wasm: don’t deadlock on event processing

emscripten_async_run_in_main_runtime_thread_ schedules
an async call on the on the main thread. However, the
calls are ordered, also in respect to _synchronous_ calls
to the main thread (for example those made during file
write/flush).

Making a synchronous call from a secondary thread may
then cause Emscripten to service previously scheduled
async calls during the synchronous call. This can cause
a deadlock if:

- a secondary thread makes a sync call while holding a lock, and
- a previously scheduled async call attempt to acquire the same
  lock on the main thread.

(See https://github.com/emscripten-core/emscripten/issues/10155
for sample code)

Avoid this case by adding a second zero-timer async call;
this way Qt should process events when the main thread
becomes idle.

Change-Id: I221fe4e25bbb1a56627e63c3d1809e40ccefb030
Pick-to: 5.15
Reviewed-by: Lorn Potter <lorn.potter@gmail.com>

0 files changed, 0 insertions, 0 deletions