bpo-31479: Always reset the signal alarm in tests (#3588)

* bpo-31479: Always reset the signal alarm in tests

Use "try: ... finally: signal.signal(0)" pattern to make sure that
tests don't "leak" a pending fatal signal alarm.

* Move two more alarm() calls into the try block

Fix also typo: replace signal.signal(0) with signal.alarm(0)

* Move another signal.alarm() into the try block

1 files changed, 22 insertions, 18 deletions

diff --git a/Lib/test/test_selectors.py b/Lib/test/test_selectors.py
index f2594a6b12..79ac25c12d 100644
--- a/Lib/test/test_selectors.py
+++ b/Lib/test/test_selectors.py
@@ -399,17 +399,19 @@ class BaseSelectorTestCase(unittest.TestCase):
 
         orig_alrm_handler = signal.signal(signal.SIGALRM, handler)
         self.addCleanup(signal.signal, signal.SIGALRM, orig_alrm_handler)
-        self.addCleanup(signal.alarm, 0)
 
-        signal.alarm(1)
+        try:
+            signal.alarm(1)
 
-        s.register(rd, selectors.EVENT_READ)
-        t = time()
-        # select() is interrupted by a signal which raises an exception
-        with self.assertRaises(InterruptSelect):
-            s.select(30)
-        # select() was interrupted before the timeout of 30 seconds
-        self.assertLess(time() - t, 5.0)
+            s.register(rd, selectors.EVENT_READ)
+            t = time()
+            # select() is interrupted by a signal which raises an exception
+            with self.assertRaises(InterruptSelect):
+                s.select(30)
+            # select() was interrupted before the timeout of 30 seconds
+            self.assertLess(time() - t, 5.0)
+        finally:
+            signal.alarm(0)
 
     @unittest.skipUnless(hasattr(signal, "alarm"),
                          "signal.alarm() required for this test")
@@ -421,17 +423,19 @@ class BaseSelectorTestCase(unittest.TestCase):
 
         orig_alrm_handler = signal.signal(signal.SIGALRM, lambda *args: None)
         self.addCleanup(signal.signal, signal.SIGALRM, orig_alrm_handler)
-        self.addCleanup(signal.alarm, 0)
 
-        signal.alarm(1)
+        try:
+            signal.alarm(1)
 
-        s.register(rd, selectors.EVENT_READ)
-        t = time()
-        # select() is interrupted by a signal, but the signal handler doesn't
-        # raise an exception, so select() should by retries with a recomputed
-        # timeout
-        self.assertFalse(s.select(1.5))
-        self.assertGreaterEqual(time() - t, 1.0)
+            s.register(rd, selectors.EVENT_READ)
+            t = time()
+            # select() is interrupted by a signal, but the signal handler doesn't
+            # raise an exception, so select() should by retries with a recomputed
+            # timeout
+            self.assertFalse(s.select(1.5))
+            self.assertGreaterEqual(time() - t, 1.0)
+        finally:
+            signal.alarm(0)
 
 
 class ScalableSelectorMixIn: