251 lines
7.4 KiB
Go
251 lines
7.4 KiB
Go
// Copyright 2018 The gVisor Authors.
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//
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// Licensed under the Apache License, Version 2.0 (the "License");
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// you may not use this file except in compliance with the License.
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// You may obtain a copy of the License at
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//
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// http://www.apache.org/licenses/LICENSE-2.0
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//
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// Unless required by applicable law or agreed to in writing, software
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// distributed under the License is distributed on an "AS IS" BASIS,
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// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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// See the License for the specific language governing permissions and
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// limitations under the License.
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// Package waiter provides the implementation of a wait queue, where waiters can
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// be enqueued to be notified when an event of interest happens.
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//
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// Becoming readable and/or writable are examples of events. Waiters are
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// expected to use a pattern similar to this to make a blocking function out of
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// a non-blocking one:
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//
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// func (o *object) blockingRead(...) error {
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// err := o.nonBlockingRead(...)
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// if err != ErrAgain {
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// // Completed with no need to wait!
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// return err
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// }
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//
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// e := createOrGetWaiterEntry(...)
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// o.EventRegister(&e, waiter.EventIn)
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// defer o.EventUnregister(&e)
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//
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// // We need to try to read again after registration because the
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// // object may have become readable between the last attempt to
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// // read and read registration.
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// err = o.nonBlockingRead(...)
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// for err == ErrAgain {
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// wait()
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// err = o.nonBlockingRead(...)
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// }
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//
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// return err
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// }
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//
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// Another goroutine needs to notify waiters when events happen. For example:
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//
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// func (o *object) Write(...) ... {
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// // Do write work.
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// [...]
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//
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// if oldDataAvailableSize == 0 && dataAvailableSize > 0 {
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// // If no data was available and now some data is
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// // available, the object became readable, so notify
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// // potential waiters about this.
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// o.Notify(waiter.EventIn)
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// }
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// }
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package waiter
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import (
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"sync"
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)
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// EventMask represents io events as used in the poll() syscall.
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type EventMask uint16
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// Events that waiters can wait on. The meaning is the same as those in the
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// poll() syscall.
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const (
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EventIn EventMask = 0x01 // POLLIN
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EventPri EventMask = 0x02 // POLLPRI
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EventOut EventMask = 0x04 // POLLOUT
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EventErr EventMask = 0x08 // POLLERR
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EventHUp EventMask = 0x10 // POLLHUP
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allEvents EventMask = 0x1f
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)
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// EventMaskFromLinux returns an EventMask representing the supported events
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// from the Linux events e, which is in the format used by poll(2).
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func EventMaskFromLinux(e uint32) EventMask {
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// Our flag definitions are currently identical to Linux.
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return EventMask(e) & allEvents
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}
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// ToLinux returns e in the format used by Linux poll(2).
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func (e EventMask) ToLinux() uint32 {
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// Our flag definitions are currently identical to Linux.
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return uint32(e)
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}
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// Waitable contains the methods that need to be implemented by waitable
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// objects.
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type Waitable interface {
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// Readiness returns what the object is currently ready for. If it's
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// not ready for a desired purpose, the caller may use EventRegister and
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// EventUnregister to get notifications once the object becomes ready.
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//
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// Implementations should allow for events like EventHUp and EventErr
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// to be returned regardless of whether they are in the input EventMask.
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Readiness(mask EventMask) EventMask
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// EventRegister registers the given waiter entry to receive
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// notifications when an event occurs that makes the object ready for
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// at least one of the events in mask.
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EventRegister(e *Entry, mask EventMask)
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// EventUnregister unregisters a waiter entry previously registered with
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// EventRegister().
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EventUnregister(e *Entry)
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}
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// EntryCallback provides a notify callback.
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type EntryCallback interface {
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// Callback is the function to be called when the waiter entry is
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// notified. It is responsible for doing whatever is needed to wake up
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// the waiter.
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//
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// The callback is supposed to perform minimal work, and cannot call
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// any method on the queue itself because it will be locked while the
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// callback is running.
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Callback(e *Entry)
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}
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// Entry represents a waiter that can be add to the a wait queue. It can
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// only be in one queue at a time, and is added "intrusively" to the queue with
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// no extra memory allocations.
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//
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// +stateify savable
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type Entry struct {
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// Context stores any state the waiter may wish to store in the entry
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// itself, which may be used at wake up time.
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//
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// Note that use of this field is optional and state may alternatively be
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// stored in the callback itself.
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Context interface{}
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Callback EntryCallback
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// The following fields are protected by the queue lock.
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mask EventMask
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waiterEntry
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}
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type channelCallback struct{}
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// Callback implements EntryCallback.Callback.
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func (*channelCallback) Callback(e *Entry) {
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ch := e.Context.(chan struct{})
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select {
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case ch <- struct{}{}:
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default:
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}
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}
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// NewChannelEntry initializes a new Entry that does a non-blocking write to a
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// struct{} channel when the callback is called. It returns the new Entry
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// instance and the channel being used.
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//
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// If a channel isn't specified (i.e., if "c" is nil), then NewChannelEntry
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// allocates a new channel.
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func NewChannelEntry(c chan struct{}) (Entry, chan struct{}) {
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if c == nil {
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c = make(chan struct{}, 1)
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}
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return Entry{Context: c, Callback: &channelCallback{}}, c
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}
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// Queue represents the wait queue where waiters can be added and
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// notifiers can notify them when events happen.
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//
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// The zero value for waiter.Queue is an empty queue ready for use.
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//
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// +stateify savable
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type Queue struct {
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list waiterList `state:"zerovalue"`
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mu sync.RWMutex `state:"nosave"`
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}
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// EventRegister adds a waiter to the wait queue; the waiter will be notified
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// when at least one of the events specified in mask happens.
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func (q *Queue) EventRegister(e *Entry, mask EventMask) {
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q.mu.Lock()
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e.mask = mask
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q.list.PushBack(e)
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q.mu.Unlock()
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}
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// EventUnregister removes the given waiter entry from the wait queue.
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func (q *Queue) EventUnregister(e *Entry) {
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q.mu.Lock()
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q.list.Remove(e)
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q.mu.Unlock()
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}
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// Notify notifies all waiters in the queue whose masks have at least one bit
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// in common with the notification mask.
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func (q *Queue) Notify(mask EventMask) {
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q.mu.RLock()
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for e := q.list.Front(); e != nil; e = e.Next() {
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if mask&e.mask != 0 {
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e.Callback.Callback(e)
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}
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}
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q.mu.RUnlock()
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}
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// Events returns the set of events being waited on. It is the union of the
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// masks of all registered entries.
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func (q *Queue) Events() EventMask {
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ret := EventMask(0)
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q.mu.RLock()
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for e := q.list.Front(); e != nil; e = e.Next() {
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ret |= e.mask
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}
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q.mu.RUnlock()
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return ret
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}
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// IsEmpty returns if the wait queue is empty or not.
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func (q *Queue) IsEmpty() bool {
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q.mu.Lock()
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defer q.mu.Unlock()
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return q.list.Front() == nil
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}
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// AlwaysReady implements the Waitable interface but is always ready. Embedding
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// this struct into another struct makes it implement the boilerplate empty
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// functions automatically.
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type AlwaysReady struct {
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}
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// Readiness always returns the input mask because this object is always ready.
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func (*AlwaysReady) Readiness(mask EventMask) EventMask {
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return mask
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}
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// EventRegister doesn't do anything because this object doesn't need to issue
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// notifications because its readiness never changes.
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func (*AlwaysReady) EventRegister(*Entry, EventMask) {
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}
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// EventUnregister doesn't do anything because this object doesn't need to issue
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// notifications because its readiness never changes.
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func (*AlwaysReady) EventUnregister(e *Entry) {
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}
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