http2: allow prioritization to be disabled using DisableClientPriority field

As part of adding support for HTTP/2 stream prioritization, a
DisableClientPriority field will be added to http.Server to allow users
to completely disable client prioritization if desired. When
DisableClientPriority is set to true, HTTP/2 server will revert back to
the old behavior where streams are processed in a round-robin manner.

For golang/go#75500

Change-Id: Ida083b3ac17a953e5ddb3ad7ab8a81f9cde2bfc1
Reviewed-on: https://go-review.googlesource.com/c/net/+/737521
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Nicholas Husin <husin@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

http2/client_priority_go126.go

@@ -0,0 +1,20 @@
+// Copyright 2026 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build !go1.27
+
+package http2
+
+import "net/http"
+
+// Support for go.dev/issue/75500 is added in Go 1.27. In case anyone uses
+// x/net with versions before Go 1.27, we return true here so that their write
+// scheduler will still be the round-robin write scheduler rather than the RFC
+// 9218 write scheduler. That way, older users of Go will not see a sudden
+// change of behavior just from importing x/net.
+//
+// TODO(nsh): remove this file after x/net go.mod is at Go 1.27.
+func clientPriorityDisabled(_ *http.Server) bool {
+	return true
+}

http2/client_priority_go127.go

@@ -0,0 +1,13 @@
+// Copyright 2026 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build go1.27
+
+package http2
+
+import "net/http"
+
+func clientPriorityDisabled(s *http.Server) bool {
+	return s.DisableClientPriority
+}

http2/server.go

@@ -472,10 +472,13 @@ func (s *Server) serveConn(c net.Conn, opts *ServeConnOpts, newf func(*serverCon
 		sc.conn.SetWriteDeadline(time.Time{})
 	}
 
-	if s.NewWriteScheduler != nil {
+	switch {
+	case s.NewWriteScheduler != nil:
 		sc.writeSched = s.NewWriteScheduler()
-	} else {
+	case clientPriorityDisabled(http1srv):
 		sc.writeSched = newRoundRobinWriteScheduler()
+	default:
+		sc.writeSched = newPriorityWriteSchedulerRFC9218()
 	}
 
 	// These start at the RFC-specified defaults. If there is a higher