accounting: fix locking in Transfer to avoid deadlock with --progress

Before this change, using -P occasionally deadlocked on the Transfer
mutex when Transfer.Done() was called with a non nil error and the
StatsInfo mutex since they mutually call each other.

This was fixed by making sure that the Transfer mutex is always
released before calling any StatsInfo methods.

This improves on: 6f87267b34

Fixes #3505

fs/accounting/transfer.go

@@ -48,6 +48,10 @@ type Transfer struct {
 	checking  bool
 
 	// Protects all below
+	//
+	// NB to avoid deadlocks we must release this lock before
+	// calling any methods on Transfer.stats.  This is because
+	// StatsInfo calls back into Transfer.
 	mu          sync.RWMutex
 	acc         *Account
 	err         error
@@ -79,20 +83,26 @@ func newTransferRemoteSize(stats *StatsInfo, remote string, size int64, checking
 // Done ends the transfer.
 // Must be called after transfer is finished to run proper cleanups.
 func (tr *Transfer) Done(err error) {
-	tr.mu.Lock()
-
 	if err != nil {
 		tr.stats.Error(err)
+
+		tr.mu.Lock()
 		tr.err = err
+		tr.mu.Unlock()
 	}
-	if tr.acc != nil {
-		if err := tr.acc.Close(); err != nil {
+
+	tr.mu.RLock()
+	acc := tr.acc
+	tr.mu.RUnlock()
+
+	if acc != nil {
+		if err := acc.Close(); err != nil {
 			fs.LogLevelPrintf(fs.Config.StatsLogLevel, nil, "can't close account: %+v\n", err)
 		}
 	}
 
+	tr.mu.Lock()
 	tr.completedAt = time.Now()
-
 	tr.mu.Unlock()
 
 	if tr.checking {