[Python-checkins] bpo-35125: remove inner callback on outer cancellation in asyncio shield (GH-10340)

[Miss Islington (bot)]

https://github.com/python/cpython/commit/b35acc5b3a0148c5fd4462968b310fb436726d5a
commit: b35acc5b3a0148c5fd4462968b310fb436726d5a
branch: master
author: Romain Picard <romain.picard at oakbits.com>
committer: Miss Islington (bot) <31488909+miss-islington at users.noreply.github.com>
date: 2019-05-07T11:58:24-07:00
summary:

bpo-35125: remove inner callback on outer cancellation in asyncio shield (GH-10340)



When the future returned by shield is cancelled, its completion callback of the
inner future is not removed. This makes the callback list of inner inner future
grow each time a shield is created and cancelled.

This change unregisters the callback from the inner future when the outer
future is cancelled.



https://bugs.python.org/issue35125

files:
A Misc/NEWS.d/next/Library/2019-02-15-17-18-50.bpo-35125.h0xk0f.rst
M Lib/asyncio/tasks.py
M Lib/test/test_asyncio/test_tasks.py

diff --git a/Lib/asyncio/tasks.py b/Lib/asyncio/tasks.py
index b007b74344ed..211b9126b011 100644
--- a/Lib/asyncio/tasks.py
+++ b/Lib/asyncio/tasks.py
@@ -818,7 +818,7 @@ def shield(arg, *, loop=None):
     loop = futures._get_loop(inner)
     outer = loop.create_future()
 
-    def _done_callback(inner):
+    def _inner_done_callback(inner):
         if outer.cancelled():
             if not inner.cancelled():
                 # Mark inner's result as retrieved.
@@ -834,7 +834,13 @@ def _done_callback(inner):
             else:
                 outer.set_result(inner.result())
 
-    inner.add_done_callback(_done_callback)
+
+    def _outer_done_callback(outer):
+        if not inner.done():
+            inner.remove_done_callback(_inner_done_callback)
+
+    inner.add_done_callback(_inner_done_callback)
+    outer.add_done_callback(_outer_done_callback)
     return outer
 
 
diff --git a/Lib/test/test_asyncio/test_tasks.py b/Lib/test/test_asyncio/test_tasks.py
index c4f6d703549c..fa9783f2ff21 100644
--- a/Lib/test/test_asyncio/test_tasks.py
+++ b/Lib/test/test_asyncio/test_tasks.py
@@ -1777,7 +1777,7 @@ def test_shield_exception(self):
         test_utils.run_briefly(self.loop)
         self.assertIs(outer.exception(), exc)
 
-    def test_shield_cancel(self):
+    def test_shield_cancel_inner(self):
         inner = self.new_future(self.loop)
         outer = asyncio.shield(inner)
         test_utils.run_briefly(self.loop)
@@ -1785,6 +1785,15 @@ def test_shield_cancel(self):
         test_utils.run_briefly(self.loop)
         self.assertTrue(outer.cancelled())
 
+    def test_shield_cancel_outer(self):
+        inner = self.new_future(self.loop)
+        outer = asyncio.shield(inner)
+        test_utils.run_briefly(self.loop)
+        outer.cancel()
+        test_utils.run_briefly(self.loop)
+        self.assertTrue(outer.cancelled())
+        self.assertEqual(0, 0 if outer._callbacks is None else len(outer._callbacks))
+
     def test_shield_shortcut(self):
         fut = self.new_future(self.loop)
         fut.set_result(42)
diff --git a/Misc/NEWS.d/next/Library/2019-02-15-17-18-50.bpo-35125.h0xk0f.rst b/Misc/NEWS.d/next/Library/2019-02-15-17-18-50.bpo-35125.h0xk0f.rst
new file mode 100644
index 000000000000..2e28a25d2415
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2019-02-15-17-18-50.bpo-35125.h0xk0f.rst
@@ -0,0 +1 @@
+Asyncio: Remove inner callback on outer cancellation in shield