, remaining);
+            position += remaining;
+            return remaining;
+        }
+
+        private byte[] ensureCapacity(int capacity) {
+            var contents = Nullability.assertNonNull(entry.contents);
+            if (capacity >= entry.length) {
+                var newCapacity = Math.max(capacity, contents.length << 1);
+                contents = entry.contents = Arrays.copyOf(contents, newCapacity);
+            }
+
+            return contents;
+        }
+
+        @Override
+        public int write(ByteBuffer src) throws IOException {
+            var toWrite = src.remaining();
+            var endPosition = position + toWrite;
+            if (endPosition > 1 << 30) throw new IOException("File is too large");
+
+            var contents = ensureCapacity((int) endPosition);
+            src.get(contents, (int) position, toWrite);
+            position = endPosition;
+            if (endPosition > entry.length) entry.length = (int) endPosition;
+            return toWrite;
+        }
+
+        @Override
+        public long position() throws IOException {
+            checkClosed();
+            return position;
+        }
+
+        @Override
+        public SeekableByteChannel position(long newPosition) throws IOException {
+            checkClosed();
+            if (newPosition < 0) throw new IllegalArgumentException("Position out of bounds");
+            this.position = newPosition;
+            return this;
+        }
+
+        @Override
+        public long size() throws IOException {
+            checkClosed();
+            return entry.length;
+        }
+
+        @Override
+        public SeekableByteChannel truncate(long size) {
+            throw new UnsupportedOperationException();
+        }
+
+        @Override
+        public boolean isOpen() {
+            return isOpen && entry.contents != null;
+        }
+
+        @Override
+        public void close() throws IOException {
+            checkClosed();
+            isOpen = false;
+        }
+    }
+}

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

@@ -27,7 +27,7 @@
 public class WritableFileMount extends FileMount implements WritableMount {
     private static final Logger LOG = LoggerFactory.getLogger(WritableFileMount.class);
 
-    private static final long MINIMUM_FILE_SIZE = 500;
+    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);
 

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

@@ -12,10 +12,9 @@
 import dan200.computercraft.core.ComputerContext;
 import dan200.computercraft.core.computer.mainthread.MainThread;
 import dan200.computercraft.core.computer.mainthread.MainThreadConfig;
+import dan200.computercraft.core.filesystem.MemoryMount;
 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 +36,7 @@ public static void run(String program, Consumer<Computer> setup, int maxTimes) {
             .addFile("test.lua", program)
             .addFile("startup.lua", "assertion.assert(pcall(loadfile('test.lua', nil, _ENV))) os.shutdown()");
 
-        run(new ReadOnlyWritableMount(mount), setup, maxTimes);
+        run(mount, setup, maxTimes);
     }
 
     public static void run(String program, int maxTimes) {

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

@@ -0,0 +1,46 @@
+// SPDX-FileCopyrightText: 2023 The CC: Tweaked Developers
+//
+// SPDX-License-Identifier: MPL-2.0
+
+package dan200.computercraft.core.filesystem;
+
+import dan200.computercraft.api.filesystem.Mount;
+import dan200.computercraft.api.filesystem.WritableMount;
+import dan200.computercraft.test.core.filesystem.MountContract;
+import dan200.computercraft.test.core.filesystem.WritableMountContract;
+import org.opentest4j.TestAbortedException;
+
+public class MemoryMountTest implements MountContract, WritableMountContract {
+    @Override
+    public Mount createSkeleton() {
+        var mount = new MemoryMount();
+        mount.addFile("f.lua", "");
+        mount.addFile("dir/file.lua", "print('testing')", EPOCH, MODIFY_TIME);
+        return mount;
+    }
+
+    @Override
+    public MountAccess createMount(long capacity) {
+        var mount = new MemoryMount(capacity);
+        return new MountAccess() {
+            @Override
+            public WritableMount mount() {
+                return mount;
+            }
+
+            @Override
+            public void makeReadOnly(String path) {
+                throw new TestAbortedException("Not supported for MemoryMount");
+            }
+
+            @Override
+            public void ensuresExist() {
+            }
+
+            @Override
+            public long computeRemainingSpace() {
+                return mount.getRemainingSpace();
+            }
+        };
+    }
+}

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

@@ -10,10 +10,10 @@
 import java.util.Objects;
 
 /**
- * An {@link AutoCloseable} implementation which can be used to combine other [AutoCloseable] instances.
+ * An {@link AutoCloseable} implementation which can be used to combine other {@link 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.
+ * Values which implement {@link AutoCloseable} can be dynamically registered with {@link CloseScope#add(AutoCloseable)}.
+ * 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.

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

@@ -0,0 +1,23 @@
+// SPDX-FileCopyrightText: 2023 The CC: Tweaked Developers
+//
+// SPDX-License-Identifier: MPL-2.0
+
+package dan200.computercraft.test.core;
+
+import org.junit.jupiter.api.DisplayNameGeneration;
+import org.junit.jupiter.api.DisplayNameGenerator;
+
+import java.lang.reflect.Method;
+
+/**
+ * A {@link DisplayNameGenerator} which replaces underscores with spaces. This is equivalent to
+ * {@link DisplayNameGenerator.ReplaceUnderscores}, but excludes the parameter types.
+ *
+ * @see DisplayNameGeneration
+ */
+public class ReplaceUnderscoresDisplayNameGenerator extends DisplayNameGenerator.ReplaceUnderscores {
+    @Override
+    public String generateDisplayNameForMethod(Class<?> testClass, Method testMethod) {
+        return testMethod.getName().replace('_', ' ');
+    }
+}

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

@@ -11,10 +11,9 @@
 import dan200.computercraft.core.computer.GlobalEnvironment;
 import dan200.computercraft.core.filesystem.FileMount;
 import dan200.computercraft.core.filesystem.JarMount;
+import dan200.computercraft.core.filesystem.MemoryMount;
 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 +31,7 @@ public class BasicEnvironment implements ComputerEnvironment, GlobalEnvironment,
     private final WritableMount mount;
 
     public BasicEnvironment() {
-        this(new ReadOnlyWritableMount(new MemoryMount()));
+        this(new MemoryMount());
     }
 
     public BasicEnvironment(WritableMount mount) {

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

@@ -1,50 +0,0 @@
-// SPDX-FileCopyrightText: 2019 The CC: Tweaked Developers
-//
-// SPDX-License-Identifier: MPL-2.0
-
-package dan200.computercraft.test.core.filesystem;
-
-import dan200.computercraft.api.filesystem.FileOperationException;
-import dan200.computercraft.api.filesystem.Mount;
-import dan200.computercraft.core.apis.handles.ArrayByteChannel;
-import dan200.computercraft.core.filesystem.AbstractInMemoryMount;
-import dan200.computercraft.core.util.Nullability;
-
-import javax.annotation.Nullable;
-import java.io.IOException;
-import java.nio.channels.SeekableByteChannel;
-import java.nio.charset.StandardCharsets;
-
-/**
- * A read-only mount {@link Mount} which provides a list of in-memory set of files.
- */
-public class MemoryMount extends AbstractInMemoryMount<MemoryMount.FileEntry> {
-    public MemoryMount() {
-        root = new FileEntry("");
-    }
-
-    public MemoryMount addFile(String file, String contents) {
-        getOrCreateChild(Nullability.assertNonNull(root), file, FileEntry::new).contents = contents.getBytes(StandardCharsets.UTF_8);
-        return this;
-    }
-
-    @Override
-    protected long getSize(FileEntry file) {
-        return file.contents == null ? 0 : file.contents.length;
-    }
-
-    @Override
-    protected SeekableByteChannel openForRead(FileEntry file) throws IOException {
-        if (file.contents == null) throw new FileOperationException(file.path, "File is a directory");
-        return new ArrayByteChannel(file.contents);
-    }
-
-    protected static class FileEntry extends AbstractInMemoryMount.FileEntry<FileEntry> {
-        @Nullable
-        byte[] contents;
-
-        protected FileEntry(String path) {
-            super(path);
-        }
-    }
-}

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

@@ -34,7 +34,10 @@ public interface MountContract {
      * Create a skeleton mount. This should contain the following files:
      *
      * <ul>
-     *     <li>{@code dir/file.lua}, containing {@code print('testing')}.</li>
+     *     <li>
+     *         {@code dir/file.lua}, containing {@code print('testing')}. If {@linkplain #hasFileTimes() file times are
+     *         supported}, it should have a modification time of {@link #MODIFY_TIME}.
+     *     </li>
      *     <li>{@code f.lua}, containing nothing.</li>
      * </ul>
      *

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

@@ -0,0 +1,33 @@
+// SPDX-FileCopyrightText: 2023 The CC: Tweaked Developers
+//
+// SPDX-License-Identifier: MPL-2.0
+
+package dan200.computercraft.test.core.filesystem;
+
+import dan200.computercraft.api.filesystem.WritableMount;
+
+import java.io.IOException;
+import java.nio.channels.Channels;
+import java.nio.charset.StandardCharsets;
+
+/**
+ * Utility functions for working with mounts.
+ */
+public final class Mounts {
+    private Mounts() {
+    }
+
+    /**
+     * Write a file to this mount.
+     *
+     * @param mount    The mount to modify.
+     * @param path     The path to write to.
+     * @param contents The contents of this path.
+     * @throws IOException If writing fails.
+     */
+    public static void writeFile(WritableMount mount, String path, String contents) throws IOException {
+        try (var handle = Channels.newWriter(mount.openForWrite(path), StandardCharsets.UTF_8)) {
+            handle.write(contents);
+        }
+    }
+}

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

@@ -1,91 +0,0 @@
-// SPDX-FileCopyrightText: 2022 The CC: Tweaked Developers
-//
-// SPDX-License-Identifier: MPL-2.0
-
-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

@@ -5,10 +5,16 @@
 package dan200.computercraft.test.core.filesystem;
 
 import dan200.computercraft.api.filesystem.WritableMount;
+import dan200.computercraft.api.lua.LuaValues;
+import dan200.computercraft.test.core.ReplaceUnderscoresDisplayNameGenerator;
+import org.junit.jupiter.api.DisplayNameGeneration;
 import org.junit.jupiter.api.Test;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.MethodSource;
 import org.opentest4j.TestAbortedException;
 
 import java.io.IOException;
+import java.util.stream.Stream;
 
 import static org.junit.jupiter.api.Assertions.*;
 
@@ -17,9 +23,16 @@
  *
  * @see MountContract
  */
+@DisplayNameGeneration(ReplaceUnderscoresDisplayNameGenerator.class)
 public interface WritableMountContract {
     long CAPACITY = 1_000_000;
 
+    String LONG_CONTENTS = "This is some example text.\n".repeat(100);
+
+    static Stream<String> fileContents() {
+        return Stream.of("", LONG_CONTENTS);
+    }
+
     /**
      * Create a new empty mount.
      *
@@ -43,18 +56,30 @@ default MountAccess createExisting(long capacity) throws IOException {
     }
 
     @Test
-    default void testRootWritable() throws IOException {
+    default void Root_is_writable() throws IOException {
         assertFalse(createExisting(CAPACITY).mount().isReadOnly("/"));
         assertFalse(createMount(CAPACITY).mount().isReadOnly("/"));
     }
 
     @Test
-    default void testMissingDirWritable() throws IOException {
+    default void Missing_dir_is_writable() throws IOException {
         assertFalse(createExisting(CAPACITY).mount().isReadOnly("/foo/bar/baz/qux"));
     }
 
     @Test
-    default void testDirReadOnly() throws IOException {
+    default void Make_directory_recursive() throws IOException {
+        var access = createMount(CAPACITY);
+        var mount = access.mount();
+        mount.makeDirectory("a/b/c");
+
+        assertTrue(mount.isDirectory("a/b/c"));
+
+        assertEquals(CAPACITY - 500 * 3, mount.getRemainingSpace());
+        assertEquals(access.computeRemainingSpace(), access.mount().getRemainingSpace(), "Free space is inconsistent");
+    }
+
+    @Test
+    default void Can_make_read_only() throws IOException {
         var root = createMount(CAPACITY);
         var mount = root.mount();
         mount.makeDirectory("read-only");
@@ -66,18 +91,97 @@ default void testDirReadOnly() throws IOException {
     }
 
     @Test
-    default void testMovePreservesSpace() throws IOException {
+    default void Initial_free_space_and_capacity() throws IOException {
+        var mount = createExisting(CAPACITY).mount();
+        assertEquals(CAPACITY, mount.getCapacity());
+        assertEquals(CAPACITY, mount.getRemainingSpace());
+    }
+
+    @Test
+    default void Write_updates_size_and_free_space() throws IOException {
         var access = createExisting(CAPACITY);
         var mount = access.mount();
-        mount.openForWrite("foo").close();
+
+        Mounts.writeFile(mount, "hello.txt", LONG_CONTENTS);
+        assertEquals(LONG_CONTENTS.length(), mount.getSize("hello.txt"));
+        assertEquals(CAPACITY - LONG_CONTENTS.length(), mount.getRemainingSpace());
+
+        Mounts.writeFile(mount, "hello.txt", "");
+        assertEquals(0, mount.getSize("hello.txt"));
+        assertEquals(CAPACITY - 500, mount.getRemainingSpace());
+        assertEquals(access.computeRemainingSpace(), access.mount().getRemainingSpace(), "Free space is inconsistent");
+    }
+
+    @Test
+    default void Append_jumps_to_file_end() throws IOException {
+        var access = createExisting(CAPACITY);
+        var mount = access.mount();
+
+        Mounts.writeFile(mount, "a.txt", "example");
+
+        try (var handle = mount.openForAppend("a.txt")) {
+            assertEquals(7, handle.position());
+            handle.write(LuaValues.encode(" text"));
+            assertEquals(12, handle.position());
+        }
+
+        assertEquals(12, mount.getSize("a.txt"));
+    }
+
+    @ParameterizedTest(name = "\"{0}\"")
+    @MethodSource("fileContents")
+    default void Move_file(String contents) throws IOException {
+        var access = createExisting(CAPACITY);
+        var mount = access.mount();
+        Mounts.writeFile(mount, "src.txt", contents);
 
         var remainingSpace = mount.getRemainingSpace();
-        mount.rename("foo", "bar");
 
+        mount.rename("src.txt", "dest.txt");
+        assertFalse(mount.exists("src.txt"));
+        assertTrue(mount.exists("dest.txt"));
+
+        assertEquals(contents.length(), mount.getSize("dest.txt"));
         assertEquals(remainingSpace, mount.getRemainingSpace(), "Free space has changed after moving");
         assertEquals(access.computeRemainingSpace(), access.mount().getRemainingSpace(), "Free space is inconsistent");
     }
 
+    @ParameterizedTest(name = "\"{0}\"")
+    @MethodSource("fileContents")
+    default void Move_file_fails_when_destination_exists(String contents) throws IOException {
+        var access = createExisting(CAPACITY);
+        var mount = access.mount();
+        Mounts.writeFile(mount, "src.txt", contents);
+        Mounts.writeFile(mount, "dest.txt", "dest");
+
+        var remainingSpace = mount.getRemainingSpace();
+
+        assertThrows(IOException.class, () -> mount.rename("src.txt", "dest.txt"));
+
+        assertEquals(contents.length(), mount.getSize("src.txt"));
+        assertEquals(4, mount.getSize("dest.txt"));
+
+        assertEquals(remainingSpace, mount.getRemainingSpace(), "Free space has changed despite no move occurred.");
+        assertEquals(access.computeRemainingSpace(), access.mount().getRemainingSpace(), "Free space is inconsistent");
+    }
+
+    @Test
+    default void Move_file_fails_when_source_does_not_exist() throws IOException {
+        var access = createExisting(CAPACITY);
+        var mount = access.mount();
+        Mounts.writeFile(mount, "dest.txt", "dest");
+
+        var remainingSpace = mount.getRemainingSpace();
+
+        assertThrows(IOException.class, () -> mount.rename("src.txt", "dest.txt"));
+
+        assertFalse(mount.exists("src.txt"));
+        assertEquals(4, mount.getSize("dest.txt"));
+
+        assertEquals(remainingSpace, mount.getRemainingSpace(), "Free space has changed despite no move occurred.");
+        assertEquals(access.computeRemainingSpace(), access.mount().getRemainingSpace(), "Free space is inconsistent");
+    }
+
     /**
      * Wraps a {@link WritableMount} with additional operations.
      */
@@ -90,7 +194,7 @@ interface MountAccess {
         WritableMount mount();
 
         /**
-         * Make a path read-only. This may throw a {@link TestAbortedException} if
+         * Make a path read-only. This may throw a {@link TestAbortedException} if this operation is not supported.
          *
          * @param path The mount-relative path.
          */

projects/lints/src/main/kotlin/cc/tweaked/linter/LoaderOverride.kt

@@ -23,7 +23,7 @@
     private const val FABRIC_ANNOTATION: String = "dan200.computercraft.annotations.FabricOverride"
 
     fun hasOverrideAnnotation(symbol: Symbol.MethodSymbol, state: VisitorState) =
-        ASTHelpers.hasAnnotation(symbol, Override::class.java, state)
+        ASTHelpers.hasAnnotation(symbol, "java.lang.Override", state)
 
     fun getAnnotation(flags: ErrorProneFlags) = when (flags.get("ModLoader").orElse(null)) {
         "forge" -> FORGE_ANNOTATION