WebGL with PROXY_TO_PTHREAD runtime error issue

[Greg Baxes]

Hi everyone,

I have been working successfully with Emscripten over the past few weeks porting a fairly heavy C++ GLES2 3D terrain-rendering engine and small glut-based application layer. It uses pthreads. Everything runs incredibly well in Chrome! I have always seen the JS console throw the warning: "Blocking on the main thread is very dangerous." To correct this blocking-on-main issue and to be able to add websockets support to the application, I have added the -s PROXY_TO_PTHREAD=1 flag to my build. With this, the application crashes with a pthread error when it first encounters a glGetString call. I have tried using the -s OFFSCREEN_FRAMEBUFFER=1 flag and things get further, but then crash on a glTexImage2D call.

To emulate the problem in a simpler context, I went back to my first simple GLES2 test, standard es2gears.c, which I successfully built using:

emcc -s WASM=1 -g es2gears.c -o demo.html

Similarly with it, when I add the -s PROXY_TO_PTHREAD=1 and -s USE_PTHREADS=1 flags, it crashes the same way when it encounters a glEnable call, throwing the same kind of error:

"pthread sent an error! ... Uncaught TypeError: Cannot read property 'enable' of undefined at _glEnable"

So, I think the es2gears.c build is a good test case of the issue I'm seeing.

Any suggestions to fix this? Is WebGL not able to run in a thread under main? Thank you for any insights!

[Jukka Jylänki]

WebGL API is only available in main browser thread by default.

If you build with -s OFFSCREEN_FRAMEBUFFER=1, then WebGL will be
available from pthreads via the Emscripten WebGL runtime proxying all
the GL calls to the main thread.

If you build with -s OFFSCREENCANVAS_SUPPORT=1, then WebGL will be
available from pthreads via the OffscreenCanvas web api. Check
https://caniuse.com/#feat=offscreencanvas for availability.

Browsers are really bad at printing JS errors: the error "Cannot read
property 'enable' of undefined" should read something like "GLctx is
undefined in GLctx.enable(x) at function _glEnable". That is, there is
no active GL context in the current thread where the GL call was
attempted.

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[Greg Baxes]

Hi jj,

Thanks for your reply, very helpful in that building with -s OFFSCREEN_FRAMEBUFFER=1 allows WebGL to be available from pthreads.

However, when I build with -s OFFSCREEN_FRAMEBUFFER=1, it throws "emscripten_async_queue_call_on_thread" errors from pthread at runtime. This occurs in both my terrain renderer builds as well as the simple es2gears.c example. As a simple test case, I have attached es2gears.c. It builds and runs nicely in chrome if I build it with:

  emcc -s WASM=1 -s USE_PTHREADS=1 -g es2gears.c -o demo.html

However, it crashes at runtime with the "emscripten_async_queue_call_on_thread" errors from pthreads if I build it with:

  emcc -s WASM=1 -s USE_PTHREADS=1 -s PROXY_TO_PTHREAD=1 -s OFFSCREEN_FRAMEBUFFER=1 -g es2gears.c -o demo.html

Can you advise what I'm missing to get es2gears.c running with WebGL and pthreads? Thank you.

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[Jukka Jylänki]

Oh, the code uses GLUT to initialize the GL context. Unfortunately
OFFSCREEN_FRAMEBUFFER (and OFFSCREENCANVAS_SUPPORT) features are only
available for contexts created with HTML5 API and SDL2, but won't work
for contexts created with GLUT, GLEW, GLFW, EGL or SDL 1.

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[Greg Baxes]

jj,

OK, thanks, that answers what I'm seeing! We'll migrate it over to SDL2.

Best, Greg

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