downloader: Create concept of an "important" channel

In our usage, we have one channel where we really care / want to know if it's down,
but also a bunch of other channels where they're expected to not be streaming most/all of the time.

To prevent these extra channels making a ton of noise, we introduce the concept of an "important"
channel, indicated by appending a '!' to the channel name in the command line.

So for example, you might specify channels as "foo! foo_backup foo_behindthescenes".

Important channels have the same behaviour as previously.

Non-important channels:
* Have a 20-second retry on a master playlist fetch failure, instead of 5
* Log at debug when the stream is down, instead of info.

downloader/downloader/main.py

@@ -109,11 +109,14 @@ class StreamsManager(object):
 	* A worker is older than MAX_WORKER_AGE
 	"""
 
-	FETCH_MIN_INTERVAL = 5
+	# Important streams are retried more aggressively when down
+	IMPORTANT_FETCH_MIN_INTERVAL = 5
+	FETCH_MIN_INTERVAL = 20
+
 	FETCH_TIMEOUTS = 5, 30
 	MAX_WORKER_AGE = 20*60*60 # 20 hours, twitch's media playlist links expire after 24 hours
 
-	def __init__(self, channel, base_dir, qualities):
+	def __init__(self, channel, base_dir, qualities, important=False):
 		self.channel = channel
 		self.logger = logging.getLogger("StreamsManager({})".format(channel))
 		self.base_dir = base_dir
@@ -122,6 +125,9 @@ class StreamsManager(object):
 		self.latest_urls_changed = gevent.event.Event() # set when latest_urls changes
 		self.refresh_needed = gevent.event.Event() # set to tell main loop to refresh now
 		self.stopping = gevent.event.Event() # set to tell main loop to stop
+		self.important = important
+		if self.important:
+			self.FETCH_MIN_INTERVAL = self.IMPORTANT_FETCH_MIN_INTERVAL
 
 	def mark_working(self, worker):
 		"""Notify the manager that the given worker is up and running,
@@ -219,7 +225,9 @@ class StreamsManager(object):
 					self.start_worker(quality)
 		except Exception as e:
 			if isinstance(e, requests.HTTPError) and e.response is not None and e.response.status_code == 404:
-				self.logger.info("Stream is not up. Retrying.")
+				# Log about important streams being down at info, but others at debug.
+				level = logging.INFO if self.important else logging.DEBUG
+				self.logger.log(level, "Stream is not up. Retrying.")
 				self.trigger_refresh()
 			else:
 				self.logger.exception("Failed to fetch master playlist")
@@ -563,12 +571,15 @@ class SegmentGetter(object):
 			stat.set(max(stat._value.get(), timestamp)) # NOTE: not thread-safe but is gevent-safe
 
 
-@argh.arg('channels', nargs="+", help="Twitch channels to watch")
+@argh.arg('channels', nargs="+", help=
+	"Twitch channels to watch. Add a '!' suffix to indicate they're expected to be always up. "
+	"This affects retry interval, error reporting and monitoring."
+)
 def main(channels, base_dir=".", qualities="source", metrics_port=8001, backdoor_port=0):
 	qualities = qualities.split(",") if qualities else []
 
 	managers = [
-		StreamsManager(channel, base_dir, qualities)
+		StreamsManager(channel.rstrip('!'), base_dir, qualities, important=channel.endswith('!'))
 		for channel in channels
 	]