downloader: Pool connections when we can

To preserve independence between workers and ensure that a
retry (a worker re-create) actually starts from scratch, we only pool connections
on a per-worker basis.
Furthermore, for the same reason, we only let SegmentGetters use the worker's
pool on their first attempt. After that, they create a new pool to ensure they have a clean retry.

Despite this, the result should be that we're almost always re-using an existing connection
when getting segments or media playlists, unless something goes wrong.

SSL connection setup was measured as almost half the CPU time used by the process,
so this change should result in a signifigant CPU usage reduction.

downloader/downloader/main.py

@@ -265,6 +265,12 @@ class StreamWorker(object):
 		self.stopping = gevent.event.Event() # set to stop main loop
 		self.getters = {} # map from url to SegmentGetter
 		self.done = gevent.event.Event() # set when stopped and all getters are done
+		# Set up a Session for connection pooling. Note that if we have an issue,
+		# a new worker is created and so it gets a new session, just in case there's a problem
+		# with our connection pool.
+		# This worker's SegmentGetters will use its session by default for performance,
+		# but will fall back to a new one if something goes wrong.
+		self.session = requests.Session()
 
 	def __repr__(self):
 		return "<{} at 0x{:x} for stream {!r}>".format(type(self).__name__, id(self), self.stream)
@@ -307,7 +313,7 @@ class StreamWorker(object):
 			self.logger.debug("Getting media playlist {}".format(self.url))
 			try:
 				with soft_hard_timeout(self.logger, "getting media playlist", self.FETCH_TIMEOUTS, self.trigger_new_worker):
-					playlist = twitch.get_media_playlist(self.url)
+					playlist = twitch.get_media_playlist(self.url, session=self.session)
 			except Exception as e:
 				self.logger.warning("Failed to fetch media playlist {}".format(self.url), exc_info=True)
 				self.trigger_new_worker()
@@ -332,7 +338,15 @@ class StreamWorker(object):
 				if segment.uri not in self.getters:
 					if date is None:
 						raise ValueError("Cannot determine date of segment")
-					self.getters[segment.uri] = SegmentGetter(self.logger, self.manager.base_dir, self.manager.channel, self.stream, segment, date)
+					self.getters[segment.uri] = SegmentGetter(
+						self.logger,
+						self.session,
+						self.manager.base_dir,
+						self.manager.channel,
+						self.stream,
+						segment,
+						date,
+					)
 					gevent.spawn(self.getters[segment.uri].run)
 				if date is not None:
 					date += datetime.timedelta(seconds=segment.duration)
@@ -376,7 +390,7 @@ class SegmentGetter(object):
 	FETCH_HEADERS_TIMEOUTS = 5, 30
 	FETCH_FULL_TIMEOUTS = 15, 240
 
-	def __init__(self, parent_logger, base_dir, channel, stream, segment, date):
+	def __init__(self, parent_logger, session, base_dir, channel, stream, segment, date):
 		self.logger = parent_logger.getChild("SegmentGetter@{:x}".format(id(self)))
 		self.base_dir = base_dir
 		self.channel = channel
@@ -386,6 +400,8 @@ class SegmentGetter(object):
 		self.prefix = self.make_path_prefix()
 		self.retry = None # Event, set to begin retrying
 		self.done = gevent.event.Event() # set when file exists or we give up
+		# Our parent's connection pool, but we'll replace it if there's any issues
+		self.session = session
 
 	def run(self):
 		try:
@@ -410,6 +426,9 @@ class SegmentGetter(object):
 			# If worker has returned, and return value is true, we're done
 			if worker.ready() and worker.value:
 				break
+			# Create a new session, so we don't reuse a connection from the old session
+			# which had an error / some other issue. This is mostly just out of paranoia.
+			self.session = requests.Session()
 			# if retry not set, wait for FETCH_RETRY first
 			self.retry.wait(common.jitter(self.FETCH_RETRY))
 		self.logger.debug("Getter is done")
@@ -470,7 +489,7 @@ class SegmentGetter(object):
 			self.logger.debug("Downloading segment {} to {}".format(self.segment, temp_path))
 			with soft_hard_timeout(self.logger, "getting and writing segment", self.FETCH_FULL_TIMEOUTS, retry.set):
 				with soft_hard_timeout(self.logger, "getting segment headers", self.FETCH_HEADERS_TIMEOUTS, retry.set):
-					resp = requests.get(self.segment.uri, stream=True)
+					resp = self.session.get(self.segment.uri, stream=True)
 				# twitch returns 403 for expired segment urls, and 404 for very old urls where the original segment is gone.
 				# the latter can happen if we have a network issue that cuts us off from twitch for some time.
 				if resp.status_code in (403, 404):

downloader/downloader/twitch.py

@@ -10,9 +10,9 @@ import hls_playlist
 logger = logging.getLogger(__name__)
 
 
-def get_master_playlist(channel):
+def get_master_playlist(channel, session=requests):
 	"""Get the master playlist for given channel from twitch"""
-	resp = requests.get(
+	resp = session.get(
 		"https://api.twitch.tv/api/channels/{}/access_token.json".format(channel),
 		params={'as3': 't'},
 		headers={
@@ -22,7 +22,7 @@ def get_master_playlist(channel):
 	)
 	resp.raise_for_status() # getting access token
 	token = resp.json()
-	resp = requests.get(
+	resp = session.get(
 		"https://usher.ttvnw.net/api/channel/hls/{}.m3u8".format(channel),
 		params={
 			# Taken from streamlink. Unsure what's needed and what changing things can do.
@@ -93,7 +93,7 @@ def get_media_playlist_uris(master_playlist, target_qualities):
 	return {name: variant.uri for name, variant in variants.items()}
 
 
-def get_media_playlist(uri):
+def get_media_playlist(uri, session=requests):
-	resp = requests.get(uri)
+	resp = session.get(uri)
 	resp.raise_for_status()
 	return hls_playlist.load(resp.text, base_uri=resp.url)