[release-branch.go1.22] net/http: add missing call to decConnsPerHost

A recent change to Transport.dialConnFor introduced an early return that
skipped dialing. This path did not call decConnsPerHost, which can cause
subsequent HTTP calls to hang if Transport.MaxConnsPerHost is set.

For golang#65705
Fixes golang#65759

Change-Id: I157591114b02a3a66488d3ead7f1e6dbd374a41c
Reviewed-on: https://go-review.googlesource.com/c/go/+/564036
Reviewed-by: Damien Neil <dneil@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Damien Neil <dneil@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>
(cherry picked from commit 098a87f)
Reviewed-on: https://go-review.googlesource.com/c/go/+/566536
Reviewed-by: Carlos Amedee <carlos@golang.org>

src/net/http/transport.go

@@ -1483,6 +1483,7 @@ func (t *Transport) dialConnFor(w *wantConn) {
 	defer w.afterDial()
 	ctx := w.getCtxForDial()
 	if ctx == nil {
+		t.decConnsPerHost(w.key)
 		return
 	}
 

src/net/http/transport_test.go

@@ -730,6 +730,56 @@ func testTransportMaxConnsPerHost(t *testing.T, mode testMode) {
 	}
 }
 
+func TestTransportMaxConnsPerHostDialCancellation(t *testing.T) {
+	run(t, testTransportMaxConnsPerHostDialCancellation,
+		testNotParallel, // because test uses SetPendingDialHooks
+		[]testMode{http1Mode, https1Mode, http2Mode},
+	)
+}
+
+func testTransportMaxConnsPerHostDialCancellation(t *testing.T, mode testMode) {
+	CondSkipHTTP2(t)
+
+	h := HandlerFunc(func(w ResponseWriter, r *Request) {
+		_, err := w.Write([]byte("foo"))
+		if err != nil {
+			t.Fatalf("Write: %v", err)
+		}
+	})
+
+	cst := newClientServerTest(t, mode, h)
+	defer cst.close()
+	ts := cst.ts
+	c := ts.Client()
+	tr := c.Transport.(*Transport)
+	tr.MaxConnsPerHost = 1
+
+	// This request is cancelled when dial is queued, which preempts dialing.
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+	SetPendingDialHooks(cancel, nil)
+	defer SetPendingDialHooks(nil, nil)
+
+	req, _ := NewRequestWithContext(ctx, "GET", ts.URL, nil)
+	_, err := c.Do(req)
+	if !errors.Is(err, context.Canceled) {
+		t.Errorf("expected error %v, got %v", context.Canceled, err)
+	}
+
+	// This request should succeed.
+	SetPendingDialHooks(nil, nil)
+	req, _ = NewRequest("GET", ts.URL, nil)
+	resp, err := c.Do(req)
+	if err != nil {
+		t.Fatalf("request failed: %v", err)
+	}
+	defer resp.Body.Close()
+	_, err = io.ReadAll(resp.Body)
+	if err != nil {
+		t.Fatalf("read body failed: %v", err)
+	}
+}
+
 func TestTransportRemovesDeadIdleConnections(t *testing.T) {
 	run(t, testTransportRemovesDeadIdleConnections, []testMode{http1Mode})
 }