bpo-31731: Fix test_io.check_interrupted_write()

Lib/test/test_io.py

@@ -4149,18 +4149,24 @@ def check_interrupted_write(self, item, bytes, **fdopen_kwargs):
         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()
         fdopen_kwargs["closefd"] = False
         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

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.