bpo-31731: Fix test_io.check_interrupted_write() (GH-11225)

Fix a race condition in check_interrupted_write() of test_io:
create directly the thread with SIGALRM signal blocked,
rather than blocking the signal later from the thread. Previously, it
was possible that the thread gets the signal before the signal is
blocked.
(cherry picked from commit 05c9d31eb62cc45dc3c55a5cdb7cbc713d0421db)

Co-authored-by: Victor Stinner <vstinner@redhat.com>

diff --git a/Lib/test/test_io.py b/Lib/test/test_io.py
index 4c206d287f27dbb544494bc5adef577c9c80cb9e..405f2d38b494a414450c009dc0b05033f80e104d 100644 (file)
--- a/Lib/test/test_io.py
+++ b/Lib/test/test_io.py
@@ -4121,10 +4121,9 @@ class SignalsTest(unittest.TestCase):
         in the latter."""
         read_results = []
         def _read():
-            if hasattr(signal, 'pthread_sigmask'):
-                signal.pthread_sigmask(signal.SIG_BLOCK, [signal.SIGALRM])
             s = os.read(r, 1)
             read_results.append(s)
+
         t = threading.Thread(target=_read)
         t.daemon = True
         r, w = os.pipe()
@@ -4132,7 +4131,14 @@ class SignalsTest(unittest.TestCase):
         large_data = item * (support.PIPE_MAX_SIZE // len(item) + 1)
         try:
             wio = self.io.open(w, **fdopen_kwargs)
-            t.start()
+            if hasattr(signal, 'pthread_sigmask'):
+                # create the thread with SIGALRM signal blocked
+                signal.pthread_sigmask(signal.SIG_BLOCK, [signal.SIGALRM])
+                t.start()
+                signal.pthread_sigmask(signal.SIG_UNBLOCK, [signal.SIGALRM])
+            else:
+                t.start()
+
             # Fill the pipe enough that the write will be blocking.
             # It will be interrupted by the timer armed above.  Since the
             # other thread has read one byte, the low-level write will

diff --git a/Misc/NEWS.d/next/Tests/2018-12-18-23-20-39.bpo-31731.tcv85C.rst b/Misc/NEWS.d/next/Tests/2018-12-18-23-20-39.bpo-31731.tcv85C.rst
new file mode 100644 (file)
index 0000000..530977c
--- /dev/null
+++ b/Misc/NEWS.d/next/Tests/2018-12-18-23-20-39.bpo-31731.tcv85C.rst
@@ -0,0 +1,4 @@
+Fix a race condition in ``check_interrupted_write()`` of test_io: create
+directly the thread with SIGALRM signal blocked, rather than blocking the
+signal later from the thread. Previously, it was possible that the thread gets
+the signal before the signal is blocked.