Taskgroup tweaks (GH-31559)

Now uses .cancel()/.uncancel(), for even fewer broken edge cases.

Author: Tinche

Lib/asyncio/taskgroups.py

@@ -66,31 +66,28 @@ async def __aexit__(self, et, exc, tb):
                 self._base_error is None):
             self._base_error = exc
 
-        if et is exceptions.CancelledError:
-            if self._parent_cancel_requested:
-                # Only if we did request task to cancel ourselves
-                # we mark it as no longer cancelled.
-                self._parent_task.uncancel()
-            else:
-                propagate_cancellation_error = et
-
-        if et is not None and not self._aborting:
-            # Our parent task is being cancelled:
-            #
-            #    async with TaskGroup() as g:
-            #        g.create_task(...)
-            #        await ...  # <- CancelledError
-            #
+        if et is not None:
             if et is exceptions.CancelledError:
-                propagate_cancellation_error = et
-
-            # or there's an exception in "async with":
-            #
-            #    async with TaskGroup() as g:
-            #        g.create_task(...)
-            #        1 / 0
-            #
-            self._abort()
+                if self._parent_cancel_requested and not self._parent_task.uncancel():
+                    # Do nothing, i.e. swallow the error.
+                    pass
+                else:
+                    propagate_cancellation_error = exc
+
+            if not self._aborting:
+                # Our parent task is being cancelled:
+                #
+                #    async with TaskGroup() as g:
+                #        g.create_task(...)
+                #        await ...  # <- CancelledError
+                #
+                # or there's an exception in "async with":
+                #
+                #    async with TaskGroup() as g:
+                #        g.create_task(...)
+                #        1 / 0
+                #
+                self._abort()
 
         # We use while-loop here because "self._on_completed_fut"
         # can be cancelled multiple times if our parent task
@@ -118,7 +115,6 @@ async def __aexit__(self, et, exc, tb):
             self._on_completed_fut = None
 
         assert self._unfinished_tasks == 0
-        self._on_completed_fut = None  # no longer needed
 
         if self._base_error is not None:
             raise self._base_error
@@ -199,8 +195,7 @@ def _on_task_done(self, task):
             })
             return
 
-        self._abort()
-        if not self._parent_task.cancelling():
+        if not self._aborting and not self._parent_cancel_requested:
             # If parent task *is not* being cancelled, it means that we want
             # to manually cancel it to abort whatever is being run right now
             # in the TaskGroup.  But we want to mark parent task as
@@ -219,5 +214,6 @@ def _on_task_done(self, task):
             #            pass
             #        await something_else     # this line has to be called
             #                                 # after TaskGroup is finished.
+            self._abort()
             self._parent_cancel_requested = True
             self._parent_task.cancel()

Lib/test/test_asyncio/test_taskgroups.py

@@ -120,7 +120,11 @@ async def runner():
         self.assertTrue(t2_cancel)
         self.assertTrue(t2.cancelled())
 
-    async def test_taskgroup_05(self):
+    async def test_cancel_children_on_child_error(self):
+        """
+        When a child task raises an error, the rest of the children
+        are cancelled and the errors are gathered into an EG.
+        """
 
         NUM = 0
         t2_cancel = False
@@ -165,7 +169,7 @@ async def runner():
         self.assertTrue(t2_cancel)
         self.assertTrue(runner_cancel)
 
-    async def test_taskgroup_06(self):
+    async def test_cancellation(self):
 
         NUM = 0
 
@@ -186,10 +190,12 @@ async def runner():
         await asyncio.sleep(0.1)
 
         self.assertFalse(r.done())
-        r.cancel()
-        with self.assertRaises(asyncio.CancelledError):
+        r.cancel("test")
+        with self.assertRaises(asyncio.CancelledError) as cm:
             await r
 
+        self.assertEqual(cm.exception.args, ('test',))
+
         self.assertEqual(NUM, 5)
 
     async def test_taskgroup_07(self):
@@ -226,7 +232,7 @@ async def runner():
 
         self.assertEqual(NUM, 15)
 
-    async def test_taskgroup_08(self):
+    async def test_cancellation_in_body(self):
 
         async def foo():
             await asyncio.sleep(0.1)
@@ -246,10 +252,12 @@ async def runner():
         await asyncio.sleep(0.1)
 
         self.assertFalse(r.done())
-        r.cancel()
-        with self.assertRaises(asyncio.CancelledError):
+        r.cancel("test")
+        with self.assertRaises(asyncio.CancelledError) as cm:
             await r
 
+        self.assertEqual(cm.exception.args, ('test',))
+
     async def test_taskgroup_09(self):
 
         t1 = t2 = None
@@ -699,3 +707,7 @@ async def coro():
         async with taskgroups.TaskGroup() as g:
             t = g.create_task(coro(), name="yolo")
             self.assertEqual(t.get_name(), "yolo")
+
+
+if __name__ == "__main__":
+    unittest.main()