Sharing WebGL textures between main thread and background threads / offscreen canvases

[Phil Endecott]

Dear Experts,

I have an OpenGL app which I successfully ported to Emscripten back in 2021.
That port was single-threaded.

I'm now trying to make it work with multiple threads. This mostly
works, but I
have an issue with the OpenGL code.

I only do OpenGL drawing on the main thread, but I do have background threads
that create OpenGL textures. They compute the texture content on the
CPU, then
call glTexSubImage2D or similar to load the content into a texture.

On other platforms - e.g. iOS and EGL on Linux - it is possible to create
multiple OpenGL contexts that share the same set of textures - for example,
the share_context parameter to eglCreateContext(). But I don't see a way
to do this with the WebGL APIs, or their Emscripten wrappers. (The
emscripten implementation of eglCreateContext names the share_context
parameter "hmm"!).

Has anyone else addressed this? Is this a fundamental limitation of the
WebGL + OffscreenCanvas APIs? If so, I guess I need to prepare my
texture data on a background thread and then proxy to the main thread
to actually create the texture.

Thanks,

Phil.