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bpo-30703: Improve signal delivery (#2415) 

* Improve signal delivery

Avoid using Py_AddPendingCall from signal handler, to avoid calling signal-unsafe functions.

* Remove unused function

* Improve comments

* Add stress test

* Adapt for --without-threads

* Add second stress test

* Add NEWS blurb

* Address comments @haypo
This commit is contained in:
Antoine Pitrou 2017-06-28 23:29:29 +02:00 committed by GitHub
parent 9f3bdcb643
commit c08177a1cc
5 changed files with 171 additions and 37 deletions
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1
Include/ceval.h
View File

@ -54,6 +54,7 @@ PyAPI_FUNC(int) PyEval_MergeCompilerFlags(PyCompilerFlags *cf);
#endif
PyAPI_FUNC(int) Py_AddPendingCall(int (*func)(void *), void *arg);
PyAPI_FUNC(void) _PyEval_SignalReceived(void);
PyAPI_FUNC(int) Py_MakePendingCalls(void);
/* Protection against deeply nested recursive calls

96
Lib/test/test_signal.py
View File

@ -1,4 +1,5 @@
import os
import random
import signal
import socket
import subprocess
@ -941,6 +942,101 @@ class PendingSignalsTests(unittest.TestCase):
(exitcode, stdout))
class StressTest(unittest.TestCase):
"""
Stress signal delivery, especially when a signal arrives in
the middle of recomputing the signal state or executing
previously tripped signal handlers.
"""
@unittest.skipUnless(hasattr(signal, "setitimer"),
"test needs setitimer()")
def test_stress_delivery_dependent(self):
"""
This test uses dependent signal handlers.
"""
N = 10000
sigs = []
def first_handler(signum, frame):
# 1e-6 is the minimum non-zero value for `setitimer()`.
# Choose a random delay so as to improve chances of
# triggering a race condition. Ideally the signal is received
# when inside critical signal-handling routines such as
# Py_MakePendingCalls().
signal.setitimer(signal.ITIMER_REAL, 1e-6 + random.random() * 1e-5)
def second_handler(signum=None, frame=None):
sigs.append(signum)
def setsig(signum, handler):
old_handler = signal.signal(signum, handler)
self.addCleanup(signal.signal, signum, old_handler)
# Here on Linux, SIGPROF > SIGALRM > SIGUSR1. By using both
# ascending and descending sequences (SIGUSR1 then SIGALRM,
# SIGPROF then SIGALRM), we maximize chances of hitting a bug.
setsig(signal.SIGPROF, first_handler)
setsig(signal.SIGUSR1, first_handler)
setsig(signal.SIGALRM, second_handler) # for ITIMER_REAL
expected_sigs = 0
deadline = time.time() + 15.0
while expected_sigs < N:
os.kill(os.getpid(), signal.SIGPROF)
expected_sigs += 1
# Wait for handlers to run to avoid signal coalescing
while len(sigs) < expected_sigs and time.time() < deadline:
time.sleep(1e-5)
os.kill(os.getpid(), signal.SIGUSR1)
expected_sigs += 1
while len(sigs) < expected_sigs and time.time() < deadline:
time.sleep(1e-5)
# All ITIMER_REAL signals should have been delivered to the
# Python handler
self.assertEqual(len(sigs), N, "Some signals were lost")
@unittest.skipUnless(hasattr(signal, "setitimer"),
"test needs setitimer()")
def test_stress_delivery_simultaneous(self):
"""
This test uses simultaneous signal handlers.
"""
N = 10000
sigs = []
def handler(signum, frame):
sigs.append(signum)
def setsig(signum, handler):
old_handler = signal.signal(signum, handler)
self.addCleanup(signal.signal, signum, old_handler)
setsig(signal.SIGUSR1, handler)
setsig(signal.SIGALRM, handler) # for ITIMER_REAL
expected_sigs = 0
deadline = time.time() + 15.0
while expected_sigs < N:
# Hopefully the SIGALRM will be received somewhere during
# initial processing of SIGUSR1.
signal.setitimer(signal.ITIMER_REAL, 1e-6 + random.random() * 1e-5)
os.kill(os.getpid(), signal.SIGUSR1)
expected_sigs += 2
# Wait for handlers to run to avoid signal coalescing
while len(sigs) < expected_sigs and time.time() < deadline:
time.sleep(1e-5)
# All ITIMER_REAL signals should have been delivered to the
# Python handler
self.assertEqual(len(sigs), N, "Some signals were lost")
def tearDownModule():
support.reap_children()

6
Misc/NEWS.d/next/Core and Builtins/2017-06-28-21-07-32.bpo-30703.ULCdFp.rst Normal file
View File

@ -0,0 +1,6 @@
Improve signal delivery.
Avoid using Py_AddPendingCall from signal handler, to avoid calling signal-
unsafe functions. The tests I'm adding here fail without the rest of the
patch, on Linux and OS X. This means our signal delivery logic had defects
(some signals could be lost).

30
Modules/signalmodule.c
View File

@ -188,12 +188,6 @@ The default handler for SIGINT installed by Python.\n\
It raises KeyboardInterrupt.");
static int
checksignals_witharg(void * unused)
{
return PyErr_CheckSignals();
}
static int
report_wakeup_write_error(void *data)
{
@ -244,17 +238,15 @@ trip_signal(int sig_num)
Handlers[sig_num].tripped = 1;
if (!is_tripped) {
/* Set is_tripped after setting .tripped, as it gets
cleared in PyErr_CheckSignals() before .tripped. */
is_tripped = 1;
Py_AddPendingCall(checksignals_witharg, NULL);
}
/* Set is_tripped after setting .tripped, as it gets
cleared in PyErr_CheckSignals() before .tripped. */
is_tripped = 1;
_PyEval_SignalReceived();
/* And then write to the wakeup fd *after* setting all the globals and
doing the Py_AddPendingCall. We used to write to the wakeup fd and then
set the flag, but this allowed the following sequence of events
(especially on windows, where trip_signal runs in a new thread):
doing the _PyEval_SignalReceived. We used to write to the wakeup fd
and then set the flag, but this allowed the following sequence of events
(especially on windows, where trip_signal may run in a new thread):
- main thread blocks on select([wakeup_fd], ...)
- signal arrives
@ -289,6 +281,8 @@ trip_signal(int sig_num)
wakeup.send_err_set = 1;
wakeup.send_errno = errno;
wakeup.send_win_error = GetLastError();
/* Py_AddPendingCall() isn't signal-safe, but we
still use it for this exceptional case. */
Py_AddPendingCall(report_wakeup_send_error, NULL);
}
}
@ -302,6 +296,8 @@ trip_signal(int sig_num)
rc = _Py_write_noraise(fd, &byte, 1);
if (rc < 0) {
/* Py_AddPendingCall() isn't signal-safe, but we
still use it for this exceptional case. */
Py_AddPendingCall(report_wakeup_write_error,
(void *)(intptr_t)errno);
}
@ -1556,8 +1552,10 @@ PyErr_CheckSignals(void)
arglist);
Py_DECREF(arglist);
}
if (!result)
if (!result) {
is_tripped = 1;
return -1;
}
Py_DECREF(result);
}

75
Python/ceval.c
View File

@ -140,6 +140,15 @@ static long dxp[256];
do { pending_async_exc = 0; COMPUTE_EVAL_BREAKER(); } while (0)
/* This single variable consolidates all requests to break out of the fast path
in the eval loop. */
static _Py_atomic_int eval_breaker = {0};
/* Request for running pending calls. */
static _Py_atomic_int pendingcalls_to_do = {0};
/* Request for looking at the `async_exc` field of the current thread state.
Guarded by the GIL. */
static int pending_async_exc = 0;
#ifdef WITH_THREAD
#ifdef HAVE_ERRNO_H
@ -149,16 +158,8 @@ static long dxp[256];
static PyThread_type_lock pending_lock = 0; /* for pending calls */
static unsigned long main_thread = 0;
/* This single variable consolidates all requests to break out of the fast path
in the eval loop. */
static _Py_atomic_int eval_breaker = {0};
/* Request for dropping the GIL */
static _Py_atomic_int gil_drop_request = {0};
/* Request for running pending calls. */
static _Py_atomic_int pendingcalls_to_do = {0};
/* Request for looking at the `async_exc` field of the current thread state.
Guarded by the GIL. */
static int pending_async_exc = 0;
#include "ceval_gil.h"
@ -253,9 +254,6 @@ PyEval_ReInitThreads(void)
_PyThreadState_DeleteExcept(current_tstate);
}
#else
static _Py_atomic_int eval_breaker = {0};
static int pending_async_exc = 0;
#endif /* WITH_THREAD */
/* This function is used to signal that async exceptions are waiting to be
@ -330,6 +328,15 @@ PyEval_RestoreThread(PyThreadState *tstate)
#endif
*/
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();
}
#ifdef WITH_THREAD
/* The WITH_THREAD implementation is thread-safe. It allows
@ -394,6 +401,8 @@ Py_MakePendingCalls(void)
int i;
int r = 0;
assert(PyGILState_Check());
if (!pending_lock) {
/* initial allocation of the lock */
pending_lock = PyThread_allocate_lock();
@ -408,6 +417,16 @@ Py_MakePendingCalls(void)
if (busy)
return 0;
busy = 1;
/* unsignal before starting to call callbacks, so that any callback
added in-between re-signals */
UNSIGNAL_PENDING_CALLS();
/* Python signal handler doesn't really queue a callback: it only signals
that a signal was received, see _PyEval_SignalReceived(). */
if (PyErr_CheckSignals() < 0) {
goto error;
}
/* perform a bounded number of calls, in case of recursion */
for (i=0; i<NPENDINGCALLS; i++) {
int j;
@ -424,20 +443,23 @@ Py_MakePendingCalls(void)
arg = pendingcalls[j].arg;
pendingfirst = (j + 1) % NPENDINGCALLS;
}
if (pendingfirst != pendinglast)
SIGNAL_PENDING_CALLS();
else
UNSIGNAL_PENDING_CALLS();
PyThread_release_lock(pending_lock);
/* having released the lock, perform the callback */
if (func == NULL)
break;
r = func(arg);
if (r)
break;
if (r) {
goto error;
}
}
busy = 0;
return r;
error:
busy = 0;
SIGNAL_PENDING_CALLS(); /* We're not done yet */
return -1;
}
#else /* if ! defined WITH_THREAD */
@ -472,7 +494,6 @@ static struct {
} pendingcalls[NPENDINGCALLS];
static volatile int pendingfirst = 0;
static volatile int pendinglast = 0;
static _Py_atomic_int pendingcalls_to_do = {0};
int
Py_AddPendingCall(int (*func)(void *), void *arg)
@ -506,7 +527,16 @@ Py_MakePendingCalls(void)
if (busy)
return 0;
busy = 1;
/* unsignal before starting to call callbacks, so that any callback
added in-between re-signals */
UNSIGNAL_PENDING_CALLS();
/* Python signal handler doesn't really queue a callback: it only signals
that a signal was received, see _PyEval_SignalReceived(). */
if (PyErr_CheckSignals() < 0) {
goto error;
}
for (;;) {
int i;
int (*func)(void *);
@ -518,13 +548,16 @@ Py_MakePendingCalls(void)
arg = pendingcalls[i].arg;
pendingfirst = (i + 1) % NPENDINGCALLS;
if (func(arg) < 0) {
busy = 0;
SIGNAL_PENDING_CALLS(); /* We're not done yet */
return -1;
goto error:
}
}
busy = 0;
return 0;
error:
busy = 0;
SIGNAL_PENDING_CALLS(); /* We're not done yet */
return -1;
}
#endif /* WITH_THREAD */