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Commit4ae0149

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njsmithvstinner
authored andcommitted
bpo-30038: fix race condition in signal delivery + wakeup fd (#1082)
Before, it was possible to get the following sequence ofevents (especially on Windows, where the C-level signal handler forSIGINT is run in a separate thread):- SIGINT arrives- trip_signal is called- trip_signal writes to the wakeup fd- the main thread wakes up from select()-or-equivalent- the main thread checks for pending signals, but doesn't see any- the main thread drains the wakeup fd- the main thread goes back to sleep- trip_signal sets is_tripped=1 and calls Py_AddPendingCall to notify the main thread the it should run the Python-level signal handler- the main thread doesn't notice because it's asleepThis has been causing repeated failures in the Trio test suite:python-trio/trio#119
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‎Modules/signalmodule.c‎

Lines changed: 26 additions & 7 deletions
Original file line numberDiff line numberDiff line change
@@ -244,6 +244,32 @@ trip_signal(int sig_num)
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Handlers[sig_num].tripped=1;
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if (!is_tripped) {
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/* Set is_tripped after setting .tripped, as it gets
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cleared in PyErr_CheckSignals() before .tripped. */
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is_tripped=1;
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Py_AddPendingCall(checksignals_witharg,NULL);
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}
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/* And then write to the wakeup fd *after* setting all the globals and
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doing the Py_AddPendingCall. We used to write to the wakeup fd and then
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set the flag, but this allowed the following sequence of events
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(especially on windows, where trip_signal runs in a new thread):
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- main thread blocks on select([wakeup_fd], ...)
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- signal arrives
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- trip_signal writes to the wakeup fd
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- the main thread wakes up
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- the main thread checks the signal flags, sees that they're unset
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- the main thread empties the wakeup fd
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- the main thread goes back to sleep
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- trip_signal sets the flags to request the Python-level signal handler
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be run
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- the main thread doesn't notice, because it's asleep
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See bpo-30038 for more details.
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*/
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#ifdefMS_WINDOWS
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fd=Py_SAFE_DOWNCAST(wakeup.fd,SOCKET_T,int);
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#else
@@ -281,13 +307,6 @@ trip_signal(int sig_num)
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}
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}
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}
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if (!is_tripped) {
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/* Set is_tripped after setting .tripped, as it gets
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cleared in PyErr_CheckSignals() before .tripped. */
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is_tripped=1;
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Py_AddPendingCall(checksignals_witharg,NULL);
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}
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}
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staticvoid

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