job.py: Only start new jobs in a `with watcher:` block

The documentation
(https://docs.python.org/3/library/asyncio-policy.html#asyncio.AbstractChildWatcher)
is apparently missing this part, but the code mentions that new
processes should only ever be called inside a with block:
https://github.com/python/cpython/blob/99eb70a9eb9493602ff6ad8bb92df4318cf05a3e/Lib/asyncio/unix_events.py#L808

1 files changed, 5 insertions, 26 deletions

diff --git a/src/buildstream/_scheduler/jobs/job.py b/src/buildstream/_scheduler/jobs/job.py
index c5c14ff0f..71c40f397 100644
--- a/src/buildstream/_scheduler/jobs/job.py
+++ b/src/buildstream/_scheduler/jobs/job.py
@@ -1,5 +1,6 @@
 #
 #  Copyright (C) 2018 Codethink Limited
+#  Copyright (C) 2019 Bloomberg Finance LP
 #
 #  This program is free software; you can redistribute it and/or
 #  modify it under the terms of the GNU Lesser General Public
@@ -184,32 +185,10 @@ class Job:
         # process will be notified of any signal after we launch the child.
         #
         with _signals.blocked([signal.SIGINT, signal.SIGTSTP, signal.SIGTERM], ignore=False):
-            self._process.start()
-
-        # Wait for the child task to complete.
-        #
-        # This is a tricky part of python which doesnt seem to
-        # make it to the online docs:
-        #
-        #  o asyncio.get_child_watcher() will return a SafeChildWatcher() instance
-        #    which is the default type of watcher, and the instance belongs to the
-        #    "event loop policy" in use (so there is only one in the main process).
-        #
-        #  o SafeChildWatcher() will register a SIGCHLD handler with the asyncio
-        #    loop, and will selectively reap any child pids which have been
-        #    terminated.
-        #
-        #  o At registration time, the process will immediately be checked with
-        #    `os.waitpid()` and will be reaped immediately, before add_child_handler()
-        #    returns.
-        #
-        # The self._parent_child_completed callback passed here will normally
-        # be called after the child task has been reaped with `os.waitpid()`, in
-        # an event loop callback. Otherwise, if the job completes too fast, then
-        # the callback is called immediately.
-        #
-        watcher = asyncio.get_child_watcher()
-        watcher.add_child_handler(self._process.pid, self._parent_child_completed)
+            with asyncio.get_child_watcher() as watcher:
+                self._process.start()
+                # Register the process to call `_parent_child_completed` once it is done
+                watcher.add_child_handler(self._process.pid, self._parent_child_completed)
 
     # terminate()
     #