Mac crash in glDrawArrays

[asajeffrey]

This can happen when running the WebGL test suite without --headless:

  ▶ CRASH [expected ERROR] /_webgl/conformance2/rendering/uniform-block-buffer-size.html
  │ 
  │ DESCRIPTION: This test verifies an active UniformBlock should be populated with a large enough buffer object
  │ 
  │ PASS WebGL context exists
  │ 
  │ Testing drawArrays and drawArraysInstanced
  │ PASS getError was expected value: NO_ERROR : Set up succeeded
  │ FAIL getError expected: INVALID_OPERATION. Was NO_ERROR : drawArrays: UniformBlock is not backed by a buffer
  │ FAIL getError expected: INVALID_OPERATION. Was NO_ERROR : drawArraysInstanced: UniformBlock is not backed by a buffer
  │ Stack trace for thread "WebGL thread"
  │ 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::install_crash_handler::handler
  │    5: _sigtramp
  │    6: _ZN4Gen714updateDispatchEP13GLDContextRecP17GLDRenderDispatchPj
  │    7: gldUpdateDispatch
  │    8: gleDoDrawDispatchCoreGL3
  │    9: glDrawArrays_ACC_GL3Exec
  │   10: canvas::webgl_thread::WebGLImpl::apply
  │   11: canvas::webgl_thread::WebGLThread::handle_msg
  │   12: canvas::webgl_thread::WebGLThread::process
  │   13: std::sys_common::backtrace::__rust_begin_short_backtrace
  │   14: core::ops::function::FnOnce::call_once{{vtable.shim}}
  │   15: <alloc::boxed::Box<F> as core::ops::function::FnOnce<A>>::call_once
  │   16: std::sys::unix::thread::Thread::new::thread_start
  │   17: _pthread_body
  └   18: _pthread_start

[asajeffrey]

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