Fix canceling of a gather operation

asyncio/core.py

@@ -114,11 +114,11 @@ def __next__(self):
             self.state = None
             return None
         else:
-           self.exc.__traceback__ = None
-           raise self.exc
+            self.exc.__traceback__ = None
+            raise self.exc
 
-_never = _Never()
 
+_never = _Never()
 
 ################################################################################
 # Queue and poller for stream IO
@@ -260,6 +260,11 @@ def run_until_complete(main_task=None):
                 if t.state is True:
                     # "None" indicates that the task is complete and not await'ed on (yet).
                     t.state = None
+                elif callable(t.state):
+                    # The task has a callback registered to be called on completion.
+                    t.state(t, er)
+                    t.state = False
+                    waiting = True
                 else:
                     # Schedule any other tasks waiting on the completion of this task.
                     while t.state.peek():
@@ -281,7 +286,6 @@ def run_until_complete(main_task=None):
                 _exc_context["future"] = t
                 Loop.call_exception_handler(_exc_context)
 
-
 # Create a new task from a coroutine and run it until it finishes
 def run(coro):
     """Create a new task from the given coroutine and run it until it completes.

asyncio/funcs.py

@@ -85,30 +85,76 @@ def wait_for_ms(aw, timeout):
     return wait_for(aw, timeout, core.sleep_ms)
 
 
-async def gather(*aws, return_exceptions=False):
-    """Run all *aws* awaitables concurrently. Any *aws* that are not tasks
-    are promoted to tasks.
+class _Remove:
+    @staticmethod
+    def remove(t):
+        pass
 
-    Returns a list of return values of all *aws*
 
-    This is a coroutine.
-    """
+async def gather(*aws, return_exceptions=False):
+    if not aws:
+        return []
+
+    def done(t, er):
+        # Sub-task "t" has finished, with exception "er".
+        nonlocal state
+        if gather_task.data is not _Remove:
+            # The main gather task has already been scheduled, so do nothing.
+            # This happens if another sub-task already raised an exception and
+            # woke the main gather task (via this done function), or if the main
+            # gather task was cancelled externally.
+            return
+        elif not return_exceptions and not isinstance(er, StopIteration):
+            # A sub-task raised an exception, indicate that to the gather task.
+            state = er
+        else:
+            state -= 1
+            if state:
+                # Still some sub-tasks running.
+                return
+        # Gather waiting is done, schedule the main gather task.
+        core._task_queue.push_head(gather_task)
 
     ts = [core._promote_to_task(aw) for aw in aws]
     for i in range(len(ts)):
-        try:
-            # TODO handle cancel of gather itself
-            # if ts[i].coro:
-            #    iter(ts[i]).waiting.push_head(cur_task)
-            #    try:
-            #        yield
-            #    except CancelledError as er:
-            #        # cancel all waiting tasks
-            #        raise er
-            ts[i] = await ts[i]
-        except (core.CancelledError, Exception) as er:
-            if return_exceptions:
-                ts[i] = er
-            else:
-                raise er
+        if ts[i].state is not True:
+            # Task is not running, gather not currently supported for this case.
+            raise RuntimeError("can't gather")
+        # Register the callback to call when the task is done.
+        ts[i].state = done
+
+    # Set the state for execution of the gather.
+    gather_task = core.cur_task
+    state = len(ts)
+    cancel_all = False
+
+    # Wait for the a sub-task to need attention.
+    gather_task.data = _Remove
+    try:
+        core._never.state = False
+        await core._never
+    except core.CancelledError as er:
+        cancel_all = True
+        state = er
+
+    # Clean up tasks.
+    for i in range(len(ts)):
+        if ts[i].state is done:
+            # Sub-task is still running, deregister the callback and cancel if needed.
+            ts[i].state = True
+            if cancel_all:
+                ts[i].cancel()
+        elif isinstance(ts[i].data, StopIteration):
+            # Sub-task ran to completion, get its return value.
+            ts[i] = ts[i].data.value
+        else:
+            # Sub-task had an exception with return_exceptions==True, so get its exception.
+            ts[i] = ts[i].data
+
+    # Either this gather was cancelled, or one of the sub-tasks raised an exception with
+    # return_exceptions==False, so reraise the exception here.
+    if state is not 0:
+        raise state
+
+    # Return the list of return values of each sub-task.
     return ts