X neo: Fixes for "Close vs watchq"

go/neo/client.go

@@ -68,6 +68,9 @@ type Client struct {
 	at0Initialized bool                // true after .at0 is initialized
 	at0Ready       chan(struct{})      // ready after .at0 is initialized
 
+	closeOnce sync.Once
+	closed    chan struct{} // ready when Closed
+
 	ownNet bool // true if Client "owns" networker and should release it on Close
 }
 
@@ -81,6 +84,7 @@ func NewClient(clusterName, masterAddr string, net xnet.Networker) *Client {
 	c := &Client{
 		node:     newMasteredNode(proto.CLIENT, clusterName, net, masterAddr),
 		at0Ready: make(chan struct{}),
+		closed:   make(chan struct{}),
 	}
 
 	var runCtx context.Context
@@ -91,6 +95,9 @@ func NewClient(clusterName, masterAddr string, net xnet.Networker) *Client {
 
 // Close implements zodb.IStorageDriver.
 func (c *Client) Close() (err error) {
+	c.closeOnce.Do(func() {
+		close(c.closed)
+	})
 	c.runCancel()
 	err = c.runWG.Wait()
 	if errors.Is(err, context.Canceled) {
@@ -190,7 +197,7 @@ func (c *Client) invalidateObjects(msg *proto.InvalidateObjects) error {
 	defer c.at0Mu.Unlock()
 
 	// queue initial events until .at0 is initialized after register
-	// queued events will be sent to watchq by zeo ctor after initializing .at0
+	// queued events will be sent to watchq by syncMaster after initializing .at0
 	if !c.at0Initialized {
 		c.eventq0 = append(c.eventq0, event)
 		return nil
@@ -198,7 +205,13 @@ func (c *Client) invalidateObjects(msg *proto.InvalidateObjects) error {
 
 	// at0 is initialized - ok to send current event if it goes > at0
 	if tid > c.at0 {
-		c.watchq <- event
+		select {
+		case <-c.closed:
+			// closed - client does not read watchq anymore
+
+		case c.watchq <- event:
+			// ok
+		}
 	}
 	return nil
 }
@@ -250,7 +263,13 @@ func (c *Client) flushEventq0() {
 	if c.watchq != nil {
 		for _, e := range c.eventq0 {
 			if e.Tid > c.at0 {
-				c.watchq <- e
+				select {
+				case <-c.closed:
+					// closed - client does not read watchq anymore
+
+				case c.watchq <- e:
+					// ok
+				}
 			}
 		}
 	}
@@ -487,14 +506,18 @@ func openClientByURL(ctx context.Context, u *url.URL, opt *zodb.DriverOptions) (
 		// close .watchq after serve is over
 		c.at0Mu.Lock()
 		defer c.at0Mu.Unlock()
-		if c.at0Initialized {
-			c.flushEventq0()
-		}
 		if c.watchq != nil {
-			if err != nil {
-				c.watchq <- &zodb.EventError{Err: err}
+			if err != nil && /* already flushed .eventq0 */c.at0Initialized {
+				select {
+				case <-c.closed:
+					// closed - client does not read watchq anymore
+
+				case c.watchq <- &zodb.EventError{Err: err}:
+					// ok
+				}
 			}
 			close(c.watchq)
+			c.watchq = nil // prevent flushEventq0 to send to closed chan
 		}
 
 		errq <- err