Hi all,
We are leveraging Qt's fine Wasm/Emscripten platform support in a
project where we compile LibreOffice with Emscripten to run in the
browser (see <https://zetaoffice.net/>). We are currently based on a
slightly patched, old Qt 5.15.2 version (see
<https://github.com/allotropia/qtbase/tree/5.15.2%2Bwasm>).
We need Emscripten's -sPROXY_TO_PTHREAD setting (see
<https://emscripten.org/docs/tools_reference/settings_reference.html#proxy-to-pthread>,
which moves the application's main thread off the browser main thread
into a pthread, to which it transfers any canvas DOM element as an
OffscreenCanvas upfront). We need that, among others, because the
LibreOffice code wants to arbitrarily spawn further threads from within
event loop callbacks, something that would run into deadlocks without
-sPROXY_TO_PTHREAD.
Qt does not appear to support -sPROXY_TO_PTHREAD out of the box. But
for that old Qt 5.15.2, it was relatively easy to patch things up:
There was only a single qtcanvas DOM element, so Emscripten can be
instructed to transfer it with -sOFFSCREENCAVNASES_TO_PTHRAD=#qtcanvas.
And Qt code that accessed browser functionality that is only available
on the browser's main thread (so cannot be accessed directly any more
with -sPROXY_TO_PTHREAD) was relatively rare, and I patched that up by
sprinkling in some emscripten_async_run_in_main_runtime_thread etc., and
that appeared to mostly work well enough.
Now, with recent Qt 6, things have apparently changed rather
drastically. There is no longer a single #qtcanvas DOM element, but
each window is its own, dynamically added canvas element, and
interaction with browser functionality that is only available on the
browser's main thread has grown significantly. To a point that I
quickly gave up trying to carry my patching approach forward :)
So, my question is what the general opinion here is regarding support
for Emscripten's -sPROXY_TO_PTHREAD in today's Qt 6. Has this been
discussed before? Is it considered something that you wouldn't want to
support, or something that would be supported if somebody set out to
actually implement it, or...?
Thoughts welcome,
Stephan
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