fs/accounting: Fix "file already closed" on transfer retries

This was caused by the recent reworking of the accounting interface.
The Transfer object was recycling the Accounting object without
resetting the stream.

See: https://forum.rclone.org/t/error-file-already-closed/11469/
See: https://forum.rclone.org/t/rclone-b2-sync-post-error-method-not-supported/11718/

fs/accounting/accounting.go

@@ -118,11 +118,15 @@ func (acc *Account) StopBuffering() {
 // async buffer (if any) and re-adding it
 func (acc *Account) UpdateReader(in io.ReadCloser) {
 	acc.mu.Lock()
+	if acc.withBuf {
 		acc.StopBuffering()
+	}
 	acc.in = in
 	acc.close = in
 	acc.origIn = in
+	if acc.withBuf {
 		acc.WithBuffer()
+	}
 	acc.mu.Unlock()
 }
 

fs/accounting/transfer.go

@@ -117,6 +117,8 @@ func (tr *Transfer) Account(in io.ReadCloser) *Account {
 	tr.mu.Lock()
 	if tr.acc == nil {
 		tr.acc = newAccountSizeName(tr.stats, in, tr.size, tr.remote)
+	} else {
+		tr.acc.UpdateReader(in)
 	}
 	tr.mu.Unlock()
 	return tr.acc

fs/operations/operations.go

@@ -346,6 +346,7 @@ func Copy(ctx context.Context, f fs.Fs, dst fs.Object, remote string, src fs.Obj
 						} else {
 							actionTaken = "Copied (Rcat, new)"
 						}
+						// NB Rcat closes in0
 						dst, err = Rcat(ctx, f, remote, in0, src.ModTime(ctx))
 						newDst = dst
 					} else {