downloader: Support watching multiple channels

This is useful eg. for watching db_admin or other testing channels in addition to the main channel.

downloader/downloader/main.py

@@ -10,6 +10,7 @@ import uuid
 from base64 import b64encode
 from contextlib import contextmanager
 
+import argh
 import dateutil.parser
 import gevent
 import gevent.backdoor
@@ -543,15 +544,44 @@ class SegmentGetter(object):
 			segments_downloaded.labels(partial="False", stream=self.channel, variant=self.stream).inc()
 
 
-def main(channel, base_dir=".", qualities="source", metrics_port=8001, backdoor_port=0):
+@argh.arg('channels', nargs="+", help="Twitch channels to watch")
+def main(channels, base_dir=".", qualities="source", metrics_port=8001, backdoor_port=0):
 	qualities = qualities.split(",") if qualities else []
-	manager = StreamsManager(channel, base_dir, qualities)
-	gevent.signal(signal.SIGTERM, manager.stop) # shut down on sigterm
+
+	managers = [
+		StreamsManager(channel, base_dir, qualities)
+		for channel in channels
+	]
+
+	def stop():
+		for manager in managers:
+			manager.stop()
+
+	gevent.signal(signal.SIGTERM, stop) # shut down on sigterm
+
 	common.PromLogCountsHandler.install()
 	common.install_stacksampler()
 	prom.start_http_server(metrics_port)
+
 	if backdoor_port:
 		gevent.backdoor.BackdoorServer(('127.0.0.1', backdoor_port), locals=locals()).start()
+
 	logging.info("Starting up")
-	manager.run()
+
+	workers = [gevent.spawn(manager.run) for manager in managers]
+	# Wait for any to die
+	gevent.wait(workers, count=1)
+	# If one has stopped, either:
+	# 1. stop() was called and all are stopping
+	# 2. one errored and we should stop all remaining and report the error
+	# Our behaviour in both cases is the same:
+	# 1. Tell all managers to gracefully stop
+	stop()
+	# 2. Wait (with timeout) until they've stopped
+	gevent.wait(workers)
+	# 3. Check if any of them failed. If they did, report it. If mulitple failed, we report
+	#    one arbitrarily.
+	for worker in workers:
+		worker.get() # re-raise error if failed
+
 	logging.info("Gracefully stopped")