[gentoo-portage-dev] [PATCH] Eventloop: fix deadlock involving idle_add/call_soon (bug 617550)

[Zac Medico]

Guarantee that newly added idle_add/call_soon callbacks have an
opportunity to execute before the event loop decides to wait on
self._thread_condition without a timeout. This fixes a case where
the event loop would wait on self._thread_condition indefinitely,
even though a callback scheduled by the AsynchronousTask._async_wait
method needed to be executed first.

X-Gentoo-bug: 617550
X-Gentoo-bug-url: https://bugs.gentoo.org/show_bug.cgi?id=617550
---
 pym/portage/util/_eventloop/EventLoop.py | 18 ++++++++++++++++--
 1 file changed, 16 insertions(+), 2 deletions(-)

diff --git a/pym/portage/util/_eventloop/EventLoop.py b/pym/portage/util/_eventloop/EventLoop.py
index 712838e..b7c8762 100644
--- a/pym/portage/util/_eventloop/EventLoop.py
+++ b/pym/portage/util/_eventloop/EventLoop.py
@@ -108,6 +108,15 @@ class EventLoop(object):
 		self._poll_event_handler_ids = {}
 		# Increment id for each new handler.
 		self._event_handler_id = 0
+		# New call_soon callbacks must have an opportunity to
+		# execute before it's safe to wait on self._thread_condition
+		# without a timeout, since delaying its execution indefinitely
+		# could lead to a deadlock. The following attribute stores the
+		# event handler id of the most recently added call_soon callback.
+		# If this attribute has changed since the last time that the
+		# call_soon callbacks have been called, then it's not safe to
+		# wait on self._thread_condition without a timeout.
+		self._call_soon_id = 0
 		# Use OrderedDict in order to emulate the FIFO queue behavior
 		# of the AbstractEventLoop.call_soon method.
 		self._idle_callbacks = OrderedDict()
@@ -250,10 +259,15 @@ class EventLoop(object):
 
 		if not event_handlers:
 			with self._thread_condition:
+				prev_call_soon_id = self._call_soon_id
 				if self._run_timeouts():
 					events_handled += 1
 				timeouts_checked = True
-				if not event_handlers and not events_handled and may_block:
+
+				call_soon = bool(prev_call_soon_id != self._call_soon_id)
+
+				if (not call_soon and not event_handlers
+					and not events_handled and may_block):
 					# Block so that we don't waste cpu time by looping too
 					# quickly. This makes EventLoop useful for code that needs
 					# to wait for timeout callbacks regardless of whether or
@@ -457,7 +471,7 @@ class EventLoop(object):
 		@return: an integer ID
 		"""
 		with self._thread_condition:
-			source_id = self._new_source_id()
+			source_id = self._call_soon_id = self._new_source_id()
 			self._idle_callbacks[source_id] = self._idle_callback_class(
 				args=args, callback=callback, source_id=source_id)
 			self._thread_condition.notify()
-- 
2.10.2