optionally enabled debugging: Write out last 10 media playlists after fetching each segment

We have observed an issue on twitch where there will be a small time jump
(eg. with 2s segments, times will be 00:10.3, 00:12.3, 00:14.7, 00:16.7)
and all subsequent segment timestamps will be out by this amount compared
to what other downloader instances see. Our workaround for this issue is to
watch for such gaps and:
1. trigger a worker refresh (which seems to fix the issue)
2. treat all subsequent segments as "suspect" so they are still saved
but only used if no other source is available.

backfiller/backfiller/main.py

@@ -16,6 +16,7 @@ import argh
 import gevent.backdoor
 import gevent.pool
 import prometheus_client as prom
+from requests.adapters import HTTPAdapter
 
 import common
 from common import dateutil
@@ -23,8 +24,10 @@ from common import database
 from common.requests import InstrumentedSession
 from common.segments import list_segment_files, unpadded_b64_decode
 
-# Wraps all requests in some metric collection
+# Wraps all requests in some metric collection and connection pooling
 requests = InstrumentedSession()
+adapter = HTTPAdapter(pool_maxsize=100)
+requests.mount('https://', adapter)
 
 segments_backfilled = prom.Counter(
 	'segments_backfilled',
@@ -44,6 +47,12 @@ hash_mismatches = prom.Counter(
 	['remote', 'channel', 'quality', 'hour'],
 )
 
+small_difference_segments = prom.Gauge(
+	'small_difference_segments',
+	'Number of segments which were not pulled due to differing from existing segments by only a very small time difference',
+	['remote', 'channel', 'quality', 'hour'],
+)
+
 node_list_errors = prom.Counter(
 	'node_list_errors',
 	'Number of errors fetching a list of nodes',
@@ -504,9 +513,21 @@ class BackfillerWorker(object):
 				# multiple workers request the same segment at the same time
 				random.shuffle(missing_segments)
 
+				if quality != 'chat':
+					MATCH_FIELDS = ("channel", "quality", "duration", "type", "hash")
+					EPSILON = 0.001
+					local_infos = []
+					for path in local_segments:
+						path = os.path.join(channel, quality, hour, path)
+						try:
+							local_infos.append(common.parse_segment_path(path))
+						except ValueError as e:
+							self.logger.warning('Local file {} could not be parsed: {}'.format(path, e))
+	
 				pool = gevent.pool.Pool(self.download_concurrency)
 				workers = []
-	
+				small_differences = 0
+
 				for missing_segment in missing_segments:
 	
 					if self.stopping.is_set():
@@ -542,6 +563,21 @@ class BackfillerWorker(object):
 						if datetime.datetime.utcnow() - segment.start < datetime.timedelta(seconds=self.recent_cutoff):
 							self.logger.debug('Skipping {} as too recent'.format(path))
 							continue
+
+						# if any local segment is within 1ms of the missing segment and otherwise identical, ignore it
+						found = None
+						for local_segment in local_infos:
+							# if any fields differ, no match
+							if not all(getattr(segment, field) == getattr(local_segment, field) for field in MATCH_FIELDS):
+								continue
+							# if time difference > epsilon, no match
+							if abs((segment.start - local_segment.start).total_seconds()) > EPSILON:
+								continue
+							found = local_segment
+							break
+						if found is not None:
+							self.logger.debug(f'Skipping {path} as within {EPSILON}s of identical segment {found.path}')
+							continue
 	
 					# start segment as soon as a pool slot opens up, then track it in workers
 					workers.append(pool.spawn(
@@ -549,6 +585,8 @@ class BackfillerWorker(object):
 						self.base_dir, self.node, channel, quality, hour, missing_segment, self.logger
 					))
 
+				small_difference_segments.labels(self.node, channel, quality, hour).set(small_differences)
+
 				# verify that all the workers succeeded. if any failed, raise the exception from
 				# one of them arbitrarily.
 				for worker in workers:

downloader/downloader/main.py

@@ -15,6 +15,7 @@ import gevent.backdoor
 import gevent.event
 import prometheus_client as prom
 import requests
+import requests.adapters
 from monotonic import monotonic
 
 import common
@@ -49,6 +50,24 @@ ad_segments_ignored = prom.Counter(
 	["channel", "quality"],
 )
 
+suspicious_skew_count = prom.Counter(
+	"suspicious_skew_count",
+	"Number of times we've restarted a worker due to suspicious skew",
+	["channel", "quality"],
+)
+
+segment_time_skew_non_zero_sum = prom.Gauge(
+	"segment_time_skew_non_zero_sum",
+	"Sum of all observed segment skew amounts for worker",
+	["channel", "quality", "worker"],
+)
+
+segment_time_skew_non_zero_count = prom.Counter(
+	"segment_time_skew_non_zero_count",
+	"Count of segments with non-zero skew for worker",
+	["channel", "quality", "worker"],
+)
+
 
 class TimedOutError(Exception):
 	pass
@@ -122,7 +141,7 @@ class StreamsManager(object):
 
 	FETCH_TIMEOUTS = 5, 30
 
-	def __init__(self, provider, channel, base_dir, qualities, important=False):
+	def __init__(self, provider, channel, base_dir, qualities, important=False, history_size=0):
 		self.provider = provider
 		self.channel = channel
 		self.logger = logging.getLogger("StreamsManager({})".format(channel))
@@ -203,7 +222,7 @@ class StreamsManager(object):
 			self.logger.info("Ignoring worker start as we are stopping")
 			return
 		url_time, url = self.latest_urls[quality]
-		worker = StreamWorker(self, quality, url, url_time)
+		worker = StreamWorker(self, quality, url, url_time, self.history_size)
 		self.stream_workers[quality].append(worker)
 		gevent.spawn(worker.run)
 
@@ -290,7 +309,12 @@ class StreamWorker(object):
 	FETCH_RETRY_INTERVAL = 1
 	FETCH_POLL_INTERVAL = 2
 
-	def __init__(self, manager, quality, url, url_time):
+	# Max difference between a segment's time + duration and the next segment's time (in seconds)
+	# before we consider this "suspicious" and trigger a refresh.
+	# See https://github.com/dbvideostriketeam/wubloader/issues/539
+	MAX_SEGMENT_TIME_SKEW = 0.01
+
+	def __init__(self, manager, quality, url, url_time, history_size):
 		self.manager = manager
 		self.logger = manager.logger.getChild("StreamWorker({})@{:x}".format(quality, id(self)))
 		self.quality = quality
@@ -305,11 +329,16 @@ class StreamWorker(object):
 		# 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 = common.requests.InstrumentedSession()
+		adapter = requests.adapters.HTTPAdapter(pool_maxsize=100)
+		self.session.mount('https://', adapter)
 		# Map cache is a simple cache to avoid re-downloading the same map URI for every segment,
 		# since it's generally the same but may occasionally change.
 		# We expect the map data to be very small so there is no eviction here.
 		# {uri: data}
 		self.map_cache = {}
+		# If enabled, playlist history is saved after each segment fetch,
+		# showing the last N playlist fetches up until the one that resulted in that fetch.
+		self.history_size = history_size
 
 	def __repr__(self):
 		return "<{} at 0x{:x} for stream {!r}>".format(type(self).__name__, id(self), self.quality)
@@ -347,12 +376,15 @@ class StreamWorker(object):
 
 	def _run(self):
 		first = True
+		suspicious_skew = False
+		history = []
 		while not self.stopping.is_set():
 
 			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 = self.manager.provider.get_media_playlist(self.url, session=self.session)
+					playlist_time = datetime.datetime.utcnow()
+					raw_playlist, playlist = self.manager.provider.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()
@@ -368,8 +400,12 @@ class StreamWorker(object):
 			# We successfully got the playlist at least once
 			first = False
 
+			if self.history_size > 0:
+				history = [(playlist_time, raw_playlist)] + history[:self.history_size]
+
 			# Start any new segment getters
 			date = None # tracks date in case some segment doesn't include it
+			prev_segment = None
 			for segment in playlist.segments:
 				if segment.ad_reason:
 					self.logger.info("Ignoring ad segment: {}".format(segment.ad_reason))
@@ -380,7 +416,26 @@ class StreamWorker(object):
 				self.manager.mark_working(self)
 
 				if segment.date:
-					date = common.dateutil.parse(segment.date)
+					new_date = common.dateutil.parse(segment.date)
+					if date is not None:
+						# We have observed an issue on twitch where there will be a small time jump
+						# (eg. with 2s segments, times will be 00:10.3, 00:12.3, 00:14.7, 00:16.7)
+						# and all subsequent segment timestamps will be out by this amount compared
+						# to what other downloader instances see. Our workaround for this issue is to
+						# watch for such gaps and:
+						#   1. trigger a worker refresh (which seems to fix the issue)
+						#   2. treat all subsequent segments as "suspect" so they are still saved
+						#      but only used if no other source is available.
+						skew = (date - new_date).total_seconds()
+						if skew != 0:
+							segment_time_skew_non_zero_sum.labels(self.manager.channel, self.quality, f"{id(self):x}").inc(skew)
+							segment_time_skew_non_zero_count.labels(self.manager.channel, self.quality, f"{id(self):x}").inc()
+						if abs(skew) > self.MAX_SEGMENT_TIME_SKEW and not suspicious_skew:
+							self.logger.warning(f"Suspicious skew of {skew}, triggering new worker: Expected {date} after {prev_segment}, got {new_date} for {segment}")
+							self.trigger_new_worker()
+							suspicious_skew = True
+							suspicious_skew_count.labels(self.manager.channel, self.quality).inc()
+					date = new_date
 				if segment.uri not in self.getters:
 					if date is None:
 						raise ValueError("Cannot determine date of segment")
@@ -392,11 +447,14 @@ class StreamWorker(object):
 						self.quality,
 						segment,
 						date,
+						suspicious_skew,
 						self.map_cache,
+						history,
 					)
 					gevent.spawn(self.getters[segment.uri].run)
 				if date is not None:
 					date += datetime.timedelta(seconds=segment.duration)
+				prev_segment = segment
 
 			# Clean up any old segment getters.
 			# Note use of list() to make a copy to avoid modification-during-iteration
@@ -448,19 +506,21 @@ class SegmentGetter(object):
 	# or so, to be paranoid we set it to considerably longer than that.
 	GIVE_UP_TIMEOUT = 20 * 60
 
-	def __init__(self, parent_logger, session, base_dir, channel, quality, segment, date, map_cache):
+	def __init__(self, parent_logger, session, base_dir, channel, quality, segment, date, suspect, map_cache, history):
 		self.logger = parent_logger.getChild("SegmentGetter@{:x}".format(id(self)))
 		self.base_dir = base_dir
 		self.channel = channel
 		self.quality = quality
 		self.segment = segment
 		self.date = date
+		self.suspect = suspect
 		self.map_cache = map_cache
 		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
+		self.history = history
 
 	def run(self):
 		try:
@@ -517,6 +577,12 @@ class SegmentGetter(object):
 		Type may be:
 			full: Segment is complete. Hash is included.
 			suspect: Segment appears to be complete, but we suspect it is not. Hash is included.
+				This currently triggers on two conditions:
+				- If a segment takes a very long time to download, which we've observed to result in
+				  partial files even though they appeared to end correctly.
+				- If the StreamWorker has encountered a time gap, then we suspect this segment to be
+				  mis-timed. We have observed this where there is a small (~0.5s) time jump, then
+				  all segments are consistently off by that amount compared to other nodes until refresh.
 			partial: Segment is incomplete. Hash is included.
 			temp: Segment has not been downloaded yet. A random uuid is added.
 		"""
@@ -601,7 +667,9 @@ class SegmentGetter(object):
 			raise e
 		else:
 			request_duration = monotonic() - start_time
-			segment_type = "full" if request_duration < self.FETCH_SUSPECT_TIME else "suspect"
+			segment_type = "full"
+			if self.suspect or request_duration >= self.FETCH_SUSPECT_TIME:
+				segment_type = "suspect"
 			full_path = self.make_path(segment_type, hash)
 			self.logger.debug("Saving completed segment {} as {}".format(temp_path, full_path))
 			common.rename(temp_path, full_path)
@@ -612,6 +680,20 @@ class SegmentGetter(object):
 			stat = latest_segment.labels(channel=self.channel, quality=self.quality)
 			timestamp = (self.date - datetime.datetime(1970, 1, 1)).total_seconds()
 			stat.set(max(stat._value.get(), timestamp)) # NOTE: not thread-safe but is gevent-safe
+			if self.history:
+				self.write_history(full_path)
+
+	def write_history(self, segment_path):
+		segment_path = os.path.relpath(segment_path, self.base_dir)
+		history_path = os.path.join(self.base_dir, "playlist-debug", segment_path)
+		try:
+			os.makedirs(history_path)
+		except FileExistsError:
+			pass
+		for n, (timestamp, playlist) in enumerate(self.history):
+			filename = "{}_{}".format(n, timestamp.strftime("%Y-%m-%dT%H:%M:%S.%f"))
+			path = os.path.join(history_path, filename)
+			common.atomic_write(path, playlist)
 
 
 def parse_channel(channel):
@@ -630,7 +712,7 @@ def parse_channel(channel):
 	"This affects retry interval, error reporting and monitoring. "
 	"Non-twitch URLs can also be given with the form CHANNEL[!]:TYPE:URL"
 )
-def main(channels, base_dir=".", qualities="source", metrics_port=8001, backdoor_port=0, twitch_auth_file=None):
+def main(channels, base_dir=".", qualities="source", metrics_port=8001, backdoor_port=0, twitch_auth_file=None, playlist_debug=0):
 	qualities = qualities.split(",") if qualities else []
 
 	twitch_auth_token = None
@@ -651,7 +733,7 @@ def main(channels, base_dir=".", qualities="source", metrics_port=8001, backdoor
 			channel_qualities = ["source"]
 		else:
 			raise ValueError(f"Unknown type {type!r}")
-		manager = StreamsManager(provider, channel, base_dir, channel_qualities, important=important)
+		manager = StreamsManager(provider, channel, base_dir, channel_qualities, important=important, history_size=playlist_debug)
 		managers.append(manager)
 
 	def stop():

downloader/downloader/providers.py

@@ -29,7 +29,8 @@ class Provider:
 			session = InstrumentedSession()
 		resp = session.get(uri, metric_name='get_media_playlist')
 		resp.raise_for_status()
-		return hls_playlist.load(resp.text, base_uri=resp.url)
+		playlist = resp.text
+		return playlist, hls_playlist.load(playlist, base_uri=resp.url)
 
 
 class URLProvider(Provider):

restreamer/restreamer/main.py

@@ -91,10 +91,10 @@ def metrics():
 	"""Return current metrics in prometheus metrics format"""
 	return prom.generate_latest()
 
-# To make nginx proxying simpler, we want to allow /metrics/* to work
+# To make nginx proxying simpler, we want to allow /metrics/restreamer to work
-@app.route('/metrics/<trailing>')
+@app.route('/metrics/restreamer')
 @request_stats
-def metrics_with_trailing(trailing):
+def metrics_with_trailing():
 	"""Expose Prometheus metrics."""
 	return prom.generate_latest()