http2: limit client initial MAX_CONCURRENT_STREAMS

Prevent the client trying to establish more streams than the server is
willing to accept during the initial life time of a connection by
limiting maxConcurrentStreams to 100, the http2 specifications
recommended minimum, until we've received the initial SETTINGS frame
from the server.

After a SETTINGS frame has been received use the servers
MAX_CONCURRENT_STREAMS, if present, otherwise use 1000 as a reasonable
value.

For normal consumers this will have very little impact, allowing a
decent level of concurrency from the start, and for highly concurrent
consumers or large bursts it will prevent significant number of rejected
streams being attempted hence actually increasing performance.

Fixes golang/go#39389

Author: stevenh

http2/transport.go

@@ -51,6 +51,15 @@ const (
 	transportDefaultStreamMinRefresh = 4 << 10
 
 	defaultUserAgent = "Go-http-client/2.0"
+
+	// initialMaxConcurrentStreams is a connections maxConcurrentStreams until
+	// it's received servers initial SETTINGS frame, which corresponds with the
+	// spec's minimum recommended value.
+	initialMaxConcurrentStreams = 100
+
+	// defaultMaxConcurrentStreams is a connections default maxConcurrentStreams
+	// if the server doesn't include one in its initial SETTINGS frame.
+	defaultMaxConcurrentStreams = 1000
 )
 
 // Transport is an HTTP/2 Transport.
@@ -237,6 +246,7 @@ type ClientConn struct {
 	inflow          flow       // peer's conn-level flow control
 	closing         bool
 	closed          bool
+	seenSettings    bool                     // true if we've seen a settings frame, false otherwise
 	wantSettingsAck bool                     // we sent a SETTINGS frame and haven't heard back
 	goAway          *GoAwayFrame             // if non-nil, the GoAwayFrame we received
 	goAwayDebug     string                   // goAway frame's debug data, retained as a string
@@ -634,10 +644,10 @@ func (t *Transport) newClientConn(c net.Conn, singleUse bool) (*ClientConn, erro
 		tconn:                 c,
 		readerDone:            make(chan struct{}),
 		nextStreamID:          1,
-		maxFrameSize:          16 << 10,           // spec default
-		initialWindowSize:     65535,              // spec default
-		maxConcurrentStreams:  1000,               // "infinite", per spec. 1000 seems good enough.
-		peerMaxHeaderListSize: 0xffffffffffffffff, // "infinite", per spec. Use 2^64-1 instead.
+		maxFrameSize:          16 << 10,                    // spec default
+		initialWindowSize:     65535,                       // spec default
+		maxConcurrentStreams:  initialMaxConcurrentStreams, // "infinite", per spec. Use a smaller value until we have received server settings.
+		peerMaxHeaderListSize: 0xffffffffffffffff,          // "infinite", per spec. Use 2^64-1 instead.
 		streams:               make(map[uint32]*clientStream),
 		singleUse:             singleUse,
 		wantSettingsAck:       true,
@@ -2319,12 +2329,14 @@ func (rl *clientConnReadLoop) processSettings(f *SettingsFrame) error {
 		return ConnectionError(ErrCodeProtocol)
 	}
 
+	var seenMaxConcurrentStreams bool
 	err := f.ForeachSetting(func(s Setting) error {
 		switch s.ID {
 		case SettingMaxFrameSize:
 			cc.maxFrameSize = s.Val
 		case SettingMaxConcurrentStreams:
 			cc.maxConcurrentStreams = s.Val
+			seenMaxConcurrentStreams = true
 		case SettingMaxHeaderListSize:
 			cc.peerMaxHeaderListSize = uint64(s.Val)
 		case SettingInitialWindowSize:
@@ -2356,6 +2368,17 @@ func (rl *clientConnReadLoop) processSettings(f *SettingsFrame) error {
 		return err
 	}
 
+	if !cc.seenSettings {
+		if !seenMaxConcurrentStreams {
+			// This was the servers initial SETTINGS frame and it
+			// didn't contain a MAX_CONCURRENT_STREAMS field so
+			// increase the number of concurrent streams this
+			// connection can establish to our default.
+			cc.maxConcurrentStreams = defaultMaxConcurrentStreams
+		}
+		cc.seenSettings = true
+	}
+
 	cc.wmu.Lock()
 	defer cc.wmu.Unlock()