make the initial stream receive window configurable

While the initial window size is defined by the yamux specification
(256 kB), we can just send a window update as soon as a stream is
opened / accepted. In fact, that's exactly what we do already.

mux.go

@@ -31,6 +31,10 @@ type Config struct {
 	// an expectation that things will move along quickly.
 	ConnectionWriteTimeout time.Duration
 
+	// InitialStreamWindowSize is used to control the initial
+	// window size that we allow for a stream.
+	InitialStreamWindowSize uint32
+
 	// MaxStreamWindowSize is used to control the maximum
 	// window size that we allow for a stream.
 	MaxStreamWindowSize uint32
@@ -56,16 +60,17 @@ type Config struct {
 // DefaultConfig is used to return a default configuration
 func DefaultConfig() *Config {
 	return &Config{
-		AcceptBacklog:          256,
-		PingBacklog:            32,
-		EnableKeepAlive:        true,
-		KeepAliveInterval:      30 * time.Second,
-		ConnectionWriteTimeout: 10 * time.Second,
-		MaxStreamWindowSize:    maxStreamWindow,
-		LogOutput:              os.Stderr,
-		ReadBufSize:            4096,
-		MaxMessageSize:         64 * 1024,
-		WriteCoalesceDelay:     100 * time.Microsecond,
+		AcceptBacklog:           256,
+		PingBacklog:             32,
+		EnableKeepAlive:         true,
+		KeepAliveInterval:       30 * time.Second,
+		ConnectionWriteTimeout:  10 * time.Second,
+		InitialStreamWindowSize: initialStreamWindow,
+		MaxStreamWindowSize:     maxStreamWindow,
+		LogOutput:               os.Stderr,
+		ReadBufSize:             4096,
+		MaxMessageSize:          64 * 1024,
+		WriteCoalesceDelay:      100 * time.Microsecond,
 	}
 }
 
@@ -77,8 +82,11 @@ func VerifyConfig(config *Config) error {
 	if config.KeepAliveInterval == 0 {
 		return fmt.Errorf("keep-alive interval must be positive")
 	}
-	if config.MaxStreamWindowSize < initialStreamWindow {
-		return errors.New("MaxStreamWindowSize must be larger than the initialStreamWindow (256 kB)")
+	if config.InitialStreamWindowSize < initialStreamWindow {
+		return errors.New("InitialStreamWindowSize must be larger or equal 256 kB")
+	}
+	if config.MaxStreamWindowSize < config.InitialStreamWindowSize {
+		return errors.New("MaxStreamWindowSize must be larger than the InitialStreamWindowSize")
 	}
 	if config.MaxMessageSize < 1024 {
 		return fmt.Errorf("MaxMessageSize must be greater than a kilobyte")

session_test.go

@@ -6,6 +6,7 @@ import (
 	"fmt"
 	"io"
 	"io/ioutil"
+	"math/rand"
 	"net"
 	"reflect"
 	"runtime"
@@ -1683,3 +1684,51 @@ func TestReadDeadlineInterrupt(t *testing.T) {
 		}
 	}
 }
+
+// Make sure that a transfer doesn't stall, no matter what values the peers use for their InitialStreamWindow.
+func TestInitialStreamWindow(t *testing.T) {
+	for i := 0; i < 10; i++ {
+		const (
+			maxWindow    = 5 * initialStreamWindow
+			transferSize = 10 * maxWindow
+		)
+		rand.Seed(time.Now().UnixNano())
+		randomUint32 := func(min, max uint32) uint32 { return uint32(rand.Int63n(int64(max-min))) + min }
+
+		cconf := DefaultConfig()
+		cconf.InitialStreamWindowSize = randomUint32(initialStreamWindow, maxWindow)
+		sconf := DefaultConfig()
+		sconf.InitialStreamWindowSize = randomUint32(initialStreamWindow, maxWindow)
+
+		conn1, conn2 := testConn()
+		client, _ := Client(conn1, cconf)
+		server, _ := Server(conn2, sconf)
+
+		errChan := make(chan error, 1)
+		go func() {
+			defer close(errChan)
+			str, err := client.OpenStream(context.Background())
+			if err != nil {
+				errChan <- err
+				return
+			}
+			defer str.Close()
+			if _, err := str.Write(make([]byte, transferSize)); err != nil {
+				errChan <- err
+				return
+			}
+		}()
+
+		str, err := server.AcceptStream()
+		if err != nil {
+			t.Fatal(err)
+		}
+		data, err := ioutil.ReadAll(str)
+		if err != nil {
+			t.Fatal(err)
+		}
+		if uint32(len(data)) != transferSize {
+			t.Fatalf("expected %d bytes to be transferred, got %d", transferSize, len(data))
+		}
+	}
+}

stream.go

@@ -59,8 +59,11 @@ func newStream(session *Session, id uint32, state streamState) *Stream {
 		sendWindow:    initialStreamWindow,
 		readDeadline:  makePipeDeadline(),
 		writeDeadline: makePipeDeadline(),
+		// Initialize the recvBuf with initialStreamWindow, not config.InitialStreamWindowSize.
+		// The peer isn't allowed to send more data than initialStreamWindow until we've sent
+		// the first window update (which will grant it up to config.InitialStreamWindowSize).
 		recvBuf:       newSegmentedBuffer(initialStreamWindow),
-		recvWindow:    initialStreamWindow,
+		recvWindow:    session.config.InitialStreamWindowSize,
 		epochStart:    time.Now(),
 		recvNotifyCh:  make(chan struct{}, 1),
 		sendNotifyCh:  make(chan struct{}, 1),