[Python-Dev] Questions about signal handling. (original) (raw)

Victor Stinner vstinner at redhat.com
Tue Sep 25 03:44:57 EDT 2018

Please don't rely on this ugly API. By design, Py_AddPendingCall() tries 100 times to acquire the lock: if it fails to acquire the lock, it does notthing... your callback is ignored...

By the way, recently, we had to fix yet another bug in signal handling. A new function has been added:

void _PyEval_SignalReceived(void) { /* bpo-30703: Function called when the C signal handler of Python gets a signal. We cannot queue a callback using Py_AddPendingCall() since that function is not async-signal-safe. */ SIGNAL_PENDING_CALLS(); }

If you want to exchange commands between two interpreters, which I see as two threads, I suggest to use two queues and something to consume queues.

Victor Le lun. 24 sept. 2018 à 22:23, Eric Snow <ericsnowcurrently at gmail.com> a écrit :

On Mon, Sep 24, 2018 at 11:14 AM Yury Selivanov <yselivanov.ml at gmail.com> wrote: > On Fri, Sep 21, 2018 at 7:04 PM Eric Snow <ericsnowcurrently at gmail.com> wrote: > > 1. Why do we restrict calls to signal.signal() to the main thread? > > 2. Why must signal handlers run in the main thread? > > 3. Why does signal handling operate via the "pending calls" machinery > > and not distinctly? > > Here's my take on this: > > Handling signals in a multi-threaded program is hard. Some signals can > be delivered to an arbitrary thread, some to the one that caused them. > Posix provides lots of mechanisms to tune how signals are received (or > blocked) by individual threads, but (a) Python doesn't expose those > APIs, (b) using those APIs correctly is insanely hard. By restricting > that we can only receive signals in the main thread we remove all that > complexity. Just to be clear, I'm not suggesting that we allow folks to specify in which Python (or kernel) thread a Python-level signal handler is called. The reason I've asked about signals is because of the semantics under subinterpreters (where there is no "main" thread). However, I don't have any plans to introduce per-interpreter signal handlers. Mostly I want to understand about the "main" thread restriction for the possible impact on subinterpreters. FWIW, I'm also mildly curious about the value of the "main" thread restriction currently. From what I can tell the restriction was made early on and there are hints in the C code that it's no longer needed. I suspect we still have the restriction solely because no one has bothered to change it. However, I wasn't sure so I figured I'd ask. :) > Restricting that signal.signal() can only be called from > the main thread just makes this API more consistent Yeah, that's what I thought. > (and also IIRC > avoids weird sigaction() behaviour when it is called from different > threads within one program). Is there a good place where this weirdness is documented? > Next, you can only call reentrant functions in your signal handlers. > For instance, printf() function isn't safe to use. Therefore one > common practice is to set a flag that a signal was received and check > it later (exactly what we do with the pending calls machinery). We don't actually use the pending calls machinery for signals though. The only thing we do is always call PyErrCheckSignals() before making any pending calls. Wouldn't it be equivalent if we called PyErrCheckSignals() at the beginning of the eval loop right before calling PyMakePendingCalls()? This matters to me because I'd like to use "pending" calls for subinterpreters, which means dealing with signals in PyMakePendingCalls() is problematic. Pulling the PyErrCheckSignals() call out would eliminate that problem. > Therefore, IMO, the current way we handle signals in Python is the > safest, most predictable, and most cross-platform option there is. > And changing how Python signals API works with threads in any way will > actually break the world. I agree that the way we deal with signals (i.e. set a flag that is later handled in PyErrCheckSignals(), protected by the GIL) shouldn't change. My original 3 questions do not relate to that. Looks like that wasn't terribly clear. :) -eric

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