AudioBufferSourceNode: fix panic when duration exceeds buffer len

[orottier]

This fixes the last remaining panic occuring in the wpt suite:

  wpt/webaudio/the-audio-api/the-audiobuffersourcenode-interface/audiobuffersource-start.html

  √ # AUDIT TASK RUNNER STARTED.
  √ > [Tests AudioBufferSourceNode start()]
Panic occurred in Audio Processor: 'index out of bounds: the len is 8 but the index is 8'. Removing node from graph.
  √   Case 0: start(when): implicitly play whole buffer from beginning to end is identical to the array [0,1,2,3,4,5,6,7,0,0,0,0,0,0,0,0...].
  √   Case 0: start(when): implicitly play whole buffer from beginning to end: tail contains only the constant 0.
  √   Case 1: start(when, 0): play whole buffer from beginning to end explicitly giving offset of 0 is identical to the array [0,1,2,3,4,5,6,7,0,0,0,0,0,0,0,0...].
  √   Case 1: start(when, 0): play whole buffer from beginning to end explicitly giving offset of 0: tail contains only the constant 0.
  √   Case 2: start(when, 0, 8_frames): play whole buffer from beginning to end explicitly giving offset of 0 and duration of 8 frames is identical to the array [0,1,2,3,4,5,6,7,0,0,0,0,0,0,0,0...].
  √   Case 2: start(when, 0, 8_frames): play whole buffer from beginning to end explicitly giving offset of 0 and duration of 8 frames: tail contains only the constant 0.
  √   Case 3: start(when, 4_frames): play with explicit non-zero offset is identical to the array [4,5,6,7,0,0,0,0,0,0,0,0,0,0,0,0...].
  √   Case 3: start(when, 4_frames): play with explicit non-zero offset: tail contains only the constant 0.
  √   Case 4: start(when, 4_frames, 4_frames): play with explicit non-zero offset and duration is identical to the array [4,5,6,7,0,0,0,0,0,0,0,0,0,0,0,0...].
  √   Case 4: start(when, 4_frames, 4_frames): play with explicit non-zero offset and duration: tail contains only the constant 0.
  √   Case 5: start(when, 7_frames): play with explicit non-zero offset near end of buffer is identical to the array [7,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0...].
  √   Case 5: start(when, 7_frames): play with explicit non-zero offset near end of buffer: tail contains only the constant 0.
  √   Case 6: start(when, 8_frames): play with explicit offset at end of buffer is identical to the array [0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0...].
  √   Case 6: start(when, 8_frames): play with explicit offset at end of buffer: tail contains only the constant 0.
  √   Case 7: start(when, 9_frames): play with explicit offset past end of buffer is identical to the array [0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0...].
  √   Case 7: start(when, 9_frames): play with explicit offset past end of buffer: tail contains only the constant 0.
  × X Case 8: start(when, 0, 15_frames): play with whole buffer, with long duration (clipped) expected to be equal to the array [0,1,2,3,4,5,6,7,0,0,0,0,0,0,0,0...] but differs in 7 places:
  	Index	Actual			Expected
  	[1]	0.0000000000000000e+0	1.0000000000000000e+0
  	[2]	0.0000000000000000e+0	2.0000000000000000e+0
  	[3]	0.0000000000000000e+0	3.0000000000000000e+0
  	[4]	0.0000000000000000e+0	4.0000000000000000e+0
  	...and 3 more errors.

It took me a while to hunt this one down and somehow I did not succeed to write a regression test :( I just could not reproduce the bug and did not feel to port the full wpt test to rust..

[b-ma]

Hey, nice catch!

I managed to create a failing test for that:

    #[test]
    // regression test for #452
    // - fast track
    // - duration not set so `self.duration` is `f64::MAX`
    // - stop time is > buffer length
    fn test_end_of_file_fast_track_2() {
        let sample_rate = 48_000.;
        let mut context = OfflineAudioContext::new(1, RENDER_QUANTUM_SIZE, sample_rate);

        let mut buffer = context.create_buffer(1, 5, sample_rate);
        let data = vec![1.; 1];
        buffer.copy_to_channel(&data, 0);

        let mut src = context.create_buffer_source();
        src.connect(&context.destination());
        src.set_buffer(buffer);
        // play in fast track
        src.start_at(0.);
        // stop after end of buffer but before the end of render quantum
        src.stop_at(125. / sample_rate as f64);

        let result = context.start_rendering_sync();
        let channel = result.get_channel_data(0);

        let mut expected = vec![0.; 128];
        expected[0] = 1.;

        assert_float_eq!(channel[..], expected[..], abs_all <= 0.);
    }

[orottier]

Thanks, that does the trick!