operations: fix and tidy multithread code

- fix docs and error messages for multithread
- use sync/errgroup built in concurrency limiting
- re-arrange multithread code
- don't continue multi-thread uploads if one part fails

fs/operations/multithread.go

@@ -11,34 +11,12 @@ import (
 	"github.com/rclone/rclone/fs/accounting"
 	"github.com/rclone/rclone/lib/readers"
 	"golang.org/x/sync/errgroup"
-	"golang.org/x/sync/semaphore"
 )
 
 const (
 	multithreadChunkSize = 64 << 10
 )
 
-// An offsetWriter maps writes at offset base to offset base+off in the underlying writer.
-//
-// Modified from the go source code. Can be replaced with
-// io.OffsetWriter when we no longer need to support go1.19
-type offsetWriter struct {
-	w   io.WriterAt
-	off int64 // the current offset
-}
-
-// newOffsetWriter returns an offsetWriter that writes to w
-// starting at offset off.
-func newOffsetWriter(w io.WriterAt, off int64) *offsetWriter {
-	return &offsetWriter{w, off}
-}
-
-func (o *offsetWriter) Write(p []byte) (n int, err error) {
-	n, err = o.w.WriteAt(p, o.off)
-	o.off += int64(n)
-	return
-}
-
 // Return a boolean as to whether we should use multi thread copy for
 // this transfer
 func doMultiThreadCopy(ctx context.Context, f fs.Fs, src fs.Object) bool {
@@ -102,10 +80,12 @@ func (mc *multiThreadCopyState) copyStream(ctx context.Context, stream int, writ
 
 	rc, err := Open(ctx, mc.src, &fs.RangeOption{Start: start, End: end - 1})
 	if err != nil {
-		return fmt.Errorf("multipart copy: failed to open source: %w", err)
+		return fmt.Errorf("multi-thread copy: failed to open source: %w", err)
 	}
 	defer fs.CheckClose(rc, &err)
 
+	// FIXME NewRepeatableReader is allocating - need to be more careful with the memory allocations
+	// Also allocating for copy to local which doesn't need it
 	bytesWritten, err := writer.WriteChunk(stream, readers.NewRepeatableReader(rc))
 	if err != nil {
 		return err
@@ -128,7 +108,6 @@ func calculateNumChunks(size int64, chunkSize int64) int {
 	if size%chunkSize != 0 {
 		numChunks++
 	}
-
 	return int(numChunks)
 }
 
@@ -140,7 +119,7 @@ func multiThreadCopy(ctx context.Context, f fs.Fs, remote string, src fs.Object,
 	if openChunkWriter == nil {
 		openWriterAt := f.Features().OpenWriterAt
 		if openWriterAt == nil {
-			return nil, errors.New("multi-part copy: neither OpenChunkWriter nor OpenWriterAt supported")
+			return nil, errors.New("multi-thread copy: neither OpenChunkWriter nor OpenWriterAt supported")
 		}
 		openChunkWriter = openChunkWriterFromOpenWriterAt(openWriterAt, int64(ci.MultiThreadChunkSize), int64(ci.MultiThreadWriteBufferSize), f)
 	}
@@ -153,7 +132,12 @@ func multiThreadCopy(ctx context.Context, f fs.Fs, remote string, src fs.Object,
 	}
 
 	g, gCtx := errgroup.WithContext(ctx)
+	g.SetLimit(streams)
+
 	chunkSize, chunkWriter, err := openChunkWriter(ctx, remote, src)
+	if err != nil {
+		return nil, fmt.Errorf("multi-thread copy: failed to open chunk writer: %w", err)
+	}
 
 	if chunkSize > src.Size() {
 		fs.Debugf(src, "multi-thread copy: chunk size %v was bigger than source file size %v", fs.SizeSuffix(chunkSize), fs.SizeSuffix(src.Size()))
@@ -175,25 +159,18 @@ func multiThreadCopy(ctx context.Context, f fs.Fs, remote string, src fs.Object,
 		numChunks: numChunks,
 	}
 
-	if err != nil {
-		return nil, fmt.Errorf("multipart copy: failed to open chunk writer: %w", err)
-	}
-
 	// Make accounting
 	mc.acc = tr.Account(ctx, nil)
 
 	fs.Debugf(src, "Starting multi-thread copy with %d parts of size %v with %v parallel streams", mc.numChunks, fs.SizeSuffix(mc.partSize), mc.streams)
-	sem := semaphore.NewWeighted(int64(mc.streams))
 	for chunk := 0; chunk < mc.numChunks; chunk++ {
-		fs.Debugf(src, "Acquiring semaphore...")
-		if err := sem.Acquire(ctx, 1); err != nil {
-			fs.Errorf(src, "Failed to acquire semaphore: %v", err)
+		// Fail fast, in case an errgroup managed function returns an error
+		if gCtx.Err() != nil {
 			break
 		}
-		currChunk := chunk
-		g.Go(func() (err error) {
-			defer sem.Release(1)
-			return mc.copyStream(gCtx, currChunk, chunkWriter)
+		chunk := chunk
+		g.Go(func() error {
+			return mc.copyStream(gCtx, chunk, chunkWriter)
 		})
 	}
 
@@ -224,6 +201,28 @@ func multiThreadCopy(ctx context.Context, f fs.Fs, remote string, src fs.Object,
 	return obj, nil
 }
 
+// An offsetWriter maps writes at offset base to offset base+off in the underlying writer.
+//
+// Modified from the go source code. Can be replaced with
+// io.OffsetWriter when we no longer need to support go1.19
+type offsetWriter struct {
+	w   io.WriterAt
+	off int64 // the current offset
+}
+
+// newOffsetWriter returns an offsetWriter that writes to w
+// starting at offset off.
+func newOffsetWriter(w io.WriterAt, off int64) *offsetWriter {
+	return &offsetWriter{w, off}
+}
+
+func (o *offsetWriter) Write(p []byte) (n int, err error) {
+	n, err = o.w.WriteAt(p, o.off)
+	o.off += int64(n)
+	return
+}
+
+// writerAtChunkWriter converts a WriterAtCloser into a ChunkWriter
 type writerAtChunkWriter struct {
 	ctx             context.Context
 	remote          string
@@ -235,6 +234,7 @@ type writerAtChunkWriter struct {
 	f               fs.Fs
 }
 
+// WriteChunk writes chunkNumber from reader
 func (w writerAtChunkWriter) WriteChunk(chunkNumber int, reader io.ReadSeeker) (int64, error) {
 	fs.Debugf(w.remote, "writing chunk %v", chunkNumber)
 
@@ -254,21 +254,23 @@ func (w writerAtChunkWriter) WriteChunk(chunkNumber int, reader io.ReadSeeker) (
 	if n != bytesToWrite {
 		return -1, fmt.Errorf("expected to write %v bytes for chunk %v, but wrote %v bytes", bytesToWrite, chunkNumber, n)
 	}
-	// if we were buffering, flush do disk
+	// if we were buffering, flush to disk
 	switch w := writer.(type) {
 	case *bufio.Writer:
 		er2 := w.Flush()
 		if er2 != nil {
-			return -1, fmt.Errorf("multipart copy: flush failed: %w", err)
+			return -1, fmt.Errorf("multi-thread copy: flush failed: %w", err)
 		}
 	}
 	return n, nil
 }
 
+// Close the chunk writing
 func (w writerAtChunkWriter) Close() error {
 	return w.writerAt.Close()
 }
 
+// Abort the chunk writing
 func (w writerAtChunkWriter) Abort() error {
 	obj, err := w.f.NewObject(w.ctx, w.remote)
 	if err != nil {