Add Poll method to RingBuffer

Adds a Poll method like the one in Queue to do Gets with a timeout.

Author: tylertreat

queue/ring.go

@@ -19,6 +19,7 @@ package queue
 import (
 	"runtime"
 	"sync/atomic"
+	"time"
 )
 
 // roundUp takes a uint64 greater than 0 and rounds it up to the next
@@ -128,9 +129,23 @@ L:
 // to the queue or Dispose is called on the queue.  An error will be returned
 // if the queue is disposed.
 func (rb *RingBuffer) Get() (interface{}, error) {
-	var n *node
-	pos := atomic.LoadUint64(&rb.dequeue)
-	i := 0
+	return rb.Poll(0)
+}
+
+// Poll will return the next item in the queue.  This call will block
+// if the queue is empty.  This call will unblock when an item is added
+// to the queue, Dispose is called on the queue, or the timeout is reached. An
+// error will be returned if the queue is disposed or a timeout occurs.
+func (rb *RingBuffer) Poll(timeout time.Duration) (interface{}, error) {
+	var (
+		n     *node
+		pos   = atomic.LoadUint64(&rb.dequeue)
+		i     = 0
+		start time.Time
+	)
+	if timeout > 0 {
+		start = time.Now()
+	}
 L:
 	for {
 		if atomic.LoadUint64(&rb.disposed) == 1 {
@@ -150,6 +165,10 @@ L:
 			pos = atomic.LoadUint64(&rb.dequeue)
 		}
 
+		if timeout > 0 && time.Since(start) >= timeout {
+			return nil, ErrTimeout
+		}
+
 		if i == 10000 {
 			runtime.Gosched() // free up the cpu before the next iteration
 			i = 0

queue/ring_test.go

@@ -20,6 +20,7 @@ import (
 	"sync"
 	"sync/atomic"
 	"testing"
+	"time"
 
 	"github.com/stretchr/testify/assert"
 )
@@ -176,6 +177,54 @@ func TestRingGetEmpty(t *testing.T) {
 	wg.Wait()
 }
 
+func TestRingPollEmpty(t *testing.T) {
+	rb := NewRingBuffer(3)
+
+	_, err := rb.Poll(1)
+	assert.Equal(t, ErrTimeout, err)
+}
+
+func TestRingPoll(t *testing.T) {
+	rb := NewRingBuffer(10)
+
+	// should be able to Poll() before anything is present, without breaking future Puts
+	rb.Poll(time.Millisecond)
+
+	rb.Put(`test`)
+	result, err := rb.Poll(0)
+	if !assert.Nil(t, err) {
+		return
+	}
+
+	assert.Equal(t, `test`, result)
+	assert.Equal(t, uint64(0), rb.Len())
+
+	rb.Put(`1`)
+	rb.Put(`2`)
+
+	result, err = rb.Poll(time.Millisecond)
+	if !assert.Nil(t, err) {
+		return
+	}
+
+	assert.Equal(t, `1`, result)
+	assert.Equal(t, uint64(1), rb.Len())
+
+	result, err = rb.Poll(time.Millisecond)
+	if !assert.Nil(t, err) {
+		return
+	}
+
+	assert.Equal(t, `2`, result)
+
+	before := time.Now()
+	_, err = rb.Poll(5 * time.Millisecond)
+	// This delta is normally 1-3 ms but running tests in CI with -race causes
+	// this to run much slower. For now, just bump up the threshold.
+	assert.InDelta(t, 5, time.Since(before).Seconds()*1000, 10)
+	assert.Equal(t, ErrTimeout, err)
+}
+
 func TestRingLen(t *testing.T) {
 	rb := NewRingBuffer(4)
 	assert.Equal(t, uint64(0), rb.Len())