From 7e1cc8ce730fafa3f02ad0d11e0692b62ab4ba3b Mon Sep 17 00:00:00 2001
From: Mathieu Sornay <msornay@users.noreply.github.com>
Date: Fri, 9 Jun 2017 22:17:40 +0200
Subject: [PATCH] Fix waiter cancellation in asyncio.Lock (#1031)

Avoid a deadlock when the waiter who is about to take the lock is
cancelled

Issue #27585
---
 Lib/asyncio/locks.py                | 17 ++++++++++++-----
 Lib/test/test_asyncio/test_locks.py | 22 ++++++++++++++++++++++
 Misc/NEWS                           |  3 +++
 3 files changed, 37 insertions(+), 5 deletions(-)

diff --git a/Lib/asyncio/locks.py b/Lib/asyncio/locks.py
index deefc938ecfb01..92661830a06228 100644
--- a/Lib/asyncio/locks.py
+++ b/Lib/asyncio/locks.py
@@ -176,6 +176,10 @@ def acquire(self):
             yield from fut
             self._locked = True
             return True
+        except futures.CancelledError:
+            if not self._locked:
+                self._wake_up_first()
+            raise
         finally:
             self._waiters.remove(fut)
 
@@ -192,14 +196,17 @@ def release(self):
         """
         if self._locked:
             self._locked = False
-            # Wake up the first waiter who isn't cancelled.
-            for fut in self._waiters:
-                if not fut.done():
-                    fut.set_result(True)
-                    break
+            self._wake_up_first()
         else:
             raise RuntimeError('Lock is not acquired.')
 
+    def _wake_up_first(self):
+        """Wake up the first waiter who isn't cancelled."""
+        for fut in self._waiters:
+            if not fut.done():
+                fut.set_result(True)
+                break
+
 
 class Event:
     """Asynchronous equivalent to threading.Event.
diff --git a/Lib/test/test_asyncio/test_locks.py b/Lib/test/test_asyncio/test_locks.py
index 152948c8138975..c85e8b1a32f73a 100644
--- a/Lib/test/test_asyncio/test_locks.py
+++ b/Lib/test/test_asyncio/test_locks.py
@@ -176,6 +176,28 @@ def lockit(name, blocker):
         self.assertTrue(tb.cancelled())
         self.assertTrue(tc.done())
 
+    def test_finished_waiter_cancelled(self):
+        lock = asyncio.Lock(loop=self.loop)
+
+        ta = asyncio.Task(lock.acquire(), loop=self.loop)
+        test_utils.run_briefly(self.loop)
+        self.assertTrue(lock.locked())
+
+        tb = asyncio.Task(lock.acquire(), loop=self.loop)
+        test_utils.run_briefly(self.loop)
+        self.assertEqual(len(lock._waiters), 1)
+
+        # Create a second waiter, wake up the first, and cancel it.
+        # Without the fix, the second was not woken up.
+        tc = asyncio.Task(lock.acquire(), loop=self.loop)
+        lock.release()
+        tb.cancel()
+        test_utils.run_briefly(self.loop)
+
+        self.assertTrue(lock.locked())
+        self.assertTrue(ta.done())
+        self.assertTrue(tb.cancelled())
+
     def test_release_not_acquired(self):
         lock = asyncio.Lock(loop=self.loop)
 
diff --git a/Misc/NEWS b/Misc/NEWS
index 78ec9f0ccae64a..af50b0f96b568b 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -56,6 +56,9 @@ Extension Modules
 Library
 -------
 
+- bpo-27585: Fix waiter cancellation in asyncio.Lock.
+  Patch by Mathieu Sornay.
+
 - bpo-30418: On Windows, subprocess.Popen.communicate() now also ignore EINVAL
   on stdin.write() if the child process is still running but closed the pipe.