Repository: pion/interceptor (go)
Issue: Priority queue error
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### Your environment.
  - Version: v0.1.28

### What did you do?
	jitterBuffer := jitterbuffer.New(jitterbuffer.WithMinimumPacketCount(16))
	jbEvent := make(chan struct{})
	eventFunc := func(event jitterbuffer.Event, jb *jitterbuffer.JitterBuffer) {
		jbEvent <- struct{}{}
	}
	jitterBuffer.Listen(jitterbuffer.BeginPlayback, eventFunc)

	go func() {
		for {
			select {
			case <-jbEvent:
				{
					packet, _ := jitterBuffer.Pop()
                                        ...
				}
                        ...
			}
		}
	}()

	go func() {
		for {
			packet, _, err := remoteTrack.ReadRTP()
                        ...
			jitterBuffer.Push(packet)
		}
	}()

### What did you expect?
It seemed to me that I would receive `jitterbuffer.BeginPlayback` event whenever the JitterBuffer packet count became 16.

### What happened?
`func (q *PriorityQueue) PopAt(sqNum uint16)` and `func (q *PriorityQueue) PopAtTimestamp(timestamp uint32)` don't reduce the PriorityQueue length.

It would also be great to have an example of using JitterBuffer in practice.

Pull request description: JitterBuffer: Fix queue not properly decrementing packet count in some instances
#### Description
This fixes a few issues with the priority queue not decrementing its count and adds some more test coverage.

#### Reference issue
Fixes #248 


Patch:
diff --git a/pkg/jitterbuffer/priority_queue.go b/pkg/jitterbuffer/priority_queue.go
index 366ff10..50e70c8 100644
--- a/pkg/jitterbuffer/priority_queue.go
+++ b/pkg/jitterbuffer/priority_queue.go
@@ -127,6 +127,7 @@ func (q *PriorityQueue) PopAt(sqNum uint16) (*rtp.Packet, error) {
 	if q.next.priority == sqNum {
 		val := q.next.val
 		q.next = q.next.next
+		q.length--
 		return val, nil
 	}
 	pos := q.next
@@ -138,6 +139,7 @@ func (q *PriorityQueue) PopAt(sqNum uint16) (*rtp.Packet, error) {
 			if prev.next != nil {
 				prev.next.prev = prev
 			}
+			q.length--
 			return val, nil
 		}
 		prev = pos
@@ -155,6 +157,7 @@ func (q *PriorityQueue) PopAtTimestamp(timestamp uint32) (*rtp.Packet, error) {
 	if q.next.val.Timestamp == timestamp {
 		val := q.next.val
 		q.next = q.next.next
+		q.length--
 		return val, nil
 	}
 	pos := q.next
@@ -166,6 +169,7 @@ func (q *PriorityQueue) PopAtTimestamp(timestamp uint32) (*rtp.Packet, error) {
 			if prev.next != nil {
 				prev.next.prev = prev
 			}
+			q.length--
 			return val, nil
 		}
 		prev = pos


Test patch:
diff --git a/pkg/jitterbuffer/jitter_buffer_test.go b/pkg/jitterbuffer/jitter_buffer_test.go
index 0ed7302..253d3e7 100644
--- a/pkg/jitterbuffer/jitter_buffer_test.go
+++ b/pkg/jitterbuffer/jitter_buffer_test.go
@@ -45,6 +45,10 @@ func TestJitterBuffer(t *testing.T) {
 	})
 	t.Run("Appends packets and begins playout", func(*testing.T) {
 		jb := New(WithMinimumPacketCount(1))
+		events := make([]Event, 0)
+		jb.Listen(BeginPlayback, func(event Event, _ *JitterBuffer) {
+			events = append(events, event)
+		})
 		for i := 0; i < 2; i++ {
 			jb.Push(&rtp.Packet{Header: rtp.Header{SequenceNumber: uint16(5012 + i), Timestamp: uint32(512 + i)}, Payload: []byte{0x02}})
 		}
@@ -54,6 +58,8 @@ func TestJitterBuffer(t *testing.T) {
 		head, err := jb.Pop()
 		assert.Equal(head.SequenceNumber, uint16(5012))
 		assert.Equal(err, nil)
+		assert.Equal(1, len(events))
+		assert.Equal(Event(BeginPlayback), events[0])
 	})
 
 	t.Run("Wraps playout correctly", func(*testing.T) {
diff --git a/pkg/jitterbuffer/priority_queue_test.go b/pkg/jitterbuffer/priority_queue_test.go
index 44d0e01..d28cc72 100644
--- a/pkg/jitterbuffer/priority_queue_test.go
+++ b/pkg/jitterbuffer/priority_queue_test.go
@@ -81,6 +81,26 @@ func TestPriorityQueue(t *testing.T) {
 		assert.Equal(pkt.SequenceNumber, uint16(5012))
 		assert.Equal(err, nil)
 	})
+
+	t.Run("Updates the length when PopAt* are called", func(*testing.T) {
+		pkt := &rtp.Packet{Header: rtp.Header{SequenceNumber: 5000, Timestamp: 500}, Payload: []byte{0x02}}
+		q := NewQueue()
+		q.Push(pkt, pkt.SequenceNumber)
+		pkt2 := &rtp.Packet{Header: rtp.Header{SequenceNumber: 5004, Timestamp: 500}, Payload: []byte{0x02}}
+		q.Push(pkt2, pkt2.SequenceNumber)
+		for i := 0; i < 100; i++ {
+			q.Push(&rtp.Packet{Header: rtp.Header{SequenceNumber: uint16(5012 + i), Timestamp: uint32(512 + i)}, Payload: []byte{0x02}}, uint16(5012+i))
+		}
+		assert.Equal(uint16(102), q.Length())
+		popped, _ := q.PopAt(uint16(5012))
+		assert.Equal(popped.SequenceNumber, uint16(5012))
+		assert.Equal(uint16(101), q.Length())
+
+		popped, err := q.PopAtTimestamp(uint32(500))
+		assert.Equal(popped.SequenceNumber, uint16(5000))
+		assert.Equal(uint16(100), q.Length())
+		assert.Equal(err, nil)
+	})
 }
 
 func TestPriorityQueue_Find(t *testing.T) {