STINNER Victor <vstin...@python.org> added the comment:
First of all, I know that mod_wsgi uses subinterpreters, but I didn't know that
it uses daemon threads. When I hack Python to better isolate subinterpreters, I
keep mod_wsgi in mind ;-)
My commit 066e5b1a917ec2134e8997d2cadd815724314252 message says: "Daemon
threads were never supported in subinterpreters." That's inaccurate. Only the
second sentence is correct: "Previously, the subinterpreter finalization
crashed with a Pyton fatal error if a daemon thread was still running."
In Python 3.8, it is possible to use daemon thread but only if they complete
before Py_EndInterpreter() is called.
I'm not comfortable with Python "crashing" (call Py_FatalError() which calls
abort() which may generate a coredump) depending if all daemon threads complete
or not. I'm trying to make Python finalization more deterministic.
My concern is this Py_EndInterpreter() check:
if (tstate != interp->tstate_head || tstate->next != NULL) {
Py_FatalError("not the last thread");
}
This check is as old as subinterpreters, it was added at the same time than the
Py_EndInterpreter() function:
commit 25ce566661c1b7446b3ddb4076513a62f93ce08d
Author: Guido van Rossum <gu...@python.org>
Date: Sat Aug 2 03:10:38 1997 +0000
The last of the mass checkins for separate (sub)interpreters.
Everything should now work again.
See the comments for the .h files mass checkin (e.g. pystate.h) for
more detail.
My concern is that Py_FatalError() exits the whole process, not only the thread
calling Py_EndInterpreter(). The caller cannot catch Py_FatalError() call and
decide how to handle it :-( That doesn't seem like a good API when Python is
embedded in an application.
Recently, I had to deal with (fix) many bugs related to daemon threads in
Python finalization:
* https://vstinner.github.io/gil-bugfixes-daemon-threads-python39.html
* https://vstinner.github.io/threading-shutdown-race-condition.html
* https://vstinner.github.io/daemon-threads-python-finalization-python32.html
Simply denying daemon threads in subinterpreters is the safest and simplest
option. But it's also a backward incompatible change.
I'm open to allow again daemon threads in subinterpreter in Python 3.9 but emit
a DeprecationWarning to announce that this feature is going to be remove
"later" (ex: Python 3.11) since it's "broken by design" (in my opinion).
> We could also deprecate use of daemon threads in *all* subinterpreters, with
> the goal of dropping support after a while.
It sounds like a good idea to make Python finalization more reliable. But
daemon threads are still widely used in the Python standard library. For
example, by the multiprocessing module.
concurrent.futures was only modified recently to stop using daemon threads,
after I denied spawned daemon threads in subinterpreters:
* bpo-39812
* commit b61b818d916942aad1f8f3e33181801c4a1ed14b
So if someone wants to ban daemon threads, we should start by modifying the
stdlib to avoid them, then deprecate the feature, and then we can start talking
about removing the feature.
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