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package queue
import (
"errors"
"sync"
)
var (
ErrQueueClosed = errors.New("the queue is closed for reading and writing")
ErrQueueEmpty = errors.New("the queue is empty")
)
// MessageQueue represents a threadsafe message queue to be used to retrieve or
// write messages to.
type MessageQueue struct {
m *sync.RWMutex
c *sync.Cond
messages []interface{}
closed bool
}
// NewMessageQueue returns a new MessageQueue.
func NewMessageQueue() *MessageQueue {
m := &sync.RWMutex{}
return &MessageQueue{
m: m,
c: sync.NewCond(m),
messages: []interface{}{},
}
}
// Write writes `msg` to the queue.
func (mq *MessageQueue) Write(msg interface{}) error {
mq.m.Lock()
defer mq.m.Unlock()
if mq.closed {
return ErrQueueClosed
}
mq.messages = append(mq.messages, msg)
// Signal a waiter that there is now a value available in the queue.
mq.c.Signal()
return nil
}
// Read will read a value from the queue if available, otherwise return an error.
func (mq *MessageQueue) Read() (interface{}, error) {
mq.m.Lock()
defer mq.m.Unlock()
if mq.closed {
return nil, ErrQueueClosed
}
if mq.isEmpty() {
return nil, ErrQueueEmpty
}
val := mq.messages[0]
mq.messages[0] = nil
mq.messages = mq.messages[1:]
return val, nil
}
// ReadOrWait will read a value from the queue if available, else it will wait for a
// value to become available. This will block forever if nothing gets written or until
// the queue gets closed.
func (mq *MessageQueue) ReadOrWait() (interface{}, error) {
mq.m.Lock()
if mq.closed {
mq.m.Unlock()
return nil, ErrQueueClosed
}
if mq.isEmpty() {
for !mq.closed && mq.isEmpty() {
mq.c.Wait()
}
mq.m.Unlock()
return mq.Read()
}
val := mq.messages[0]
mq.messages[0] = nil
mq.messages = mq.messages[1:]
mq.m.Unlock()
return val, nil
}
// IsEmpty returns if the queue is empty
func (mq *MessageQueue) IsEmpty() bool {
mq.m.RLock()
defer mq.m.RUnlock()
return len(mq.messages) == 0
}
// Nonexported empty check that doesn't lock so we can call this in Read and Write.
func (mq *MessageQueue) isEmpty() bool {
return len(mq.messages) == 0
}
// Close closes the queue for future writes or reads. Any attempts to read or write from the
// queue after close will return ErrQueueClosed. This is safe to call multiple times.
func (mq *MessageQueue) Close() {
mq.m.Lock()
defer mq.m.Unlock()
// Already closed
if mq.closed {
return
}
mq.messages = nil
mq.closed = true
// If there's anybody currently waiting on a value from ReadOrWait, we need to
// broadcast so the read(s) can return ErrQueueClosed.
mq.c.Broadcast()
}
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