grpc: ensure transports are closed when the channel enters IDLE (#6620)

clientconn.go

@@ -1091,8 +1091,8 @@ func (ac *addrConn) updateAddrs(addrs []resolver.Address) {
 	ac.cancel()
 	ac.ctx, ac.cancel = context.WithCancel(ac.cc.ctx)
 
-	// We have to defer here because GracefulClose => Close => onClose, which
-	// requires locking ac.mu.
+	// We have to defer here because GracefulClose => onClose, which requires
+	// locking ac.mu.
 	if ac.transport != nil {
 		defer ac.transport.GracefulClose()
 		ac.transport = nil
@@ -1680,16 +1680,7 @@ func (ac *addrConn) tearDown(err error) {
 	ac.updateConnectivityState(connectivity.Shutdown, nil)
 	ac.cancel()
 	ac.curAddr = resolver.Address{}
-	if err == errConnDrain && curTr != nil {
-		// GracefulClose(...) may be executed multiple times when
-		// i) receiving multiple GoAway frames from the server; or
-		// ii) there are concurrent name resolver/Balancer triggered
-		// address removal and GoAway.
-		// We have to unlock and re-lock here because GracefulClose => Close => onClose, which requires locking ac.mu.
-		ac.mu.Unlock()
-		curTr.GracefulClose()
-		ac.mu.Lock()
-	}
+
 	channelz.AddTraceEvent(logger, ac.channelzID, 0, &channelz.TraceEventDesc{
 		Desc:     "Subchannel deleted",
 		Severity: channelz.CtInfo,
@@ -1703,6 +1694,29 @@ func (ac *addrConn) tearDown(err error) {
 	// being deleted right away.
 	channelz.RemoveEntry(ac.channelzID)
 	ac.mu.Unlock()
+
+	// We have to release the lock before the call to GracefulClose/Close here
+	// because both of them call onClose(), which requires locking ac.mu.
+	if curTr != nil {
+		if err == errConnDrain {
+			// Close the transport gracefully when the subConn is being shutdown.
+			//
+			// GracefulClose() may be executed multiple times if:
+			// - multiple GoAway frames are received from the server
+			// - there are concurrent name resolver or balancer triggered
+			//   address removal and GoAway
+			curTr.GracefulClose()
+		} else {
+			// Hard close the transport when the channel is entering idle or is
+			// being shutdown. In the case where the channel is being shutdown,
+			// closing of transports is also taken care of by cancelation of cc.ctx.
+			// But in the case where the channel is entering idle, we need to
+			// explicitly close the transports here. Instead of distinguishing
+			// between these two cases, it is simpler to close the transport
+			// unconditionally here.
+			curTr.Close(err)
+		}
+	}
 }
 
 func (ac *addrConn) getState() connectivity.State {

internal/idle/idle_e2e_test.go

@@ -142,7 +142,8 @@ func (s) TestChannelIdleness_Disabled_NoActivity(t *testing.T) {
 }
 
 // Tests the case where channel idleness is enabled by passing a small value for
-// idle_timeout. Verifies that a READY channel with no RPCs moves to IDLE.
+// idle_timeout. Verifies that a READY channel with no RPCs moves to IDLE, and
+// the connection to the backend is closed.
 func (s) TestChannelIdleness_Enabled_NoActivity(t *testing.T) {
 	// Create a ClientConn with a short idle_timeout.
 	r := manual.NewBuilderWithScheme("whatever")
@@ -159,7 +160,8 @@ func (s) TestChannelIdleness_Enabled_NoActivity(t *testing.T) {
 	t.Cleanup(func() { cc.Close() })
 
 	// Start a test backend and push an address update via the resolver.
-	backend := stubserver.StartTestService(t, nil)
+	lis := testutils.NewListenerWrapper(t, nil)
+	backend := stubserver.StartTestService(t, &stubserver.StubServer{Listener: lis})
 	t.Cleanup(backend.Stop)
 	r.UpdateState(resolver.State{Addresses: []resolver.Address{{Addr: backend.Address}}})
 
@@ -168,13 +170,25 @@ func (s) TestChannelIdleness_Enabled_NoActivity(t *testing.T) {
 	defer cancel()
 	testutils.AwaitState(ctx, t, cc, connectivity.Ready)
 
+	// Retrieve the wrapped conn from the listener.
+	v, err := lis.NewConnCh.Receive(ctx)
+	if err != nil {
+		t.Fatalf("Failed to retrieve conn from test listener: %v", err)
+	}
+	conn := v.(*testutils.ConnWrapper)
+
 	// Verify that the ClientConn moves to IDLE as there is no activity.
 	testutils.AwaitState(ctx, t, cc, connectivity.Idle)
 
 	// Verify idleness related channelz events.
 	if err := channelzTraceEventFound(ctx, "entering idle mode"); err != nil {
 		t.Fatal(err)
 	}
+
+	// Verify that the previously open connection is closed.
+	if _, err := conn.CloseCh.Receive(ctx); err != nil {
+		t.Fatalf("Failed when waiting for connection to be closed after channel entered IDLE: %v", err)
+	}
 }
 
 // Tests the case where channel idleness is enabled by passing a small value for