net/http: only decrement connection count if we removed a connection

The connection count must only be decremented if the persistent
connection was also removed.

Fixes #34941

Change-Id: I5070717d5d9effec78016005fa4910593500c8cf
Reviewed-on: https://go-review.googlesource.com/c/go/+/202087Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/net/http/transport.go

@@ -545,9 +545,10 @@ func (t *Transport) roundTrip(req *Request) (*Response, error) {
 
 		_, isH2DialError := pconn.alt.(http2erringRoundTripper)
 		if http2isNoCachedConnError(err) || isH2DialError {
-			t.removeIdleConn(pconn)
+			if t.removeIdleConn(pconn) {
 				t.decConnsPerHost(pconn.cacheKey)
 			}
+		}
 		if !pconn.shouldRetryRequest(req, err) {
 			// Issue 16465: return underlying net.Conn.Read error from peek,
 			// as we've historically done.
@@ -958,26 +959,28 @@ func (t *Transport) queueForIdleConn(w *wantConn) (delivered bool) {
 }
 
 // removeIdleConn marks pconn as dead.
-func (t *Transport) removeIdleConn(pconn *persistConn) {
+func (t *Transport) removeIdleConn(pconn *persistConn) bool {
 	t.idleMu.Lock()
 	defer t.idleMu.Unlock()
-	t.removeIdleConnLocked(pconn)
+	return t.removeIdleConnLocked(pconn)
 }
 
 // t.idleMu must be held.
-func (t *Transport) removeIdleConnLocked(pconn *persistConn) {
+func (t *Transport) removeIdleConnLocked(pconn *persistConn) bool {
 	if pconn.idleTimer != nil {
 		pconn.idleTimer.Stop()
 	}
 	t.idleLRU.remove(pconn)
 	key := pconn.cacheKey
 	pconns := t.idleConn[key]
+	var removed bool
 	switch len(pconns) {
 	case 0:
 		// Nothing
 	case 1:
 		if pconns[0] == pconn {
 			delete(t.idleConn, key)
+			removed = true
 		}
 	default:
 		for i, v := range pconns {
@@ -988,9 +991,11 @@ func (t *Transport) removeIdleConnLocked(pconn *persistConn) {
 			// conns at the end.
 			copy(pconns[i:], pconns[i+1:])
 			t.idleConn[key] = pconns[:len(pconns)-1]
+			removed = true
 			break
 		}
 	}
+	return removed
 }
 
 func (t *Transport) setReqCanceler(r *Request, fn func(error)) {

src/net/http/transport_test.go

@@ -5893,3 +5893,59 @@ func TestDontCacheBrokenHTTP2Conn(t *testing.T) {
 		t.Errorf("GotConn calls = %v; want %v", got, want)
 	}
 }
+
+// Issue 34941
+// When the client has too many concurrent requests on a single connection,
+// http.http2noCachedConnError is reported on multiple requests. There should
+// only be one decrement regardless of the number of failures.
+func TestTransportDecrementConnWhenIdleConnRemoved(t *testing.T) {
+	defer afterTest(t)
+
+	h := HandlerFunc(func(w ResponseWriter, r *Request) {
+		_, err := w.Write([]byte("foo"))
+		if err != nil {
+			t.Fatalf("Write: %v", err)
+		}
+	})
+
+	ts := httptest.NewUnstartedServer(h)
+	ts.EnableHTTP2 = true
+	ts.StartTLS()
+	defer ts.Close()
+
+	c := ts.Client()
+	tr := c.Transport.(*Transport)
+	tr.MaxConnsPerHost = 1
+	if err := ExportHttp2ConfigureTransport(tr); err != nil {
+		t.Fatalf("ExportHttp2ConfigureTransport: %v", err)
+	}
+
+	errCh := make(chan error, 300)
+	doReq := func() {
+		resp, err := c.Get(ts.URL)
+		if err != nil {
+			errCh <- fmt.Errorf("request failed: %v", err)
+			return
+		}
+		defer resp.Body.Close()
+		_, err = ioutil.ReadAll(resp.Body)
+		if err != nil {
+			errCh <- fmt.Errorf("read body failed: %v", err)
+		}
+	}
+
+	var wg sync.WaitGroup
+	for i := 0; i < 300; i++ {
+		wg.Add(1)
+		go func() {
+			defer wg.Done()
+			doReq()
+		}()
+	}
+	wg.Wait()
+	close(errCh)
+
+	for err := range errCh {
+		t.Errorf("error occurred: %v", err)
+	}
+}