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mojo/public/go/bindings/async_waiter.go

-// Copyright 2015 The Chromium 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 bindings
-
-import (
-        "fmt"
-        "runtime"
-        "sync"
-        "sync/atomic"
-
-        "mojo/public/go/system"
-)
-
-var defaultWaiter *asyncWaiterImpl
-var once sync.Once
-
-// GetAsyncWaiter returns a default implementation of |AsyncWaiter| interface.
-func GetAsyncWaiter() AsyncWaiter {
-        once.Do(func() {
-                defaultWaiter = newAsyncWaiter()
-        })
-        return defaultWaiter
-}
-
-// AsyncWaitId is an id returned by |AsyncWait()| used to cancel it.
-type AsyncWaitId uint64
-
-// WaitResponse is a struct sent to a channel waiting for |AsyncWait()| to
-// finish. It contains the same information as if |Wait()| was called on a
-// handle.
-type WaitResponse struct {
-        Result system.MojoResult
-        State  system.MojoHandleSignalsState
-}
-
-// AsyncWaiter defines an interface for asynchronously waiting (and cancelling
-// asynchronous waits) on a handle.
-type AsyncWaiter interface {
-        // AsyncWait asynchronously waits on a given handle until a signal
-        // indicated by |signals| is satisfied or it becomes known that no
-        // signal indicated by |signals| will ever be satisified. The wait
-        // response will be sent to |responseChan|.
-        //
-        // |handle| must not be closed or transferred until the wait response
-        // is received from |responseChan|.
-        AsyncWait(handle system.Handle, signals system.MojoHandleSignals, responseChan chan<- WaitResponse) AsyncWaitId
-
-        // CancelWait cancels an outstanding async wait (specified by |id|)
-        // initiated by |AsyncWait()|. A response with Mojo result
-        // |MOJO_RESULT_ABORTED| is sent to the corresponding |responseChan|.
-        CancelWait(id AsyncWaitId)
-}
-
-// waitRequest is a struct sent to asyncWaiterWorker to add another handle to
-// the list of waiting handles.
-type waitRequest struct {
-        handle  system.Handle
-        signals system.MojoHandleSignals
-
-        // Used for |CancelWait()| calls. The worker should issue IDs so that
-        // you can't cancel the wait until the worker received the wait request.
-        idChan chan<- AsyncWaitId
-
-        // A channel end to send wait results.
-        responseChan chan<- WaitResponse
-}
-
-// asyncWaiterWorker does the actual work, in its own goroutine. It calls
-// |WaitMany()| on all provided handles. New handles a added via |waitChan|
-// and removed via |cancelChan| messages. To wake the worker asyncWaiterImpl
-// sends mojo messages to a dedicated message pipe, the other end of which has
-// index 0 in all slices of the worker.
-type asyncWaiterWorker struct {
-        // |handles| and |signals| are used to make |WaitMany()| calls directly.
-        // All these arrays should be operated simultaneously; i-th element
-        // of each refers to i-th handle.
-        handles      []system.Handle
-        signals      []system.MojoHandleSignals
-        asyncWaitIds []AsyncWaitId
-        responses    []chan<- WaitResponse
-
-        // Flag shared between waiterImpl and worker that is 1 iff the worker is
-        // already notified by waiterImpl. The worker sets it to 0 as soon as
-        // |WaitMany()| succeeds.
-        isNotified *int32
-        waitChan   <-chan waitRequest // should have a non-empty buffer
-        cancelChan <-chan AsyncWaitId // should have a non-empty buffer
-        ids        uint64             // is incremented each |AsyncWait()| call
-}
-
-// removeHandle removes handle at provided index without sending response by
-// swapping all information associated with index-th handle with the last one
-// and removing the last one.
-func (w *asyncWaiterWorker) removeHandle(index int) {
-        l := len(w.handles) - 1
-        // Swap with the last and remove last.
-        w.handles[index] = w.handles[l]
-        w.handles = w.handles[0:l]
-        w.signals[index] = w.signals[l]
-        w.signals = w.signals[0:l]
-
-        w.asyncWaitIds[index] = w.asyncWaitIds[l]
-        w.asyncWaitIds = w.asyncWaitIds[0:l]
-        w.responses[index] = w.responses[l]
-        w.responses = w.responses[0:l]
-}
-
-// sendWaitResponseAndRemove send response to corresponding channel and removes
-// index-th waiting handle.
-func (w *asyncWaiterWorker) sendWaitResponseAndRemove(index int, result system.MojoResult, state system.MojoHandleSignalsState) {
-        w.responses[index] <- WaitResponse{
-                result,
-                state,
-        }
-        w.removeHandle(index)
-}
-
-// respondToSatisfiedWaits responds to all wait requests that have at least
-// one satisfied signal and removes them.
-func (w *asyncWaiterWorker) respondToSatisfiedWaits(states []system.MojoHandleSignalsState) {
-        // Don't touch handle at index 0 as it is the waking handle.
-        for i := 1; i < len(states); {
-                if (states[i].SatisfiedSignals & w.signals[i]) != 0 {
-                        // Respond and swap i-th with last and remove last.
-                        w.sendWaitResponseAndRemove(i, system.MOJO_RESULT_OK, states[i])
-                        // Swap i-th with last and remove last.
-                        states[i] = states[len(states)-1]
-                        states = states[:len(states)-1]
-                } else {
-                        i++
-                }
-        }
-}
-
-// processIncomingRequests processes all queued async wait or cancel requests
-// sent by asyncWaiterImpl.
-func (w *asyncWaiterWorker) processIncomingRequests() {
-        for {
-                select {
-                case request := <-w.waitChan:
-                        w.handles = append(w.handles, request.handle)
-                        w.signals = append(w.signals, request.signals)
-                        w.responses = append(w.responses, request.responseChan)
-
-                        w.ids++
-                        id := AsyncWaitId(w.ids)
-                        w.asyncWaitIds = append(w.asyncWaitIds, id)
-                        request.idChan <- id
-                case AsyncWaitId := <-w.cancelChan:
-                        // Zero index is reserved for the waking message pipe handle.
-                        index := 0
-                        for i := 1; i < len(w.asyncWaitIds); i++ {
-                                if w.asyncWaitIds[i] == AsyncWaitId {
-                                        index = i
-                                        break
-                                }
-                        }
-                        // Do nothing if the id was not found as wait response may be
-                        // already sent if the async wait was successful.
-                        if index > 0 {
-                                w.sendWaitResponseAndRemove(index, system.MOJO_RESULT_ABORTED, system.MojoHandleSignalsState{})
-                        }
-                default:
-                        return
-                }
-        }
-}
-
-// runLoop run loop of the asyncWaiterWorker. Blocks on |WaitMany()|. If the
-// wait is interrupted by waking handle (index 0) then it means that the worker
-// was woken by waiterImpl, so the worker processes incoming requests from
-// waiterImpl; otherwise responses to corresponding wait request.
-func (w *asyncWaiterWorker) runLoop() {
-        for {
-                result, index, states := system.GetCore().WaitMany(w.handles, w.signals, system.MOJO_DEADLINE_INDEFINITE)
-                // Set flag to 0, so that the next incoming request to
-                // waiterImpl would explicitly wake worker by sending a message
-                // to waking message pipe.
-                atomic.StoreInt32(w.isNotified, 0)
-                if index == -1 {
-                        panic(fmt.Sprintf("error waiting on handles: %v", result))
-                        break
-                }
-                // Zero index means that the worker was signaled by asyncWaiterImpl.
-                if index == 0 {
-                        if result != system.MOJO_RESULT_OK {
-                                panic(fmt.Sprintf("error waiting on waking handle: %v", result))
-                        }
-                        w.handles[0].(system.MessagePipeHandle).ReadMessage(system.MOJO_READ_MESSAGE_FLAG_NONE)
-                        w.processIncomingRequests()
-                } else if result != system.MOJO_RESULT_OK {
-                        w.sendWaitResponseAndRemove(index, result, system.MojoHandleSignalsState{})
-                } else {
-                        w.respondToSatisfiedWaits(states)
-                }
-        }
-}
-
-// asyncWaiterImpl is an implementation of |AsyncWaiter| interface.
-// Runs a worker in a separate goroutine and comunicates with it by sending a
-// message to |wakingHandle| to wake worker from |WaitMany()| call and
-// sending request via |waitChan| and |cancelChan|.
-type asyncWaiterImpl struct {
-        wakingHandle system.MessagePipeHandle
-
-        // Flag shared between waiterImpl and worker that is 1 iff the worker is
-        // already notified by waiterImpl. The worker sets it to 0 as soon as
-        // |WaitMany()| succeeds.
-        isWorkerNotified *int32
-        waitChan         chan<- waitRequest // should have a non-empty buffer
-        cancelChan       chan<- AsyncWaitId // should have a non-empty buffer
-}
-
-func finalizeWorker(worker *asyncWaiterWorker) {
-        // Close waking handle on worker side.
-        worker.handles[0].Close()
-}
-
-func finalizeAsyncWaiter(waiter *asyncWaiterImpl) {
-        waiter.wakingHandle.Close()
-}
-
-// newAsyncWaiter creates an asyncWaiterImpl and starts its worker goroutine.
-func newAsyncWaiter() *asyncWaiterImpl {
-        result, h0, h1 := system.GetCore().CreateMessagePipe(nil)
-        if result != system.MOJO_RESULT_OK {
-                panic(fmt.Sprintf("can't create message pipe %v", result))
-        }
-        waitChan := make(chan waitRequest, 10)
-        cancelChan := make(chan AsyncWaitId, 10)
-        isNotified := new(int32)
-        worker := &asyncWaiterWorker{
-                []system.Handle{h1},
-                []system.MojoHandleSignals{system.MOJO_HANDLE_SIGNAL_READABLE},
-                []AsyncWaitId{0},
-                []chan<- WaitResponse{make(chan WaitResponse)},
-                isNotified,
-                waitChan,
-                cancelChan,
-                0,
-        }
-        runtime.SetFinalizer(worker, finalizeWorker)
-        go worker.runLoop()
-        waiter := &asyncWaiterImpl{
-                wakingHandle:     h0,
-                isWorkerNotified: isNotified,
-                waitChan:         waitChan,
-                cancelChan:       cancelChan,
-        }
-        runtime.SetFinalizer(waiter, finalizeAsyncWaiter)
-        return waiter
-}
-
-// wakeWorker wakes the worker from |WaitMany()| call. This should be called
-// after sending a message to |waitChan| or |cancelChan| to avoid deadlock.
-func (w *asyncWaiterImpl) wakeWorker() {
-        if atomic.CompareAndSwapInt32(w.isWorkerNotified, 0, 1) {
-                result := w.wakingHandle.WriteMessage([]byte{0}, nil, system.MOJO_WRITE_MESSAGE_FLAG_NONE)
-                if result != system.MOJO_RESULT_OK {
-                        panic("can't write to a message pipe")
-                }
-        }
-}
-
-func (w *asyncWaiterImpl) AsyncWait(handle system.Handle, signals system.MojoHandleSignals, responseChan chan<- WaitResponse) AsyncWaitId {
-        idChan := make(chan AsyncWaitId, 1)
-        w.waitChan <- waitRequest{
-                handle,
-                signals,
-                idChan,
-                responseChan,
-        }
-        w.wakeWorker()
-        return <-idChan
-}
-
-func (w *asyncWaiterImpl) CancelWait(id AsyncWaitId) {
-        w.cancelChan <- id
-        w.wakeWorker()
-}