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// Copyright 2010 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 textproto
import (
"sync"
)
// A Pipeline manages a pipelined in-order request/response sequence.
//
// To use a Pipeline p to manage multiple clients on a connection,
// each client should run:
//
// id := p.Next() // take a number
//
// p.StartRequest(id) // wait for turn to send request
// «send request»
// p.EndRequest(id) // notify Pipeline that request is sent
//
// p.StartResponse(id) // wait for turn to read response
// «read response»
// p.EndResponse(id) // notify Pipeline that response is read
//
// A pipelined server can use the same calls to ensure that
// responses computed in parallel are written in the correct order.
type Pipeline struct {
mu sync.Mutex
id uint
request sequencer
response sequencer
}
// Next returns the next id for a request/response pair.
func (p *Pipeline) Next() uint {
p.mu.Lock()
id := p.id
p.id++
p.mu.Unlock()
return id
}
// StartRequest blocks until it is time to send (or, if this is a server, receive)
// the request with the given id.
func (p *Pipeline) StartRequest(id uint) {
p.request.Start(id)
}
// EndRequest notifies p that the request with the given id has been sent
// (or, if this is a server, received).
func (p *Pipeline) EndRequest(id uint) {
p.request.End(id)
}
// StartResponse blocks until it is time to receive (or, if this is a server, send)
// the request with the given id.
func (p *Pipeline) StartResponse(id uint) {
p.response.Start(id)
}
// EndResponse notifies p that the response with the given id has been received
// (or, if this is a server, sent).
func (p *Pipeline) EndResponse(id uint) {
p.response.End(id)
}
// A sequencer schedules a sequence of numbered events that must
// happen in order, one after the other. The event numbering must start
// at 0 and increment without skipping. The event number wraps around
// safely as long as there are not 2^32 simultaneous events pending.
type sequencer struct {
mu sync.Mutex
id uint
wait map[uint]chan struct{}
}
// Start waits until it is time for the event numbered id to begin.
// That is, except for the first event, it waits until End(id-1) has
// been called.
func (s *sequencer) Start(id uint) {
s.mu.Lock()
if s.id == id {
s.mu.Unlock()
return
}
c := make(chan struct{})
if s.wait == nil {
s.wait = make(map[uint]chan struct{})
}
s.wait[id] = c
s.mu.Unlock()
<-c
}
// End notifies the sequencer that the event numbered id has completed,
// allowing it to schedule the event numbered id+1. It is a run-time error
// to call End with an id that is not the number of the active event.
func (s *sequencer) End(id uint) {
s.mu.Lock()
if s.id != id {
s.mu.Unlock()
panic("out of sync")
}
id++
s.id = id
if s.wait == nil {
s.wait = make(map[uint]chan struct{})
}
c, ok := s.wait[id]
if ok {
delete(s.wait, id)
}
s.mu.Unlock()
if ok {
close(c)
}
}
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