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moby--moby/vendor/github.com/docker/go-events/queue.go

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package events
import (
"container/list"
"sync"
"github.com/sirupsen/logrus"
)
// Queue accepts all messages into a queue for asynchronous consumption
// by a sink. It is unbounded and thread safe but the sink must be reliable or
// events will be dropped.
type Queue struct {
dst Sink
events *list.List
cond *sync.Cond
mu sync.Mutex
closed bool
}
// NewQueue returns a queue to the provided Sink dst.
func NewQueue(dst Sink) *Queue {
eq := Queue{
dst: dst,
events: list.New(),
}
eq.cond = sync.NewCond(&eq.mu)
go eq.run()
return &eq
}
// Write accepts the events into the queue, only failing if the queue has
// been closed.
func (eq *Queue) Write(event Event) error {
eq.mu.Lock()
defer eq.mu.Unlock()
if eq.closed {
return ErrSinkClosed
}
eq.events.PushBack(event)
eq.cond.Signal() // signal waiters
return nil
}
// Close shutsdown the event queue, flushing
func (eq *Queue) Close() error {
eq.mu.Lock()
defer eq.mu.Unlock()
if eq.closed {
return nil
}
// set closed flag
eq.closed = true
eq.cond.Signal() // signal flushes queue
eq.cond.Wait() // wait for signal from last flush
return eq.dst.Close()
}
// run is the main goroutine to flush events to the target sink.
func (eq *Queue) run() {
for {
event := eq.next()
if event == nil {
return // nil block means event queue is closed.
}
if err := eq.dst.Write(event); err != nil {
// TODO(aaronl): Dropping events could be bad depending
// on the application. We should have a way of
// communicating this condition. However, logging
// at a log level above debug may not be appropriate.
// Eventually, go-events should not use logrus at all,
// and should bubble up conditions like this through
// error values.
logrus.WithFields(logrus.Fields{
"event": event,
"sink": eq.dst,
}).WithError(err).Debug("eventqueue: dropped event")
}
}
}
// next encompasses the critical section of the run loop. When the queue is
// empty, it will block on the condition. If new data arrives, it will wake
// and return a block. When closed, a nil slice will be returned.
func (eq *Queue) next() Event {
eq.mu.Lock()
defer eq.mu.Unlock()
for eq.events.Len() < 1 {
if eq.closed {
eq.cond.Broadcast()
return nil
}
eq.cond.Wait()
}
front := eq.events.Front()
block := front.Value.(Event)
eq.events.Remove(front)
return block
}