glContextSwitchingQueryIssue.cpp

int main()
{
    //
    //
    // Setup globals and hidden shared OpenGL context (for resource sharing)
    // (uses `SDL_GLContext` internally)
    auto                 graphicsContext = sf::GraphicsContext::create().value();
    sf::priv::GlContext& sharedCtxPtr    = sf::WindowContext::getSharedGlContextPtr();

    //
    //
    // Create window (uses `SDL_Window` and `SDL_GLContext` internally)
    sf::RenderWindow window({.size = {256u, 256u}});
    assert(window.setActive()); // calls `SDL_GL_MakeCurrent`
    sf::priv::GlContext* windowCtxPtr = sf::WindowContext::getActiveThreadLocalGlContextPtr();

    //
    //
    // Create OpenGL texture on the shared context, then revert to previous context and bind it
    GLuint glTexture{};

    assert(sharedCtxPtr.makeCurrent(true)); // calls `SDL_GL_MakeCurrent`

    glCheck(glGenTextures(1, &glTexture));
    assert(glTexture);

    assert(windowCtxPtr->makeCurrent(true)); // calls `SDL_GL_MakeCurrent`

    glCheck(glBindTexture(GL_TEXTURE_2D, glTexture));
    glCheck(glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, 256, 256, 0, GL_RGBA, GL_UNSIGNED_BYTE, nullptr));

    //
    //
    // Set window context to active and calls `glClearColor` + `glClear`
    window.clear(sf::Color::Red);
    // Intentionally not calling `window.display()` to avoid `SDL_GL_SwapWindow` call

    //
    //
    // Create destination framebuffer and bind the previously created texture to it
    GLuint destFrameBuffer{};
    glCheck(glGenFramebuffers(1, &destFrameBuffer));
    assert(destFrameBuffer);

    // Read from the window FBO, write to the texture FBO
    glCheck(glBindFramebuffer(GL_READ_FRAMEBUFFER, 0u /* default FBO */));
    glCheck(glBindFramebuffer(GL_DRAW_FRAMEBUFFER, destFrameBuffer));

    glCheck(glFramebufferTexture2D(GL_DRAW_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, glTexture, 0));

    assert(glCheckFramebufferStatus(GL_READ_FRAMEBUFFER) == GL_FRAMEBUFFER_COMPLETE);
    assert(glCheckFramebufferStatus(GL_DRAW_FRAMEBUFFER) == GL_FRAMEBUFFER_COMPLETE);

    //
    //
    // This context switching should be idempotent, but commenting out `sharedCtxPtr.makeCurrent(true)` FIXES THE ISSUE
    assert(sharedCtxPtr.makeCurrent(true));  // calls `SDL_GL_MakeCurrent`
    assert(windowCtxPtr->makeCurrent(true)); // calls `SDL_GL_MakeCurrent`

    //
    //
    // This query should be a noop, but commenting out `glGetIntegerv` FIXES THE ISSUE
    GLint out{};
    glCheck(glGetIntegerv(GL_READ_FRAMEBUFFER_BINDING, &out));

    // (!) EITHER (!) the context switching or the query must be commented out to FIX THE ISSUE

    //
    //
    // Blit the framebuffer from the window FBO to the texture FBO, then delete the framebuffer
    glCheck(glBindFramebuffer(GL_READ_FRAMEBUFFER, 0u));
    glCheck(glBindFramebuffer(GL_DRAW_FRAMEBUFFER, destFrameBuffer));
    glCheck(glBlitFramebuffer(0, 0, 256, 256, 0, 0, 256, 256, GL_COLOR_BUFFER_BIT, GL_NEAREST));
    glCheck(glDeleteFramebuffers(1, &destFrameBuffer));

    //
    //
    // Write texture data into a pixel array and read it back
    glCheck(glBindTexture(GL_TEXTURE_2D, glTexture));

    sf::base::U8 pixels[256 * 256 * 4]{};

    GLuint imageFBO{};
    glCheck(glGenFramebuffers(1, &imageFBO));
    assert(imageFBO);

    glCheck(glBindFramebuffer(GL_FRAMEBUFFER, imageFBO));
    glCheck(glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, glTexture, 0));
    glCheck(glReadPixels(0, 0, 256, 256, GL_RGBA, GL_UNSIGNED_BYTE, pixels));
    glCheck(glDeleteFramebuffers(1, &imageFBO));

    const auto          index = (64 + 64 * 256) * 4; // pixel at (64, 64)
    const sf::base::U8* pixel = &pixels[index];

    //
    //
    // These assertions will fail unless EITHER the context switching or the query is commented out
    assert(pixel[0] == 255); // Red
    assert(pixel[1] == 0);   // Green
    assert(pixel[2] == 0);   // Blue
    assert(pixel[3] == 255); // Alpha
}