// 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"
// frameWriteMsg is a request to write a frame.
type frameWriteMsg struct {
// write is the interface value that does the writing, once the
// writeScheduler (below) has decided to select this frame
// to write. The write functions are all defined in write.go.
write writeFramer
stream *stream // used for prioritization. nil for non-stream frames.
// 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
}
// for debugging only:
func (wm frameWriteMsg) String() string {
var streamID uint32
if wm.stream != nil {
streamID = wm.stream.id
}
var des string
if s, ok := wm.write.(fmt.Stringer); ok {
des = s.String()
} else {
des = fmt.Sprintf("%T", wm.write)
}
return fmt.Sprintf("[frameWriteMsg stream=%d, ch=%v, type: %v]", streamID, wm.done != nil, des)
}
// writeScheduler tracks pending frames to write, priorities, and decides
// the next one to use. It is not thread-safe.
type writeScheduler struct {
// zero are frames not associated with a specific stream.
// They're sent before any stream-specific freams.
zero writeQueue
// maxFrameSize is the maximum size of a DATA frame
// we'll write. Must be non-zero and between 16K-16M.
maxFrameSize uint32
// sq contains the stream-specific queues, keyed by stream ID.
// when a stream is idle, it's deleted from the map.
sq map[uint32]*writeQueue
// canSend is a slice of memory that's reused between frame
// scheduling decisions to hold the list of writeQueues (from sq)
// which have enough flow control data to send. After canSend is
// built, the best is selected.
canSend []*writeQueue
// pool of empty queues for reuse.
queuePool []*writeQueue
}
func (ws *writeScheduler) putEmptyQueue(q *writeQueue) {
if len(q.s) != 0 {
panic("queue must be empty")
}
ws.queuePool = append(ws.queuePool, q)
}
func (ws *writeScheduler) getEmptyQueue() *writeQueue {
ln := len(ws.queuePool)
if ln == 0 {
return new(writeQueue)
}
q := ws.queuePool[ln-1]
ws.queuePool = ws.queuePool[:ln-1]
return q
}
func (ws *writeScheduler) empty() bool { return ws.zero.empty() && len(ws.sq) == 0 }
func (ws *writeScheduler) add(wm frameWriteMsg) {
st := wm.stream
if st == nil {
ws.zero.push(wm)
} else {
ws.streamQueue(st.id).push(wm)
}
}
func (ws *writeScheduler) streamQueue(streamID uint32) *writeQueue {
if q, ok := ws.sq[streamID]; ok {
return q
}
if ws.sq == nil {
ws.sq = make(map[uint32]*writeQueue)
}
q := ws.getEmptyQueue()
ws.sq[streamID] = q
return q
}
// take returns the most important frame to write and removes it from the scheduler.
// It is illegal to call this if the scheduler is empty or if there are no connection-level
// flow control bytes available.
func (ws *writeScheduler) take() (wm frameWriteMsg, ok bool) {
if ws.maxFrameSize == 0 {
panic("internal error: ws.maxFrameSize not initialized or invalid")
}
// If there any frames not associated with streams, prefer those first.
// These are usually SETTINGS, etc.
if !ws.zero.empty() {
return ws.zero.shift(), true
}
if len(ws.sq) == 0 {
return
}
// Next, prioritize frames on streams that aren't DATA frames (no cost).
for id, q := range ws.sq {
if q.firstIsNoCost() {
return ws.takeFrom(id, q)
}
}
// Now, all that remains are DATA frames with non-zero bytes to
// send. So pick the best one.
if len(ws.canSend) != 0 {
panic("should be empty")
}
for _, q := range ws.sq {
if n := ws.streamWritableBytes(q); n > 0 {
ws.canSend = append(ws.canSend, q)
}
}
if len(ws.canSend) == 0 {
return
}
defer ws.zeroCanSend()
// TODO: find the best queue
q := ws.canSend[0]
return ws.takeFrom(q.streamID(), q)
}
// zeroCanSend is defered from take.
func (ws *writeScheduler) zeroCanSend() {
for i := range ws.canSend {
ws.canSend[i] = nil
}
ws.canSend = ws.canSend[:0]
}
// streamWritableBytes returns the number of DATA bytes we could write
// from the given queue's stream, if this stream/queue were
// selected. It is an error to call this if q's head isn't a
// *writeData.
func (ws *writeScheduler) streamWritableBytes(q *writeQueue) int32 {
wm := q.head()
ret := wm.stream.flow.available() // max we can write
if ret == 0 {
return 0
}
if int32(ws.maxFrameSize) < ret {
ret = int32(ws.maxFrameSize)
}
if ret == 0 {
panic("internal error: ws.maxFrameSize not initialized or invalid")
}
wd := wm.write.(*writeData)
if len(wd.p) < int(ret) {
ret = int32(len(wd.p))
}
return ret
}
func (ws *writeScheduler) takeFrom(id uint32, q *writeQueue) (wm frameWriteMsg, ok bool) {
wm = q.head()
// If the first item in this queue costs flow control tokens
// and we don't have enough, write as much as we can.
if wd, ok := wm.write.(*writeData); ok && len(wd.p) > 0 {
allowed := wm.stream.flow.available() // max we can write
if allowed == 0 {
// No quota available. Caller can try the next stream.
return frameWriteMsg{}, false
}
if int32(ws.maxFrameSize) < allowed {
allowed = int32(ws.maxFrameSize)
}
// TODO: further restrict the allowed size, because even if
// the peer says it's okay to write 16MB data frames, we might
// want to write smaller ones to properly weight competing
// streams' priorities.
if len(wd.p) > int(allowed) {
wm.stream.flow.take(allowed)
chunk := wd.p[:allowed]
wd.p = wd.p[allowed:]
// Make up a new write message of a valid size, rather
// than shifting one off the queue.
return frameWriteMsg{
stream: wm.stream,
write: &writeData{
streamID: wd.streamID,
p: chunk,
// even if the original had endStream set, there
// arebytes remaining because len(wd.p) > allowed,
// so we know endStream is false:
endStream: false,
},
// our caller is blocking on the final DATA frame, not
// these intermediates, so no need to wait:
done: nil,
}, true
}
wm.stream.flow.take(int32(len(wd.p)))
}
q.shift()
if q.empty() {
ws.putEmptyQueue(q)
delete(ws.sq, id)
}
return wm, true
}
func (ws *writeScheduler) forgetStream(id uint32) {
q, ok := ws.sq[id]
if !ok {
return
}
delete(ws.sq, id)
// But keep it for others later.
for i := range q.s {
q.s[i] = frameWriteMsg{}
}
q.s = q.s[:0]
ws.putEmptyQueue(q)
}
type writeQueue struct {
s []frameWriteMsg
}
// streamID returns the stream ID for a non-empty stream-specific queue.
func (q *writeQueue) streamID() uint32 { return q.s[0].stream.id }
func (q *writeQueue) empty() bool { return len(q.s) == 0 }
func (q *writeQueue) push(wm frameWriteMsg) {
q.s = append(q.s, wm)
}
// head returns the next item that would be removed by shift.
func (q *writeQueue) head() frameWriteMsg {
if len(q.s) == 0 {
panic("invalid use of queue")
}
return q.s[0]
}
func (q *writeQueue) shift() frameWriteMsg {
if len(q.s) == 0 {
panic("invalid use of queue")
}
wm := q.s[0]
// TODO: less copy-happy queue.
copy(q.s, q.s[1:])
q.s[len(q.s)-1] = frameWriteMsg{}
q.s = q.s[:len(q.s)-1]
return wm
}
func (q *writeQueue) firstIsNoCost() bool {
if df, ok := q.s[0].write.(*writeData); ok {
return len(df.p) == 0
}
return true
}