make Depth() return last cached depth after diskQueue closed

instead of 0 (or, before the preceding change, blocking indefinitely)

Author: ploxiln

diskqueue.go

@@ -138,7 +138,12 @@ func New(name string, dataPath string, maxBytesPerFile int64,
 
 // Depth returns the depth of the queue
 func (d *diskQueue) Depth() int64 {
-	return <-d.depthChan
+	depth, ok := <-d.depthChan
+	if !ok {
+		// ioLoop exited
+		depth = d.depth
+	}
+	return depth
 }
 
 // ReadChan returns the receive-only []byte channel for reading data