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// Copyright 2023 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 go1.21
package quic
import (
// Limits on the number of open streams.
// Every connection has separate limits for bidirectional and unidirectional streams.
// Note that the MAX_STREAMS limit includes closed as well as open streams.
// Closing a stream doesn't enable an endpoint to open a new one;
// only an increase in the MAX_STREAMS limit does.
// localStreamLimits are limits on the number of open streams created by us.
type localStreamLimits struct {
gate gate
max int64 // peer-provided MAX_STREAMS
opened int64 // number of streams opened by us, -1 when conn is closed
func (lim *localStreamLimits) init() {
lim.gate = newGate()
// open creates a new local stream, blocking until MAX_STREAMS quota is available.
func (lim *localStreamLimits) open(ctx context.Context, c *Conn) (num int64, err error) {
// TODO: Send a STREAMS_BLOCKED when blocked.
if err := lim.gate.waitAndLock(ctx, c.testHooks); err != nil {
return 0, err
if lim.opened < 0 {
return 0, errConnClosed
num = lim.opened
lim.gate.unlock(lim.opened < lim.max)
return num, nil
// connHasClosed indicates the connection has been closed, locally or by the peer.
func (lim *localStreamLimits) connHasClosed() {
lim.opened = -1
// setMax sets the MAX_STREAMS provided by the peer.
func (lim *localStreamLimits) setMax(maxStreams int64) {
lim.max = max(lim.max, maxStreams)
lim.gate.unlock(lim.opened < lim.max)
// remoteStreamLimits are limits on the number of open streams created by the peer.
type remoteStreamLimits struct {
max int64 // last MAX_STREAMS sent to the peer
opened int64 // number of streams opened by the peer (including subsequently closed ones)
closed int64 // number of peer streams in the "closed" state
maxOpen int64 // how many streams we want to let the peer simultaneously open
sendMax sentVal // set when we should send MAX_STREAMS
func (lim *remoteStreamLimits) init(maxOpen int64) {
lim.maxOpen = maxOpen
lim.max = min(maxOpen, implicitStreamLimit) // initial limit sent in transport parameters
lim.opened = 0
// open handles the peer opening a new stream.
func (lim *remoteStreamLimits) open(id streamID) error {
num := id.num()
if num >= lim.max {
return localTransportError{
code: errStreamLimit,
reason: "stream limit exceeded",
if num >= lim.opened {
lim.opened = num + 1
return nil
// close handles the peer closing an open stream.
func (lim *remoteStreamLimits) close() {
// maybeUpdateMax updates the MAX_STREAMS value we will send to the peer.
func (lim *remoteStreamLimits) maybeUpdateMax() {
newMax := min(
// Max streams the peer can have open at once.
// Max streams the peer can open with a single frame.
avail := lim.max - lim.opened
if newMax > lim.max && (avail < 8 || newMax-lim.max >= 2*avail) {
// If the peer has less than 8 streams, or if increasing the peer's
// stream limit would double it, then send a MAX_STREAMS.
lim.max = newMax
// appendFrame appends a MAX_STREAMS frame to the current packet, if necessary.
// It returns true if no more frames need appending,
// false if not everything fit in the current packet.
func (lim *remoteStreamLimits) appendFrame(w *packetWriter, typ streamType, pnum packetNumber, pto bool) bool {
if lim.sendMax.shouldSendPTO(pto) {
if !w.appendMaxStreamsFrame(typ, lim.max) {
return false
return true