Use more defensive checks for ratelimits (#176)

Troi is used in LB radio in production where it can be invoked in an api request
context. Multiple api requests modifying the global domain_ratelimit_lookup
concurrently causes issues with the ratelimiting logic. For instance, one thread
can check that the key exists in the dict but another thread can delete the key
from the dict before the first has accessed the key leading to issues. Since, the
key is only deleted after the sleep it can be safely ignored by using more
defensive accesses.

Author: amCap1712

troi/http_request.py

@@ -60,18 +60,21 @@ def http_fetch(url, method, headers=None, params=None, **kwargs):
     parse = urlparse(url)
     while True:
         _key = parse.scheme + parse.netloc
-        if _key in domain_ratelimit_lookup:
-            (limit, remaining, reset) = domain_ratelimit_lookup[_key]
+        ratelimit = domain_ratelimit_lookup.get(_key, None)
+        if ratelimit is not None:
+            (limit, remaining, reset) = ratelimit
 
             # MB's rate limit headers are borked, so for the time being, use nearly 1s
             if parse.netloc.startswith("musicbrainz.org"):
                 time_left = .9
             else:
                 time_left = reset - time()
-                if time_left > 0:
-                    time_to_wait = time_left / remaining
-                    sleep(time_to_wait)
-            del domain_ratelimit_lookup[_key]
+
+            if time_left > 0:
+                time_to_wait = time_left / remaining
+                sleep(time_to_wait)
+
+            domain_ratelimit_lookup.pop(_key, None)
 
         if method == "GET":
             r = session.get(url, params=params, headers=headers, **kwargs)