SpawnProcess: cancel _main_task in _unregister (bug 711174)

Cancel _main_task in _unregister, in order to ensure that the build
log is properly closed. Note that is analogous to the fix for bug
658806 in commit a9a62e5, but behavior related to coroutine
cancellation now plays a role since the PipeLogger is now cancelled
indirectly via _main_task. We still need to verify that coroutine
cancellation will raise an appropriate exception in the coroutine
(asyncio.CancelledError, GeneratorExit, or StopIteration) and also
that the specific coroutine will react appropriately to the
specific exception that will be raised.

Fixes: 8074127 ("SpawnProcess: add _main coroutine")
Bug: https://bugs.gentoo.org/711174
Signed-off-by: Zac Medico <zmedico@gentoo.org>

lib/_emerge/SpawnProcess.py

@@ -218,6 +218,8 @@ def _unregister(self):
 		if self.cgroup is not None:
 			self._cgroup_cleanup()
 			self.cgroup = None
+		if self._main_task is not None:
+			self._main_task.done() or self._main_task.cancel()
 
 	def _cancel(self):
 		SubProcess._cancel(self)