net/http: deflake TestServerSetKeepAlivesEnabledClosesConns

Fixes #17754
Updates #9478 (details in here)

Change-Id: Iae2c1ca05a18ed266b53b2594c22fc57fab33c5e
Reviewed-on: https://go-review.googlesource.com/32587
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/net/http/export_test.go

@@ -186,3 +186,15 @@ func ExportHttp2ConfigureTransport(t *Transport) error {
 }
 
 var Export_shouldCopyHeaderOnRedirect = shouldCopyHeaderOnRedirect
+
+func (s *Server) ExportAllConnsIdle() bool {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	for c := range s.activeConn {
+		st, ok := c.curState.Load().(ConnState)
+		if !ok || st != StateIdle {
+			return false
+		}
+	}
+	return true
+}

src/net/http/serve_test.go

@@ -4875,6 +4875,9 @@ func TestServerSetKeepAlivesEnabledClosesConns(t *testing.T) {
 	}) {
 		t.Fatalf("idle count before SetKeepAlivesEnabled called = %v; want 1", idle0)
 	}
+	if !waitCondition(2*time.Second, 10*time.Millisecond, ts.Config.ExportAllConnsIdle) {
+		t.Fatalf("test server has active conns")
+	}
 
 	ts.Config.SetKeepAlivesEnabled(false)