http2: fix race on ClientConn.maxFrameSize

This fixes TestTrailersClientToServer_h2. Before this CL, the following
command reliably fails. With this CL merged into net/http, the following
command reliably succeeds.

go test -race -run=TestTrailersClientToServer_h2 -count 1000 net/http

Updates golang/go#22721

Change-Id: I05d1504c60854fcf3ae9531f36a126e94b00f0b7
Reviewed-on: https://go-review.googlesource.com/79238
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com>

http2/transport.go

@@ -811,7 +811,7 @@ func (cc *ClientConn) roundTrip(req *http.Request) (res *http.Response, gotErrAf
 
 	cc.wmu.Lock()
 	endStream := !hasBody && !hasTrailers
-	werr := cc.writeHeaders(cs.ID, endStream, hdrs)
+	werr := cc.writeHeaders(cs.ID, endStream, int(cc.maxFrameSize), hdrs)
 	cc.wmu.Unlock()
 	traceWroteHeaders(cs.trace)
 	cc.mu.Unlock()
@@ -964,13 +964,12 @@ func (cc *ClientConn) awaitOpenSlotForRequest(req *http.Request) error {
 }
 
 // requires cc.wmu be held
-func (cc *ClientConn) writeHeaders(streamID uint32, endStream bool, hdrs []byte) error {
+func (cc *ClientConn) writeHeaders(streamID uint32, endStream bool, maxFrameSize int, hdrs []byte) error {
 	first := true // first frame written (HEADERS is first, then CONTINUATION)
-	frameSize := int(cc.maxFrameSize)
 	for len(hdrs) > 0 && cc.werr == nil {
 		chunk := hdrs
-		if len(chunk) > frameSize {
-			chunk = chunk[:frameSize]
+		if len(chunk) > maxFrameSize {
+			chunk = chunk[:maxFrameSize]
 		}
 		hdrs = hdrs[len(chunk):]
 		endHeaders := len(hdrs) == 0
@@ -1087,13 +1086,17 @@ func (cs *clientStream) writeRequestBody(body io.Reader, bodyCloser io.Closer) (
 		}
 	}
 
+	cc.mu.Lock()
+	maxFrameSize := int(cc.maxFrameSize)
+	cc.mu.Unlock()
+
 	cc.wmu.Lock()
 	defer cc.wmu.Unlock()
 
 	// Two ways to send END_STREAM: either with trailers, or
 	// with an empty DATA frame.
 	if len(trls) > 0 {
-		err = cc.writeHeaders(cs.ID, true, trls)
+		err = cc.writeHeaders(cs.ID, true, maxFrameSize, trls)
 	} else {
 		err = cc.fr.WriteData(cs.ID, true, nil)
 	}