cutter: Implement TranscodeChecker

It runs on an interval, fetching all videos in TRANSCODING from the DB,
checking them against youtube, and then updating any that are done.

It should be noted that youtube somewhat lies about what being "done" means,
but this is a better approximation than nothing.

cutter/cutter/main.py

@@ -31,6 +31,9 @@ class Cutter(object):
 
 
 class TranscodeChecker(object):
+	NO_VIDEOS_RETRY_INTERVAL = 5
+	ERROR_RETRY_INTERVAL = 5
+
 	def __init__(self, youtube, conn, stop):
 		"""
 		youtube is an authenticated and initialized youtube api client.
@@ -42,13 +45,60 @@ class TranscodeChecker(object):
 		self.stop = stop
 		self.logger = logging.getLogger(type(self).__name__)
 
+	def wait(self, interval):
+		"""Wait for INTERVAL with jitter, unless we're stopping"""
+		self.stop.wait(common.jitter(interval))
+
 	def run(self):
 		while not self.stop.is_set():
-			pass
+			try:
+				ids = self.get_ids_to_check()
+				if not ids:
+					self.wait(self.NO_VIDEOS_RETRY_INTERVAL)
+					continue
+				self.logger.info("Found {} videos in TRANSCODING".format(len(ids)))
+				ids = self.check_ids(ids)
+				if not ids:
+					self.wait(self.NO_VIDEOS_RETRY_INTERVAL)
+					continue
+				self.logger.info("{} videos are done".format(len(ids)))
+				done = self.mark_done(ids)
+				self.logger.info("Marked {} videos as done".format(done))
+			except Exception:
+				self.logger.exception("Error in TranscodeChecker")
+				self.wait(self.ERROR_RETRY_INTERVAL)
+
+	def get_ids_to_check(self):
+		result = query(self.conn, """
+			SELECT id, video_id
+			FROM events
+			WHERE state = 'TRANSCODING'
+		""")
+		return {id: video_id for id, video_id in result.fetchall()}
+
+	def check_ids(self, ids):
+		# Future work: Set error in DB if video id is not present,
+		# and/or try to get more info from yt about what's wrong.
+		statuses = self.youtube.get_video_status(ids.values())
+		return {
+			id: video_id for id, video_id in ids.items()
+			if statuses.get(video_id) == 'processed'
+		}
+
+	def mark_done(self, ids):
+		result = query(self.conn, """
+			UPDATE events
+			SET state = 'DONE'
+			WHERE id = ANY (%s::uuid[]) AND state = 'TRANSCODING'
+		""", ids.keys())
+		return result.rowcount
 
 
 def main(dbconnect, youtube_creds_file, metrics_port=8003, backdoor_port=0):
-	"""
+	"""dbconnect should be a postgres connection string, which is either a space-separated
+	list of key=value pairs, or a URI like:
+		postgresql://USER:PASSWORD@HOST/DBNAME?KEY=VALUE
+
 	youtube_creds_file should be a json file containing keys 'client_id', 'client_secret' and 'refresh_token'.
 	"""
 	common.PromLogCountsHandler.install()