From 9f4cfb12cdeb3f949f9fa47626eef799387dfcfe Mon Sep 17 00:00:00 2001
From: Victor Stinner <vstinner@redhat.com>
Date: Tue, 18 Dec 2018 23:52:39 +0100
Subject: [PATCH] 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>
---
 Lib/test/test_io.py                                  | 12 +++++++++---
 .../Tests/2018-12-18-23-20-39.bpo-31731.tcv85C.rst   |  4 ++++
 2 files changed, 13 insertions(+), 3 deletions(-)
 create mode 100644 Misc/NEWS.d/next/Tests/2018-12-18-23-20-39.bpo-31731.tcv85C.rst

diff --git a/Lib/test/test_io.py b/Lib/test/test_io.py
index 4c206d287f27db..405f2d38b494a4 100644
--- a/Lib/test/test_io.py
+++ b/Lib/test/test_io.py
@@ -4121,10 +4121,9 @@ 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()
@@ -4132,7 +4131,14 @@ def _read():
         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
index 00000000000000..530977c6e87eb1
--- /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.