webaudio panic playing Master Archer

[jdm]

joshmatthews@joshmatthews-fcmd6r servo % ./mach run -r https://playcanv.as/p/JERg21J8/
Powered by PlayCanvas 0.205.0 5ad0fae
[2020-04-22T15:40:02Z ERROR script::dom::bindings::error] Error at https://playcanv.as/p/JERg21J8/:97:34 iframe.contentWindow.focus is not a function
Message from PlayCanvas: The element component is currently in Beta. APIs may change without notice.
index out of bounds: the len is 0 but the index is 0 (thread AudioRenderThread, at /rustc/45ebd5808afd3df7ba842797c0fcd4447ddf30fb/src/libcore/slice/mod.rs:2842)
stack backtrace:
   0: <servo::backtrace::Print as core::fmt::Debug>::fmt
   1: core::fmt::write
   2: std::io::Write::write_fmt
   3: servo::backtrace::print
   4: servo::main::{{closure}}
   5: std::panicking::rust_panic_with_hook
   6: rust_begin_unwind
   7: core::panicking::panic_fmt
   8: core::panicking::panic_bounds_check
   9: servo_media_audio::buffer_source_node::AudioBuffer::interpolate
  10: <servo_media_audio::buffer_source_node::AudioBufferSourceNode as servo_media_audio::node::AudioNodeEngine>::process
  11: servo_media_audio::graph::AudioGraph::process
  12: servo_media_audio::render_thread::AudioRenderThread::event_loop
  13: servo_media_audio::render_thread::AudioRenderThread::start
  14: std::sys_common::backtrace::__rust_begin_short_backtrace
  15: core::ops::function::FnOnce::call_once{{vtable.shim}}
  16: <alloc::boxed::Box<F> as core::ops::function::FnOnce<A>>::call_once
  17: std::sys::unix::thread::Thread::new::thread_start
  18: _pthread_start
[2020-04-22T15:40:05Z ERROR servo] index out of bounds: the len is 0 but the index is 0
called `Result::unwrap()` on an `Err` value: RecvError (thread ScriptThread PipelineId { namespace_id: PipelineNamespaceId(1), index: PipelineIndex(1) }, at /Users/joshmatthews/.cargo/git/checkouts/media-c23a3cd5aa97076a/03c7420/audio/context.rs:188)
stack backtrace:
   0: <servo::backtrace::Print as core::fmt::Debug>::fmt
   1: core::fmt::write
   2: std::io::Write::write_fmt
   3: servo::backtrace::print
   4: servo::main::{{closure}}
   5: std::panicking::rust_panic_with_hook
   6: rust_begin_unwind
   7: core::panicking::panic_fmt
   8: core::option::expect_none_failed
   9: servo_media_audio::context::AudioContext::current_time
  10: <script::dom::baseaudiocontext::BaseAudioContext as script::dom::bindings::codegen::Bindings::BaseAudioContextBinding::BaseAudioContextBinding::BaseAudioContextMethods>::CurrentTime
  11: <std::panic::AssertUnwindSafe<F> as core::ops::function::FnOnce<()>>::call_once
  12: script::dom::bindings::codegen::Bindings::BaseAudioContextBinding::BaseAudioContextBinding::get_currentTime
  13: script::dom::bindings::utils::generic_call
  14: _ZN2js23InternalCallOrConstructEP9JSContextRKN2JS8CallArgsENS_14MaybeConstructENS_10CallReasonE
  15: _ZN2js10CallGetterEP9JSContextN2JS6HandleINS2_5ValueEEES5_NS2_13MutableHandleIS4_EE
  16: _ZN2js17NativeGetPropertyEP9JSContextN2JS6HandleIPNS_12NativeObjectEEENS3_INS2_5ValueEEENS3_INS2_11PropertyKeyEEENS2_13MutableHandleIS7_EE
  17: _ZN2js11GetPropertyEP9JSContextN2JS6HandleINS2_5ValueEEENS3_IPNS_12PropertyNameEEENS2_13MutableHandleIS4_EE
  18: _ZL9InterpretP9JSContextRN2js8RunStateE
  19: _ZN2js9RunScriptEP9JSContextRNS_8RunStateE
  20: _ZN2js23InternalCallOrConstructEP9JSContextRKN2JS8CallArgsENS_14MaybeConstructENS_10CallReasonE
  21: _ZN2js4CallEP9JSContextN2JS6HandleINS2_5ValueEEES5_RKNS_13AnyInvokeArgsENS2_13MutableHandleIS4_EENS_10CallReasonE
  22: _ZN2js3jit14InvokeFunctionEP9JSContextN2JS6HandleIP8JSObjectEEbbjPNS3_5ValueENS3_13MutableHandleIS8_EE
  23: _ZN2js3jit25InvokeFromInterpreterStubEP9JSContextPNS0_30InterpreterStubExitFrameLayoutE
[2020-04-22T15:40:05Z ERROR servo] called `Result::unwrap()` on an `Err` value: RecvError

[jdm]

Also seen on https://playcanv.as/p/Ikq6Uk6A/.

[Eijebong]

Removing the else here fixes the crash. https://github.com/servo/media/blob/62cd58a94c54e8d9ff3845f263356e242e951686/audio/buffer_source_node.rs#L304

We get in that method with loop_enabled = true, but both buffer.len() and pos end up being equal to 0.

(gdb) info locals
sample = 0x7fff8662c000
__next = 0x7fff8662c000
iter = core::slice::IterMut<f32> {ptr: core::ptr::non_null::NonNull<f32> {pointer: 0x7fff8662c004}, end: 0x7fff8662c200, _marker: core::marker::PhantomData<&mut f32>}
duration = inf
pos = 0
data = &mut [f32] {data_ptr: 0x7fff8662c000, length: 128}
data = &mut [f32] {data_ptr: 0x7fff8662c000, length: 128}
data = &mut [f32] {data_ptr: 0x7fff8662c000, length: 128}
chan = 0
__next = 0
iter = core::ops::range::Range<u8> {start: 1, end: 2}
block = servo_media_audio::block::Block {channels: 2, repeat: false, buffer: alloc::vec::Vec<f32> {buf: alloc::raw_vec::RawVec<f32, alloc::alloc::Global> {ptr: core::ptr::unique::Unique<f32> {pointer: 0x7fff8662c000, _marker: core::marker::PhantomData<f32>}, cap: 256, alloc: alloc::alloc::Global}, len: 256}}
frames_to_output = 128
buffer_offset_per_tick = 1
forward = true
computed_playback_rate = 1
actual_loop_start = 0
actual_loop_end = 0
buffer = 0x7fff86617180
start_at = 0
stop_at = 128

Here are the locals at the beginning of the block.

And at the end

sample = 0x7fff8662c000
__next = 0x7fff8662c000
iter = core::slice::IterMut<f32> {ptr: core::ptr::non_null::NonNull<f32> {pointer: 0x7fff8662c004}, end: 0x7fff8662c200, _marker: core::marker::PhantomData<&mut f32>}
duration = inf
pos = 0
data = &mut [f32] {data_ptr: 0x7fff8662c000, length: 128}
data = &mut [f32] {data_ptr: 0x7fff8662c000, length: 128}
data = &mut [f32] {data_ptr: 0x7fff8662c000, length: 128}
chan = 0
__next = 0
iter = core::ops::range::Range<u8> {start: 1, end: 2}
block = servo_media_audio::block::Block {channels: 2, repeat: false, buffer: alloc::vec::Vec<f32> {buf: alloc::raw_vec::RawVec<f32, alloc::alloc::Global> {ptr: core::ptr::unique::Unique<f32> {pointer: 0x7fff8662c
000, _marker: core::marker::PhantomData<f32>}, cap: 256, alloc: alloc::alloc::Global}, len: 256}}
frames_to_output = 128
buffer_offset_per_tick = 1
forward = true
computed_playback_rate = 1
actual_loop_start = 0
actual_loop_end = 0
buffer = 0x7fff86617180
start_at = 0
stop_at = 128