accounting: fix memory leak noticeable for transfers of large numbers of objects

Before this fix we weren't removing transfers from the transfer stats.
For transfers with 1000s of objects this uses a noticeable amount of
memory.

See: https://forum.rclone.org/t/rclone-memory-consumption-increasing-linearly/12244

fs/accounting/stats.go

@@ -532,3 +532,16 @@ func (s *StatsInfo) AddTransfer(transfer *Transfer) {
 	s.startedTransfers = append(s.startedTransfers, transfer)
 	s.mu.Unlock()
 }
+
+// RemoveTransfer removes a reference to the started transfer.
+func (s *StatsInfo) RemoveTransfer(transfer *Transfer) {
+	s.mu.Lock()
+	for i, tr := range s.startedTransfers {
+		if tr == transfer {
+			// remove the found entry
+			s.startedTransfers = append(s.startedTransfers[:i], s.startedTransfers[i+1:]...)
+			break
+		}
+	}
+	s.mu.Unlock()
+}

fs/accounting/transfer.go

@@ -113,6 +113,7 @@ func (tr *Transfer) Done(err error) {
 	} else {
 		tr.stats.DoneTransferring(tr.remote, err == nil)
 	}
+	tr.stats.RemoveTransfer(tr)
 }
 
 // Reset allows to switch the Account to another transfer method.