generate media playlist: Yield in chunks for performance

Flask sends a chunked response with one chunk per item yielded.
This adds a lot of overhead per yielded item.
We avoid this by collecting the lines of the media playlist into larger chunks
and only flushing once every 1000 segments.

For small playlists this means they'll be emitted as one chunk,
but for large playlists we still get the streaming behaviour.

restreamer/restreamer/generate_hls.py

@@ -69,27 +69,30 @@ def generate_media(segments, base_url):
 	if segments and segments[0] is None:
 		segments = segments[1:]
 
+	lines = []
 	for i, segment in enumerate(segments):
-		# For very large playlists, give other things a chance to run
+		# For very large playlists, flush a chunk and give other things a chance to run
 		if i % 1000 == 0:
+			yield "\n".join(lines) + "\n"
+			lines = []
 			gevent.idle()
 		if segment is None:
 			# Discontinuity. Adding this tag tells the client that we've missed something
 			# and it should start decoding fresh on the next segment. This is required when
 			# someone stops/starts a stream and a good idea if we're missing a segment in a
 			# continuous stream.
-			yield "#EXT-X-DISCONTINUITY\n"
+			lines.append("#EXT-X-DISCONTINUITY")
 		else:
 			# Each segment has two prefixes: timestamp and duration.
 			# This tells the client exactly what time the segment represents, which is important
 			# for the editor since it needs to describe cut points in these times.
 			path = '/'.join(segment.path.split('/')[-2:])
-			lines = [
+			lines += [
 				"#EXT-X-PROGRAM-DATE-TIME:{}".format(segment.start.strftime("%Y-%m-%dT%H:%M:%S.%fZ")),
 				"#EXTINF:{:.3f},live".format(segment.duration.total_seconds()),
 				urllib.parse.quote(os.path.join(base_url, path)),
 			]
-			yield "\n".join(lines) + "\n"
+	yield "\n".join(lines) + "\n"
 
 	# If stream is complete, add an ENDLIST marker to show this.
 	if not incomplete: