bpo-30796: Fix failures in signal delivery stress test (#2488)

* bpo-30796: Fix failures in signal delivery stress test

setitimer() can have a poor minimum resolution on some machines,
this would make the test reach its deadline (and a stray signal
could then kill a subsequent test).

* Make sure to clear the itimer after the test

Lib/test/test_signal.py

@@ -2,6 +2,7 @@ import os
 import random
 import signal
 import socket
+import statistics
 import subprocess
 import sys
 import time
@@ -949,13 +950,55 @@ class StressTest(unittest.TestCase):
     previously tripped signal handlers.
     """
 
+    def setsig(self, signum, handler):
+        old_handler = signal.signal(signum, handler)
+        self.addCleanup(signal.signal, signum, old_handler)
+
+    def measure_itimer_resolution(self):
+        N = 20
+        times = []
+
+        def handler(signum=None, frame=None):
+            if len(times) < N:
+                times.append(time.perf_counter())
+                # 1 µs is the smallest possible timer interval,
+                # we want to measure what the concrete duration
+                # will be on this platform
+                signal.setitimer(signal.ITIMER_REAL, 1e-6)
+
+        self.addCleanup(signal.setitimer, signal.ITIMER_REAL, 0)
+        self.setsig(signal.SIGALRM, handler)
+        handler()
+        while len(times) < N:
+            time.sleep(1e-3)
+
+        durations = [times[i+1] - times[i] for i in range(len(times) - 1)]
+        med = statistics.median(durations)
+        if support.verbose:
+            print("detected median itimer() resolution: %.6f s." % (med,))
+        return med
+
+    def decide_itimer_count(self):
+        # Some systems have poor setitimer() resolution (for example
+        # measured around 20 ms. on FreeBSD 9), so decide on a reasonable
+        # number of sequential timers based on that.
+        reso = self.measure_itimer_resolution()
+        if reso <= 1e-4:
+            return 10000
+        elif reso <= 1e-2:
+            return 100
+        else:
+            self.skipTest("detected itimer resolution (%.3f s.) too high "
+                          "(> 10 ms.) on this platform (or system too busy)"
+                          % (reso,))
+
     @unittest.skipUnless(hasattr(signal, "setitimer"),
                          "test needs setitimer()")
     def test_stress_delivery_dependent(self):
         """
         This test uses dependent signal handlers.
         """
-        N = 10000
+        N = self.decide_itimer_count()
         sigs = []
 
         def first_handler(signum, frame):
@@ -969,16 +1012,12 @@ class StressTest(unittest.TestCase):
         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)
+        self.setsig(signal.SIGPROF, first_handler)
-        setsig(signal.SIGUSR1, first_handler)
+        self.setsig(signal.SIGUSR1, first_handler)
-        setsig(signal.SIGALRM, second_handler)  # for ITIMER_REAL
+        self.setsig(signal.SIGALRM, second_handler)  # for ITIMER_REAL
 
         expected_sigs = 0
         deadline = time.time() + 15.0
@@ -1005,18 +1044,14 @@ class StressTest(unittest.TestCase):
         """
         This test uses simultaneous signal handlers.
         """
-        N = 10000
+        N = self.decide_itimer_count()
         sigs = []
 
         def handler(signum, frame):
             sigs.append(signum)
 
-        def setsig(signum, handler):
+        self.setsig(signal.SIGUSR1, handler)
-            old_handler = signal.signal(signum, handler)
+        self.setsig(signal.SIGALRM, handler)  # for ITIMER_REAL
-            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