fusefrontend: Read: use provided buffer

This will allow us to return internal buffers to a pool.

Author: rfjakob

internal/fusefrontend/file.go

@@ -132,15 +132,16 @@ func (f *file) createHeader() (fileID []byte, err error) {
 	return h.ID, err
 }
 
-// doRead - returns "length" plaintext bytes from plaintext offset "off".
+// doRead - read "length" plaintext bytes from plaintext offset "off" and append
+// to "dst".
 // Arguments "length" and "off" do not have to be block-aligned.
 //
 // doRead reads the corresponding ciphertext blocks from disk, decrypts them and
 // returns the requested part of the plaintext.
 //
 // Called by Read() for normal reading,
 // by Write() and Truncate() for Read-Modify-Write
-func (f *file) doRead(off uint64, length uint64) ([]byte, fuse.Status) {
+func (f *file) doRead(dst []byte, off uint64, length uint64) ([]byte, fuse.Status) {
 	// Make sure we have the file ID.
 	f.fileTableEntry.HeaderLock.RLock()
 	if f.fileTableEntry.ID == nil {
@@ -211,7 +212,7 @@ func (f *file) doRead(off uint64, length uint64) ([]byte, fuse.Status) {
 	}
 	// else: out stays empty, file was smaller than the requested offset
 
-	return out, fuse.OK
+	return append(dst, out...), fuse.OK
 }
 
 // Read - FUSE call
@@ -225,7 +226,7 @@ func (f *file) Read(buf []byte, off int64) (resultData fuse.ReadResult, code fus
 		serialize_reads.Wait(off, len(buf))
 	}
 
-	out, status := f.doRead(uint64(off), uint64(len(buf)))
+	out, status := f.doRead(buf[:0], uint64(off), uint64(len(buf)))
 
 	if f.fs.args.SerializeReads {
 		serialize_reads.Done()
@@ -282,7 +283,7 @@ func (f *file) doWrite(data []byte, off int64) (uint32, fuse.Status) {
 		// Incomplete block -> Read-Modify-Write
 		if b.IsPartial() {
 			// Read
-			oldData, status := f.doRead(b.BlockPlainOff(), f.contentEnc.PlainBS())
+			oldData, status := f.doRead(nil, b.BlockPlainOff(), f.contentEnc.PlainBS())
 			if status != fuse.OK {
 				tlog.Warn.Printf("ino%d fh%d: RMW read failed: %s", f.qIno.Ino, f.intFd(), status.String())
 				return 0, status

internal/fusefrontend/file_allocate_truncate.go

@@ -144,7 +144,7 @@ func (f *file) Truncate(newSize uint64) fuse.Status {
 	var data []byte
 	if lastBlockLen > 0 {
 		var status fuse.Status
-		data, status = f.doRead(plainOff, lastBlockLen)
+		data, status = f.doRead(nil, plainOff, lastBlockLen)
 		if status != fuse.OK {
 			tlog.Warn.Printf("Truncate: shrink doRead returned error: %v", err)
 			return status