Use duplicate() instead of rewind()

It's just more confusing having to keep track of where the ByteBuffer is
at. In this case, I think we were forgetting to rewind after computing
the digest.

Hopefully we'll be able to drop some of these in 1.17 as Java 16 has
a few more ByteBuffer methods

Fixes #992

src/main/java/dan200/computercraft/client/gui/ComputerScreenBase.java

@@ -173,7 +173,6 @@ public void onFilesDrop( @Nonnull List<Path> files )
                     return;
                 }
 
-                buffer.rewind();
                 toUpload.add( new FileUpload( name, buffer, digest ) );
             }
             catch( IOException e )

src/main/java/dan200/computercraft/shared/computer/core/IContainerComputer.java

@@ -56,7 +56,7 @@ public interface IContainerComputer
     void continueUpload( @Nonnull UUID uploadId, @Nonnull List<FileSlice> slices );
 
     /**
-     * Finish off an upload. This either writes the uploaded files or
+     * Finish off an upload. This either writes the uploaded files or informs the user that files will be overwritten.
      *
      * @param uploader The player uploading files.
      * @param uploadId The unique ID of this upload.

src/main/java/dan200/computercraft/shared/computer/upload/FileSlice.java

@@ -53,10 +53,9 @@ public void apply( List<FileUpload> files )
             return;
         }
 
-        bytes.rewind();
+        ByteBuffer other = file.duplicate();
-        file.position( offset );
+        other.position( offset ); // TODO: In 1.17 we can use a separate put(idx, _) method.
-        file.put( bytes );
+        other.put( bytes );
-        file.rewind();
 
         if( bytes.remaining() != 0 ) throw new IllegalStateException( "Should have read the whole buffer" );
     }

src/main/java/dan200/computercraft/shared/computer/upload/FileUpload.java

@@ -56,7 +56,7 @@ public byte[] getChecksum()
 
     public boolean checksumMatches()
     {
-        // This is meant to be a checksum. Doesn't need to be cryptographically secure, hence non constant time.
+        // This is meant to be a checksum. Doesn't need to be cryptographically secure, hence non-constant time.
         byte[] digest = getDigest( bytes );
         return digest != null && Arrays.equals( checksum, digest );
     }
@@ -66,9 +66,8 @@ public static byte[] getDigest( ByteBuffer bytes )
     {
         try
         {
-            bytes.rewind();
             MessageDigest digest = MessageDigest.getInstance( "SHA-256" );
-            digest.update( bytes );
+            digest.update( bytes.duplicate() );
             return digest.digest();
         }
         catch( NoSuchAlgorithmException e )

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

@@ -5,7 +5,6 @@
  */
 package dan200.computercraft.shared.network.server;
 
-import dan200.computercraft.ComputerCraft;
 import dan200.computercraft.shared.computer.core.IContainerComputer;
 import dan200.computercraft.shared.computer.core.ServerComputer;
 import dan200.computercraft.shared.computer.upload.FileSlice;
@@ -122,9 +121,9 @@ public void toBytes( @Nonnull PacketBuffer buf )
             buf.writeByte( slice.getFileId() );
             buf.writeVarInt( slice.getOffset() );
 
-            slice.getBytes().rewind();
+            ByteBuffer bytes = slice.getBytes().duplicate();
-            buf.writeShort( slice.getBytes().remaining() );
+            buf.writeShort( bytes.remaining() );
-            buf.writeBytes( slice.getBytes() );
+            buf.writeBytes( bytes );
         }
     }
 
@@ -158,7 +157,6 @@ public static void send( int instanceId, List<FileUpload> files )
 
                 int canWrite = Math.min( remaining, capacity - currentOffset );
 
-                ComputerCraft.log.info( "Adding slice from {} to {}", currentOffset, currentOffset + canWrite - 1 );
                 contents.position( currentOffset ).limit( currentOffset + canWrite );
                 slices.add( new FileSlice( fileId, currentOffset, contents.slice() ) );
                 currentOffset += canWrite;