restreamer: coalesce concurrent requests for the same playlist URL

restreamer/restreamer/main.py

@@ -10,6 +10,7 @@ from uuid import uuid4
 
 import gevent
 import gevent.backdoor
+import gevent.event
 import prometheus_client as prom
 from flask import Flask, url_for, request, abort, Response
 from gevent.pywsgi import WSGIServer
@@ -232,6 +233,14 @@ def generate_master_playlist(channel):
 	return generate_hls.generate_master(playlists)
 
 
+# Generating large media playlists is expensive, especially on the first run
+# where the cache is cold. And the video player will make repeated requests.
+# To avoid requests piling up and repeating work, if we get the exact same request again
+# while the old request is in progress, we piggyback on the previous request and return
+# the same result.
+# This cache object maps (hour_path, start, end) to an AsyncResult.
+_media_playlist_cache = {}
+
 @app.route('/playlist/<channel>/<quality>.m3u8')
 @request_stats
 @has_path_args
@@ -270,7 +279,22 @@ def generate_media_playlist(channel, quality):
 	# (not an error because someone might ask for a specific start, no end, but we ended up with
 	# end before start because that's the latest time we have)
 	if start < end:
-		segments = get_best_segments(hours_path, start, end)
+		cache_key = (hours_path, start, end)
+		if cache_key in _media_playlist_cache:
+			segments = _media_playlist_cache[cache_key].get()
+		else:
+			result = gevent.event.AsyncResult()
+			try:
+				# Note we don't populate the cache until we're in the try block,
+				# so there is no point where an exception won't be transferred to the result.
+				_media_playlist_cache[cache_key] = result
+				segments = list(get_best_segments(hours_path, start, end))
+				result.set(segments)
+			except BaseException as ex:
+				result.set_exception(ex)
+				raise
+			# Now we're done, remove the async result so a fresh request can start.
+			assert _media_playlist_cache.pop(cache_key) is result, "Got someone else's AsyncResult"
 	else:
 		# Note the None to indicate there was a "hole" at both start and end
 		segments = [None]