Fix out-of-bounds read in ByteBufferChannel

Introduced in fa122a56cf93adedca344a7aa9366d866cad6371 by the looks of
it, so shouldn't have ever made it into a release.

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

@@ -33,7 +33,9 @@ public class ByteBufferChannel implements SeekableByteChannel {
 
         var remaining = Math.min(backing.limit() - position, destination.remaining());
 
-        destination.put(position, backing, position, remaining);
+        var destPos = destination.position();
+        destination.put(destPos, backing, position, remaining);
+        destination.position(destPos + remaining);
         position += remaining;
         return remaining;
     }

projects/core/src/test/java/dan200/computercraft/core/apis/handles/ArrayByteChannelTest.java

@@ -0,0 +1,17 @@
+/*
+ * 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.apis.handles;
+
+import dan200.computercraft.test.core.filesystem.ReadableChannelContract;
+
+import java.nio.channels.SeekableByteChannel;
+
+public class ArrayByteChannelTest implements ReadableChannelContract {
+    @Override
+    public SeekableByteChannel wrap(byte[] contents) {
+        return new ArrayByteChannel(contents);
+    }
+}

projects/core/src/test/java/dan200/computercraft/core/apis/handles/ByteBufferChannelTest.java

@@ -0,0 +1,18 @@
+/*
+ * 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.apis.handles;
+
+import dan200.computercraft.test.core.filesystem.ReadableChannelContract;
+
+import java.nio.ByteBuffer;
+import java.nio.channels.SeekableByteChannel;
+
+public class ByteBufferChannelTest implements ReadableChannelContract {
+    @Override
+    public SeekableByteChannel wrap(byte[] contents) {
+        return new ByteBufferChannel(ByteBuffer.wrap(contents));
+    }
+}

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

@@ -0,0 +1,54 @@
+/*
+ * 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 org.junit.jupiter.api.Test;
+
+import java.io.IOException;
+import java.nio.ByteBuffer;
+import java.nio.channels.SeekableByteChannel;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+public interface ReadableChannelContract {
+    SeekableByteChannel wrap(byte[] contents);
+
+    @Test
+    default void testReadAdvancesPosition() throws IOException {
+        try (var buffer = wrap(new byte[]{ 1, 2, 3, 4 })) {
+            var read = ByteBuffer.allocate(2);
+
+            assertEquals(2, buffer.read(read));
+            assertEquals(2, read.position());
+            assertEquals(2, buffer.position());
+        }
+    }
+
+    @Test
+    default void testAtOffsetReadAdvancesPosition() throws IOException {
+        try (var buffer = wrap(new byte[]{ 1, 2, 3, 4 })) {
+            buffer.position(2);
+            var read = ByteBuffer.allocate(2);
+
+            assertEquals(2, buffer.read(read));
+            assertEquals(2, read.position());
+            assertEquals(4, buffer.position());
+        }
+    }
+
+    @Test
+    default void testReadAtOffset() throws IOException {
+        try (var buffer = wrap(new byte[]{ 1, 2, 3, 4 })) {
+            buffer.position(2);
+            var read = ByteBuffer.allocate(2);
+
+            assertEquals(2, buffer.read(read));
+            read.flip();
+            assertEquals(3, read.get(0));
+            assertEquals(4, read.get(1));
+        }
+    }
+}