Worker: patching asyncio to avoid an occasional crash

Thanks to David Keeney for helping out with this.

packages/flamenco-worker-python/flamenco_worker/cli.py

@@ -24,6 +24,10 @@ def main():
     log = logging.getLogger(__name__)
     log.debug('Starting')
 
+    # Patch AsyncIO
+    from . import patch_asyncio
+    patch_asyncio.patch_asyncio()
+
     # Construct the AsyncIO loop
     loop = construct_asyncio_loop()
     if args.verbose:

packages/flamenco-worker-python/flamenco_worker/patch_asyncio.py

+"""
+Patches a safer version of resume_reading into the asyncio.unix_events._UnixReadPipeTransport class.
+
+This prevents an error at the end of a subprocess execution:
+
+    File "/usr/lib/python3.x/asyncio/unix_events.py", line 364, in resume_reading
+       self._loop.add_reader(self._fileno, self._read_ready)
+    AttributeError: 'NoneType' object has no attribute 'add_reader'
+
+"""
+
+import asyncio.unix_events as ue
+
+
+def patch_asyncio():
+    import logging
+
+    log = logging.getLogger(__name__)
+    log.debug('Patching ue._UnixReadPipeTransport.resume_reading')
+
+    orig_resume_reading = ue._UnixReadPipeTransport.resume_reading
+
+    def resume_reading(self, *args, **kwargs):
+        if not self._loop:
+            return
+
+        return orig_resume_reading(*args, **kwargs)
+
+    ue._UnixReadPipeTransport.resume_reading = resume_reading