AsynchronousTask: add async_wait() method (bug 653856)

Since the AsynchronousTask.wait() method is prone to event loop
recursion, deprecate it, and add an async_wait() method method to
replace it. Instead of using task.wait() in order to implicitly run
the event loop, now loop.run_until_complete(task.async_wait()) will
be used to explicitly run the event loop. This explicit approach will
make it more obvious when code will trigger event loop recursion
which would not be compatible with asyncio's default event loop.

Bug: https://bugs.gentoo.org/653856

2 files changed, 24 insertions, 1 deletions

diff --git a/pym/_emerge/AsynchronousTask.py b/pym/_emerge/AsynchronousTask.py
index e29324440..7d2e6253b 100644
--- a/pym/_emerge/AsynchronousTask.py
+++ b/pym/_emerge/AsynchronousTask.py
@@ -29,6 +29,26 @@ class AsynchronousTask(SlotObject):
 		self._start_hook()
 		self._start()
 
+	def async_wait(self):
+		"""
+		Wait for returncode asynchronously. Notification is available
+		via the add_done_callback method of the returned Future instance.
+
+		@returns: Future, result is self.returncode
+		"""
+		waiter = self.scheduler.create_future()
+		exit_listener = lambda self: waiter.set_result(self.returncode)
+		self.addExitListener(exit_listener)
+		waiter.add_done_callback(lambda waiter:
+			self.removeExitListener(exit_listener) if waiter.cancelled() else None)
+		if self.returncode is not None:
+			# If the returncode is not None, it means the exit event has already
+			# happened, so use _async_wait() to guarantee that the exit_listener
+			# is called. This does not do any harm because a given exit listener
+			# is never called more than once.
+			self._async_wait()
+		return waiter
+
 	def _start(self):
 		self.returncode = os.EX_OK
 		self.wait()
@@ -47,6 +67,9 @@ class AsynchronousTask(SlotObject):
 		return self.returncode
 
 	def wait(self):
+		"""
+		Deprecated. Use async_wait() instead.
+		"""
 		if self.returncode is None:
 			if not self._waiting:
 				self._waiting = True
diff --git a/pym/portage/tests/ebuild/test_ipc_daemon.py b/pym/portage/tests/ebuild/test_ipc_daemon.py
index bc18cdf64..e6da51a76 100644
--- a/pym/portage/tests/ebuild/test_ipc_daemon.py
+++ b/pym/portage/tests/ebuild/test_ipc_daemon.py
@@ -157,6 +157,6 @@ class IpcDaemonTestCase(TestCase):
 		try:
 			task_scheduler.start()
 			event_loop.run_until_complete(self._run_done)
-			task_scheduler.wait()
+			event_loop.run_until_complete(task_scheduler.async_wait())
 		finally:
 			timeout_handle.cancel()