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|
package loggerutils
import (
"context"
"sync"
"github.com/docker/docker/daemon/logger"
"github.com/pkg/errors"
)
// MessageQueue is a queue for log messages.
//
// [MessageQueue.Enqueue] will block when the queue is full.
// To dequeue messages call [MessageQueue.Receiver] and pull messages off the
// returned channel.
//
// Closing only prevents new messages from being added to the queue.
// The queue can still be drained after close.
//
// The zero value of MessageQueue is safe to use, but does not do any internal
// buffering (queue size is 0).
type MessageQueue struct {
maxSize int
mu sync.Mutex
closing bool
closed chan struct{}
// Blocks multiple calls to [MessageQueue.Close] until the queue is actually closed
closeWait chan struct{}
// We need to be able to safely close the send channel so that [MessageQueue.Dequeue]
// can drain the queue without blocking.
// This cond var helps deal with that.
cond *sync.Cond
sendWaiters int
ch chan *logger.Message
}
// NewMessageQueue creates a new queue with the specified size.
func NewMessageQueue(maxSize int) *MessageQueue {
var q MessageQueue
q.maxSize = maxSize
q.init()
return &q
}
func (q *MessageQueue) init() {
if q.cond == nil {
q.cond = sync.NewCond(&q.mu)
}
if q.ch == nil {
q.ch = make(chan *logger.Message, q.maxSize)
}
if q.closed == nil {
q.closed = make(chan struct{})
}
if q.closeWait == nil {
q.closeWait = make(chan struct{})
}
}
var ErrQueueClosed = errors.New("queue is closed")
// Enqueue adds the provided message to the queue.
// Enqueue blocks if the queue is full.
//
// The two possible error cases are:
// 1. The provided context is cancelled
// 2. [ErrQueueClosed] when the queue has been closed.
func (q *MessageQueue) Enqueue(ctx context.Context, m *logger.Message) error {
q.mu.Lock()
q.init()
// Increment the waiter count
// This prevents the send channel from being closed while we are trying to send.
q.sendWaiters++
q.mu.Unlock()
defer func() {
q.mu.Lock()
// Decrement the waiter count and signal to any potential closer to check
// the wait count again.
// Only bother signaling if this is the last waiter.
q.sendWaiters--
if q.sendWaiters == 0 {
q.cond.Signal()
}
q.mu.Unlock()
}()
// Before trying to send on the channel, check if we care closed.
select {
case <-ctx.Done():
return ctx.Err()
case <-q.closed:
return ErrQueueClosed
default:
}
select {
case <-ctx.Done():
return ctx.Err()
case <-q.closed:
return ErrQueueClosed
case q.ch <- m:
return nil
}
}
// Close prevents any new messages from being added to the queue.
func (q *MessageQueue) Close() {
q.mu.Lock()
q.init()
if q.closing {
// unlock the mutex here so that the goroutine waiting on the cond var can
// take the lock when signaled.
q.mu.Unlock()
<-q.closeWait
return
}
defer q.mu.Unlock()
// Prevent multiple Close calls from trying to close things.
q.closing = true
close(q.closed)
// Wait for any senders to finish
// Because we closed the channel above, this shouldn't block for a long period.
for q.sendWaiters > 0 {
q.cond.Wait()
}
close(q.ch)
close(q.closeWait)
}
// Receiver returns a channel that can be used to dequeue messages
// The channel will be closed when the message queue is closed but may have
// messages buffered.
func (q *MessageQueue) Receiver() <-chan *logger.Message {
q.mu.Lock()
defer q.mu.Unlock()
q.init()
return q.ch
}
|
