net/http: in the IdleConnStrsForTesting_h2 helper, omit conns that cannot be reused

In #59155, we observed that the IdleConnStrsForTesting_h2 helper
function sometimes reported extra connections after a
"client conn not usable" failure and retry. It turns out that that
state corresponds exactly to the
http2clientConnIdleState.canTakeNewRequest field, so (with a bit of
extra nethttpomithttp2 plumbing) we can use that field in the helper
to filter out the unusable connections.

Fixes #59155.

Change-Id: Ief6283c9c8c5ec47dd9f378beb0ddf720832484e
Reviewed-on: https://go-review.googlesource.com/c/go/+/477856
Reviewed-by: Damien Neil <dneil@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Bryan Mills <bcmills@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>

src/net/http/export_test.go

@@ -142,11 +142,13 @@ func (t *Transport) IdleConnStrsForTesting_h2() []string {
 	pool.mu.Lock()
 	defer pool.mu.Unlock()
 
-	for k, cc := range pool.conns {
-		for range cc {
+	for k, ccs := range pool.conns {
+		for _, cc := range ccs {
+			if cc.idleState().canTakeNewRequest {
 				ret = append(ret, k)
 			}
 		}
+	}
 
 	sort.Strings(ret)
 	return ret

src/net/http/omithttp2.go

@@ -42,9 +42,17 @@ type http2noDialClientConnPool struct {
 
 type http2clientConnPool struct {
 	mu    *sync.Mutex
-	conns map[string][]struct{}
+	conns map[string][]*http2clientConn
 }
 
+type http2clientConn struct{}
+
+type http2clientConnIdleState struct {
+	canTakeNewRequest bool
+}
+
+func (cc *http2clientConn) idleState() http2clientConnIdleState { return http2clientConnIdleState{} }
+
 func http2configureTransports(*Transport) (*http2Transport, error) { panic(noHTTP2) }
 
 func http2isNoCachedConnError(err error) bool {

src/net/http/transport_test.go

@@ -5047,7 +5047,7 @@ timeoutLoop:
 		}
 
 		var conn string
-		doReq := func(n int) (ok bool) {
+		doReq := func(n int) (timeoutOk bool) {
 			req, _ := NewRequest("GET", cst.ts.URL, nil)
 			req = req.WithContext(httptrace.WithClientTrace(context.Background(), &httptrace.ClientTrace{
 				PutIdleConn: func(err error) {
@@ -5094,7 +5094,7 @@ timeoutLoop:
 		waitCondition(t, timeout/2, func(d time.Duration) bool {
 			if got := idleConns(); len(got) != 0 {
 				if d >= timeout*3/2 {
-					t.Logf("after %d, idle conns = %q", d, got)
+					t.Logf("after %v, idle conns = %q", d, got)
 				}
 				return false
 			}