[internal-branch.go1.16-vendor] http2: complete outstanding async write before reading the next frame

Updates golang/go#42777
Fixes golang/go#47691

Change-Id: I8d7e35ded3fe508f6c285e84ba768104c3406bdf
Reviewed-on: https://go-review.googlesource.com/c/net/+/330909
Reviewed-by: Brad Fitzpatrick <[email protected]>
Reviewed-by: Damien Neil <[email protected]>
Trust: Dmitri Shuralyov <[email protected]>
(cherry picked from commit 60bc85c)
Reviewed-on: https://go-review.googlesource.com/c/net/+/346889
Trust: Damien Neil <[email protected]>
Run-TryBot: Damien Neil <[email protected]>
TryBot-Result: Go Bot <[email protected]>
Reviewed-by: Cherry Mui <[email protected]>

Author: fraenkel

http2/server.go

@@ -866,6 +866,15 @@ func (sc *serverConn) serve() {
 		case res := <-sc.wroteFrameCh:
 			sc.wroteFrame(res)
 		case res := <-sc.readFrameCh:
+			// Process any written frames before reading new frames from the client since a
+			// written frame could have triggered a new stream to be started.
+			if sc.writingFrameAsync {
+				select {
+				case wroteRes := <-sc.wroteFrameCh:
+					sc.wroteFrame(wroteRes)
+				default:
+				}
+			}
 			if !sc.processFrameFromReader(res) {
 				return
 			}