When a foreign event loop that does not enter an alertable wait state
is running (which is also the case when a native dialog window is
modal), pipe handlers would freeze temporarily due to their APC
callbacks not being invoked.
We address this problem by moving the I/O callbacks to the Windows
thread pool, and only posting completion events to the main loop
from there. That makes the actual I/O completely independent from
any main loop, while the signal delivery works also with foreign
loops (because Qt event delivery uses Windows messages, which foreign
loops typically handle correctly).
As a nice side effect, performance (and in particular scalability)
is improved.
Several other approaches have been tried:
1) Using QWinEventNotifier was about a quarter slower and scaled much
worse. Additionally, it also required a rather egregious hack to
handle the (pathological) case of a single thread talking to both
ends of a QLocalSocket synchronously.
2) Queuing APCs from the thread pool to the main thread and also
posting wake-up events to its event loop, and handling I/O on the
main thread; this performed roughly like this solution, but scaled
half as well, and the separate wake-up path was still deemed hacky.
3) Only posting wake-up events to the main thread from the thread pool,
and still handling I/O on the main thread; this still performed
comparably to 2), and the pathological case was not handled at all.
4) Using this approach for reads and that of 3) for writes was slightly
faster with big amounts of data, but scaled slightly worse, and the
diverging implementations were deemed not desirable.
Fixes: QTBUG-64443
Change-Id: I66443c3021d6ba98639a214c3e768be97d2cf14b
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@gmx.de>
f265c87e01