Fix Sink playback after stop (#464)

Addressing issues #462 and #315 and #171

Given how the sink's queue is drained on stop I made some modifications in sink.append(). I added a check to ensure the sink was flushed completely (not sure this is necessary? I think it is, to avoid race conditions) and to restart the sink on append (when stopped.)

I added a test demonstrating correctness, as well as performing some trials on my own.

src/sink.rs

@@ -65,6 +65,14 @@ impl Sink {
         f32: FromSample<S::Item>,
         S::Item: Sample + Send,
     {
+        // Wait for queue to flush then resume stopped playback
+        if self.controls.stopped.load(Ordering::SeqCst) {
+            if self.sound_count.load(Ordering::SeqCst) > 0 {
+                self.sleep_until_end();
+            }
+            self.controls.stopped.store(false, Ordering::SeqCst);
+        }
+
         let controls = self.controls.clone();
 
         let source = source
@@ -201,6 +209,7 @@ impl Drop for Sink {
 mod tests {
     use crate::buffer::SamplesBuffer;
     use crate::{Sink, Source};
+    use std::sync::atomic::Ordering;
 
     #[test]
     fn test_pause_and_stop() {
@@ -233,6 +242,34 @@ mod tests {
         assert_eq!(sink.empty(), true);
     }
 
+    #[test]
+    fn test_stop_and_start() {
+        let (sink, mut queue_rx) = Sink::new_idle();
+
+        let v = vec![10i16, -10, 20, -20, 30, -30];
+
+        sink.append(SamplesBuffer::new(1, 1, v.clone()));
+        let mut src = SamplesBuffer::new(1, 1, v.clone()).convert_samples();
+
+        assert_eq!(queue_rx.next(), src.next());
+        assert_eq!(queue_rx.next(), src.next());
+
+        sink.stop();
+
+        assert!(sink.controls.stopped.load(Ordering::SeqCst));
+        assert_eq!(queue_rx.next(), Some(0.0));
+
+        src = SamplesBuffer::new(1, 1, v.clone()).convert_samples();
+        sink.append(SamplesBuffer::new(1, 1, v));
+
+        assert!(!sink.controls.stopped.load(Ordering::SeqCst));
+        // Flush silence
+        let mut queue_rx = queue_rx.skip_while(|v| *v == 0.0);
+
+        assert_eq!(queue_rx.next(), src.next());
+        assert_eq!(queue_rx.next(), src.next());
+    }
+
     #[test]
     fn test_volume() {
         let (sink, mut queue_rx) = Sink::new_idle();