playlist manager: serialize insert requests

We've seen cases where videos are not inserted even though the API call succeeded.
Our suspicion is that two concurrent insert calls for the same video are causing a race.

We try to avoid this by putting a lock around insert calls

playlist_manager/playlist_manager/main.py

@@ -185,6 +185,9 @@ class PlaylistManager(object):
 class YoutubeAPI(object):
 	def __init__(self, client):
 		self.client = client
+		# We've observed failures in the playlist API when doing concurrent calls for the same video.
+		# We could maybe have a per-video lock but this is easier.
+		self.insert_lock = gevent.lock.RLock()
 
 	def insert_into_playlist(self, playlist, video_id, index):
 		json = {
@@ -197,15 +200,16 @@ class YoutubeAPI(object):
 				"position": index,
 			},
 		}
-		resp = self.client.request("POST", "https://www.googleapis.com/youtube/v3/playlistItems",
-			params={"part": "snippet"},
-			json=json,
-			metric_name="playlist_insert",
-		)
-		if not resp.ok:
-			raise Exception("Failed to insert {video_id} at index {index} of {playlist} with {resp.status_code}: {resp.content}".format(
-				playlist=playlist, video_id=video_id, index=index, resp=resp,
-			))
+		with self.insert_lock:
+			resp = self.client.request("POST", "https://www.googleapis.com/youtube/v3/playlistItems",
+				params={"part": "snippet"},
+				json=json,
+				metric_name="playlist_insert",
+			)
+			if not resp.ok:
+				raise Exception("Failed to insert {video_id} at index {index} of {playlist} with {resp.status_code}: {resp.content}".format(
+					playlist=playlist, video_id=video_id, index=index, resp=resp,
+				))
 
 	def list_playlist(self, playlist):
 		"""Fetches the first page of playlist contents and returns a ListQuery object.