http2: fix request reservation when StrictMaxConcurrentStreams is enabled

Fix regression introduced in CL 617655. The awaitOpenSlotForStreamLocked()
should not take into consideration reserved requests as they have no presence
in the connection.

Fixes golang/go#70809

Change-Id: Ia23968189cbdb44dae860a1de9d32700115b28b4

Author: mmatczuk

http2/transport.go

@@ -1786,7 +1786,7 @@ func (cs *clientStream) cleanupWriteRequest(err error) {
 	close(cs.donec)
 }
 
-// awaitOpenSlotForStreamLocked waits until len(streams) < maxConcurrentStreams.
+// awaitOpenSlotForStreamLocked waits until len(streams) + pendingResets < maxConcurrentStreams.
 // Must hold cc.mu.
 func (cc *ClientConn) awaitOpenSlotForStreamLocked(cs *clientStream) error {
 	for {
@@ -1800,7 +1800,7 @@ func (cc *ClientConn) awaitOpenSlotForStreamLocked(cs *clientStream) error {
 			return errClientConnUnusable
 		}
 		cc.lastIdle = time.Time{}
-		if cc.currentRequestCountLocked() < int(cc.maxConcurrentStreams) {
+		if int64(len(cc.streams)+cc.pendingResets) < int64(cc.maxConcurrentStreams) {
 			return nil
 		}
 		cc.pendingRequests++

http2/transport_test.go

@@ -5425,6 +5425,52 @@ func TestIssue67671(t *testing.T) {
 	}
 }
 
+// Issue 70809: when StrictMaxConcurrentStreams enabled ClientConn.ReserveNewRequest() causes stalls in request processing
+func TestIssue70809(t *testing.T) {
+	ts := newTestServer(t, func(w http.ResponseWriter, r *http.Request) {}, func(s *Server) {
+		s.MaxConcurrentStreams = 1
+	})
+	tr := &Transport{
+		TLSClientConfig:            tlsConfigInsecure,
+		AllowHTTP:                  true,
+		StrictMaxConcurrentStreams: true,
+	}
+
+	var opt RoundTripOpt
+
+	ctx, cancel := context.WithTimeout(context.Background(), time.Second)
+	defer cancel()
+
+	get := func() int {
+		req, _ := http.NewRequestWithContext(ctx, "GET", ts.URL, nil)
+		res, err := tr.RoundTripOpt(req, opt)
+		if err != nil {
+			t.Error(err)
+		}
+		res.Body.Close()
+		return res.StatusCode
+	}
+
+	if get() != 200 {
+		t.Fatal("first request failed")
+	}
+
+	// Do not dial new connections.
+	opt.OnlyCachedConn = true
+
+	var wg sync.WaitGroup
+	for i := 0; i < 10; i++ {
+		wg.Add(1)
+		go func() {
+			defer wg.Done()
+			if get() != 200 {
+				t.Errorf("request failed")
+			}
+		}()
+	}
+	wg.Wait()
+}
+
 func TestTransport1xxLimits(t *testing.T) {
 	for _, test := range []struct {
 		name    string