[util] fix SimpleLimiter not limiting correctly

This would cause our GatewayClient to get rate limited when guild member
syncing was enabled for clients with a large number of guilds. The bug
had to do with the limiter releaseing all the requests after the initial
period it blocked for. This was resolved by rewriting the SimpleLimiter
to use a semaphore.

disco/util/limiter.py

@@ -6,33 +6,17 @@ class SimpleLimiter(object):
     def __init__(self, total, per):
         self.total = total
         self.per = per
+        self._lock = gevent.lock.Semaphore(total)
 
         self.count = 0
         self.reset_at = 0
-
-        self.event = None
-
-    def backoff(self):
-        self.event = gevent.event.Event()
-        gevent.sleep(self.reset_at - time.time())
-        self.count = 0
-        self.reset_at = 0
-        if self.event:
-            self.event.set()
         self.event = None
 
     def check(self):
-        if self.event:
-            self.event.wait()
-
-        self.count += 1
+        self._lock.acquire()
 
-        if not self.reset_at:
-            self.reset_at = time.time() + self.per
-            return
-        elif self.reset_at < time.time():
-            self.count = 1
-            self.reset_at = time.time()
+        def _release_lock():
+            gevent.sleep(self.per)
+            self._lock.release()
 
-        if self.count > self.total and self.reset_at > time.time():
-            self.backoff()
+        gevent.spawn(_release_lock)

tests/test_util_limiter.py

@@ -0,0 +1,46 @@
+import time
+import gevent
+from unittest import TestCase
+
+from disco.util.limiter import SimpleLimiter
+
+
+class TestSimpleLimiter(TestCase):
+    def test_many_wait_ratelimiter(self):
+        limit = SimpleLimiter(5, 1)
+        many = []
+
+        def check(lock):
+            limit.check()
+            lock.release()
+
+        start = time.time()
+        for _ in range(16):
+            lock = gevent.lock.Semaphore()
+            lock.acquire()
+            many.append(lock)
+            gevent.spawn(check, lock)
+
+        for item in many:
+            item.acquire()
+
+        self.assertGreater(time.time() - start, 3)
+
+    def test_nowait_ratelimiter(self):
+        limit = SimpleLimiter(5, 1)
+
+        start = time.time()
+        for _ in range(5):
+            limit.check()
+
+        self.assertLess(time.time() - start, 1)
+
+    def test_single_wait_ratelimiter(self):
+        limit = SimpleLimiter(5, 1)
+
+        start = time.time()
+        for _ in range(10):
+            limit.check()
+
+
+        self.assertEqual(int(time.time() - start), 1)