AudioScheduledSourceNode can return tail_time false when not scheduled

The render thread will not garbage collect them as long as the control
handle is kept. Once the control handle is dropped, and the node is
still not scheduled to start (start_at != f64::MAX) we can safely drop
the node from the audio graph.

Fixes #462

Author: orottier

src/node/audio_buffer_source.rs

@@ -422,14 +422,18 @@ impl AudioProcessor for AudioBufferSourceRenderer {
         // Return early if start_time is beyond this block
         if self.start_time >= next_block_time {
             output.make_silent();
-            return true;
+            // #462 AudioScheduledSourceNodes that have not been scheduled to start can safely
+            // return tail_time false in order to be collected if their control handle drops.
+            return self.start_time != f64::MAX;
         }
 
         // If the buffer has not been set wait for it.
         let buffer = match &self.buffer {
             None => {
                 output.make_silent();
-                return true;
+                // #462 like the above arm, we can safely return tail_time false if this node has
+                // no buffer set.
+                return false;
             }
             Some(b) => b,
         };

src/node/constant_source.rs

@@ -192,7 +192,9 @@ impl AudioProcessor for ConstantSourceRenderer {
 
         if self.start_time >= next_block_time {
             output.make_silent();
-            return true;
+            // #462 AudioScheduledSourceNodes that have not been scheduled to start can safely
+            // return tail_time false in order to be collected if their control handle drops.
+            return self.start_time != f64::MAX;
         }
 
         output.force_mono();
@@ -314,6 +316,23 @@ mod tests {
         assert_float_eq!(channel[128..], vec![1.; 128][..], abs_all <= 0.);
     }
 
+    #[test]
+    fn test_start_in_the_future_while_dropped() {
+        let sample_rate = 48000.;
+        let mut context = OfflineAudioContext::new(1, 4 * 128, sample_rate);
+
+        let mut src = context.create_constant_source();
+        src.connect(&context.destination());
+        src.start_at(258. / sample_rate as f64); // in 3rd block
+        drop(src); // explicit drop
+
+        let buffer = context.start_rendering_sync();
+        let channel = buffer.get_channel_data(0);
+
+        assert_float_eq!(channel[0..258], vec![0.; 258][..], abs_all <= 0.);
+        assert_float_eq!(channel[258..], vec![1.; 254][..], abs_all <= 0.);
+    }
+
     #[test]
     #[should_panic]
     fn test_start_twice() {

src/node/oscillator.rs

@@ -373,7 +373,9 @@ impl AudioProcessor for OscillatorRenderer {
 
         if self.start_time >= next_block_time {
             output.make_silent();
-            return true;
+            // #462 AudioScheduledSourceNodes that have not been scheduled to start can safely
+            // return tail_time false in order to be collected if their control handle drops.
+            return self.start_time != f64::MAX;
         } else if self.stop_time < scope.current_time {
             output.make_silent();