GODRIVER-3648 Make server selection block on topology updates.

x/mongo/driver/topology/topology.go

@@ -694,7 +694,6 @@ func (t *Topology) selectServerFromSubscription(
 		case <-ctx.Done():
 			return nil, ServerSelectionError{Wrapped: ctx.Err(), Desc: current}
 		case current = <-subscriptionCh:
-		default:
 		}
 
 		suitable, err := t.selectServerFromDescription(current, srvSelector)

x/mongo/driver/topology/topology_test.go

@@ -45,6 +45,18 @@ func compareErrors(err1, err2 error) bool {
 	return true
 }
 
+var _ description.ServerSelector = &mockServerSelector{}
+
+type mockServerSelector struct {
+	selectServerCalls atomic.Int64
+}
+
+func (m *mockServerSelector) SelectServer(description.Topology, []description.Server) ([]description.Server, error) {
+	m.selectServerCalls.Add(1)
+
+	return nil, nil
+}
+
 func TestServerSelection(t *testing.T) {
 	var selectFirst serverselector.Func = func(_ description.Topology, candidates []description.Server) ([]description.Server, error) {
 		if len(candidates) == 0 {
@@ -263,6 +275,30 @@ func TestServerSelection(t *testing.T) {
 		_, err = topo.SelectServer(context.Background(), &serverselector.Write{})
 		assert.Equal(t, ErrSubscribeAfterClosed, err, "expected error %v, got %v", ErrSubscribeAfterClosed, err)
 	})
+	t.Run("if no servers are suitable, block on topology updates", func(t *testing.T) {
+		// Create a connected Topology with no selectable servers.
+		topo, err := New(nil)
+		require.NoError(t, err)
+		atomic.StoreInt64(&topo.state, topologyConnected)
+
+		mss := &mockServerSelector{}
+
+		ctx, cancel := context.WithTimeout(context.Background(), 50*time.Millisecond)
+		defer cancel()
+
+		_, err = topo.SelectServer(ctx, mss)
+		assert.ErrorIs(t, err, context.DeadlineExceeded, "expected context deadline exceeded error")
+
+		// Expect SelectServer to be called twice: once for the fast path and
+		// once to select from the topology updates subscription.
+		//
+		// Note: The second call is due to Topology.Subscript pre-populating the
+		// channel with the current topology. It's not clear what the purpose of
+		// that behavior is. The main goal of this assertion is to make sure the
+		// subscription path blocks on updates and doesn't turn into a busy
+		// wait.
+		assert.Equal(t, int64(2), mss.selectServerCalls.Load(), "expected SelectServer to be called once")
+	})
 }
 
 func TestSessionTimeout(t *testing.T) {