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// Copyright 2014 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.
package http2
import "fmt"
// WriteScheduler is the interface implemented by HTTP/2 write schedulers.
// Methods are never called concurrently.
type WriteScheduler interface {
// OpenStream opens a new stream in the write scheduler.
// It is illegal to call this with streamID=0 or with a streamID that is
// already open -- the call may panic.
OpenStream(streamID uint32, options OpenStreamOptions)
// CloseStream closes a stream in the write scheduler. Any frames queued on
// this stream should be discarded. It is illegal to call this on a stream
// that is not open -- the call may panic.
CloseStream(streamID uint32)
// AdjustStream adjusts the priority of the given stream. This may be called
// on a stream that has not yet been opened or has been closed. Note that
// RFC 7540 allows PRIORITY frames to be sent on streams in any state. See:
AdjustStream(streamID uint32, priority PriorityParam)
// Push queues a frame in the scheduler. In most cases, this will not be
// called with wr.StreamID()!=0 unless that stream is currently open. The one
// exception is RST_STREAM frames, which may be sent on idle or closed streams.
Push(wr FrameWriteRequest)
// Pop dequeues the next frame to write. Returns false if no frames can
// be written. Frames with a given wr.StreamID() are Pop'd in the same
// order they are Push'd, except RST_STREAM frames. No frames should be
// discarded except by CloseStream.
Pop() (wr FrameWriteRequest, ok bool)
// OpenStreamOptions specifies extra options for WriteScheduler.OpenStream.
type OpenStreamOptions struct {
// PusherID is zero if the stream was initiated by the client. Otherwise,
// PusherID names the stream that pushed the newly opened stream.
PusherID uint32
// FrameWriteRequest is a request to write a frame.
type FrameWriteRequest struct {
// write is the interface value that does the writing, once the
// WriteScheduler has selected this frame to write. The write
// functions are all defined in write.go.
write writeFramer
// stream is the stream on which this frame will be written.
// nil for non-stream frames like PING and SETTINGS.
// nil for RST_STREAM streams, which use the StreamError.StreamID field instead.
stream *stream
// done, if non-nil, must be a buffered channel with space for
// 1 message and is sent the return value from write (or an
// earlier error) when the frame has been written.
done chan error
// StreamID returns the id of the stream this frame will be written to.
// 0 is used for non-stream frames such as PING and SETTINGS.
func (wr FrameWriteRequest) StreamID() uint32 {
if == nil {
if se, ok := wr.write.(StreamError); ok {
// (*serverConn).resetStream doesn't set
// stream because it doesn't necessarily have
// one. So special case this type of write
// message.
return se.StreamID
return 0
// isControl reports whether wr is a control frame for MaxQueuedControlFrames
// purposes. That includes non-stream frames and RST_STREAM frames.
func (wr FrameWriteRequest) isControl() bool {
return == nil
// DataSize returns the number of flow control bytes that must be consumed
// to write this entire frame. This is 0 for non-DATA frames.
func (wr FrameWriteRequest) DataSize() int {
if wd, ok := wr.write.(*writeData); ok {
return len(wd.p)
return 0
// Consume consumes min(n, available) bytes from this frame, where available
// is the number of flow control bytes available on the stream. Consume returns
// 0, 1, or 2 frames, where the integer return value gives the number of frames
// returned.
// If flow control prevents consuming any bytes, this returns (_, _, 0). If
// the entire frame was consumed, this returns (wr, _, 1). Otherwise, this
// returns (consumed, rest, 2), where 'consumed' contains the consumed bytes and
// 'rest' contains the remaining bytes. The consumed bytes are deducted from the
// underlying stream's flow control budget.
func (wr FrameWriteRequest) Consume(n int32) (FrameWriteRequest, FrameWriteRequest, int) {
var empty FrameWriteRequest
// Non-DATA frames are always consumed whole.
wd, ok := wr.write.(*writeData)
if !ok || len(wd.p) == 0 {
return wr, empty, 1
// Might need to split after applying limits.
allowed :=
if n < allowed {
allowed = n
if < allowed {
allowed =
if allowed <= 0 {
return empty, empty, 0
if len(wd.p) > int(allowed) {
consumed := FrameWriteRequest{
write: &writeData{
streamID: wd.streamID,
p: wd.p[:allowed],
// Even if the original had endStream set, there
// are bytes remaining because len(wd.p) > allowed,
// so we know endStream is false.
endStream: false,
// Our caller is blocking on the final DATA frame, not
// this intermediate frame, so no need to wait.
done: nil,
rest := FrameWriteRequest{
write: &writeData{
streamID: wd.streamID,
p: wd.p[allowed:],
endStream: wd.endStream,
done: wr.done,
return consumed, rest, 2
// The frame is consumed whole.
// NB: This cast cannot overflow because allowed is <= math.MaxInt32.
return wr, empty, 1
// String is for debugging only.
func (wr FrameWriteRequest) String() string {
var des string
if s, ok := wr.write.(fmt.Stringer); ok {
des = s.String()
} else {
des = fmt.Sprintf("%T", wr.write)
return fmt.Sprintf("[FrameWriteRequest stream=%d, ch=%v, writer=%v]", wr.StreamID(), wr.done != nil, des)
// replyToWriter sends err to wr.done and panics if the send must block
// This does nothing if wr.done is nil.
func (wr *FrameWriteRequest) replyToWriter(err error) {
if wr.done == nil {
select {
case wr.done <- err:
panic(fmt.Sprintf("unbuffered done channel passed in for type %T", wr.write))
wr.write = nil // prevent use (assume it's tainted after wr.done send)
// writeQueue is used by implementations of WriteScheduler.
type writeQueue struct {
s []FrameWriteRequest
prev, next *writeQueue
func (q *writeQueue) empty() bool { return len(q.s) == 0 }
func (q *writeQueue) push(wr FrameWriteRequest) {
q.s = append(q.s, wr)
func (q *writeQueue) shift() FrameWriteRequest {
if len(q.s) == 0 {
panic("invalid use of queue")
wr := q.s[0]
// TODO: less copy-happy queue.
copy(q.s, q.s[1:])
q.s[len(q.s)-1] = FrameWriteRequest{}
q.s = q.s[:len(q.s)-1]
return wr
// consume consumes up to n bytes from q.s[0]. If the frame is
// entirely consumed, it is removed from the queue. If the frame
// is partially consumed, the frame is kept with the consumed
// bytes removed. Returns true iff any bytes were consumed.
func (q *writeQueue) consume(n int32) (FrameWriteRequest, bool) {
if len(q.s) == 0 {
return FrameWriteRequest{}, false
consumed, rest, numresult := q.s[0].Consume(n)
switch numresult {
case 0:
return FrameWriteRequest{}, false
case 1:
case 2:
q.s[0] = rest
return consumed, true
type writeQueuePool []*writeQueue
// put inserts an unused writeQueue into the pool.
func (p *writeQueuePool) put(q *writeQueue) {
for i := range q.s {
q.s[i] = FrameWriteRequest{}
q.s = q.s[:0]
*p = append(*p, q)
// get returns an empty writeQueue.
func (p *writeQueuePool) get() *writeQueue {
ln := len(*p)
if ln == 0 {
return new(writeQueue)
x := ln - 1
q := (*p)[x]
(*p)[x] = nil
*p = (*p)[:x]
return q