Webgl calls from worker thread

[sergeyext]

Consider the following example:

GLFWwindow* w;

void threadLoopIteration(void*)
{
        glClear(GL_COLOR_BUFFER_BIT);
        glfwSwapBuffers(w);
}

void tw()
{
        glfwMakeContextCurrent(w);
        emscripten_set_main_loop_arg(threadLoopIteration, nullptr, 0, 1);
}

void mainLoopIteration(void*) {}

int main()
{
        std::cout << "TSS: " << emscripten_has_threading_support() << std::endl;

        if (glewInit() != GLEW_OK) {
                std::cout << "No glew" << std::endl;
                return 1;
        }
        if (glfwInit() != GLFW_TRUE) {
                std::cout << "No glf" << std::endl;
                return 2;
        }
        w = glfwCreateWindow(100, 100, "Foo", 0, 0);

        glfwMakeContextCurrent(w);
        glClearColor(1.0f, 0.0f, 0.0f, 1.0f);
        glfwMakeContextCurrent(nullptr);

        std::thread thread(tw);
        emscripten_set_main_loop_arg(mainLoopIteration, nullptr, 0, 1);

        return 0;
}

It worked correctly when compiled with sdk 1.38.31. After updating to sdk 1.38.36 it crashes with

Cannot read property 'clear' of undefined
    at _glClear (http://localhost:8006/ems_thread.js:6813:32)
    ...........

I'm compiling the code with

em++ --std=c++17 -s USE_PTHREADS=1 -s USE_GLFW=3 -s PTHREAD_POOL_SIZE=2 -o ems_thread.html ems_thread.cpp

Also tried:

• Adding -s OFFSCREENCANVAS_SUPPORT=1 and/or -s USE_WEBGL2=1 with same result.
• Removing emsdk, ~/.emscripten_cache/, ~/.emscripten and other sdk artifacts from home directory, reinstalling sdk from scratch. Same result.
• Altering command line parameters order and using emcc instead of em++. Same result.

Offtopic:
I failed to roll back to 1.38.31, 38.33 and 38.34. Both adding older versions to existing latest installation and installing from scratch lead to error while loading shared libraries: libtinfo.so.5: cannot open shared object file: No such file or directory when running emcc. Only reinstalling latest from scratch can fix the problem. What is the proper way to use older versions?

What am I missing here?

[kripken]

I'm not sure how it worked earlier, but WebGL from a worker requires either offscreen canvas (browser support) or offscreen framebuffer (proxying support). Does it work with -s OFFSCREEN_FRAMEBUFFER?

If that's not it, then as a regression, bisecting might be the fastest way to see what broke this.

About libtinfo, there was an issue with that on our linux builds - turns out different linux distros have different versions and they can be incompatible (see for example emscripten-core/emsdk#252). That should be fixed on current builds, but older ones may not work without installing locally some other libtinfo versions for your distro.

[sergeyext]

Updated to 1.38.39 and compiled the same code with -s OFFSCREEN_FRAMEBUFFER:

em++ --std=c++17 -s USE_PTHREADS=1 -s USE_GLFW=3 -s PTHREAD_POOL_SIZE=2 -s OFFSCREENCANVAS_SUPPORT=1 -s OFFSCREEN_FRAMEBUFFER=1 -o ems_thread.html ems_thread.cpp

It does not crash, but also does not clear the viewport with red, as expected.

Also tried to move gl initialization to working thread, got crash on glfwInit (ReferenceError: window is not defined).

Next, I removed threading from code and compiled it with -s OFFSCREENCANVAS_SUPPORT=1 -s OFFSCREEN_FRAMEBUFFER=1:

GLFWwindow* w;

void mainLoopIteration(void*)
{
        glClear(GL_COLOR_BUFFER_BIT);
        glfwSwapBuffers(w);
}

int main()
{
        std::cout << "TSS: " << emscripten_has_threading_support() << std::endl;

        if (glfwInit() != GLFW_TRUE) {
                std::cout << "No glfw" << std::endl;
                return 2;
        }
        w = glfwCreateWindow(100, 100, "Foo", 0, 0);
        if (glewInit() != GLEW_OK) {
                std::cout << "No glew" << std::endl;
                return 1;
        }


        glfwMakeContextCurrent(w);
        glClearColor(1.0f, 0.0f, 0.0f, 1.0f);
        emscripten_set_main_loop_arg(mainLoopIteration, nullptr, 0, 1);

        return 0;
}

Got black viewport. Removing OFFSCREEN_FRAMEBUFFER=1 fixed the problem. Looks like glfwSwapBuffers(); does not swap anything with OFFSCREEN_FRAMEBUFFER=1 flag. Or am I missing any other call for buffer swapping?

[sergeyext]

Replaced glfwSwapBuffers() with emscripten_webgl_commit_frame() in multithreaded test, now it works.

Probably glfwSwapBuffers() should call emscripten_webgl_commit_frame() when compiled with OFFSCREEN_FRAMEBUFFER.