[tasks] Don't update _next_iteration on retry

discord/ext/tasks/__init__.py

@@ -68,6 +68,7 @@ class Loop:
             raise ValueError('count must be greater than 0 or None.')
 
         self.change_interval(seconds=seconds, minutes=minutes, hours=hours)
+        self._last_iteration_failed = False
         self._last_iteration = None
         self._next_iteration = None
 
@@ -88,18 +89,22 @@ class Loop:
         backoff = ExponentialBackoff()
         await self._call_loop_function('before_loop')
         sleep_until = discord.utils.sleep_until
+        self._last_iteration_failed = False
         self._next_iteration = datetime.datetime.now(datetime.timezone.utc)
         try:
             await asyncio.sleep(0) # allows canceling in before_loop
             while True:
-                self._last_iteration = self._next_iteration
-                self._next_iteration = self._get_next_sleep_time()
+                if not self._last_iteration_failed:
+                    self._last_iteration = self._next_iteration
+                    self._next_iteration = self._get_next_sleep_time()
                 try:
                     await self.coro(*args, **kwargs)
+                    self._last_iteration_failed = False
                     now = datetime.datetime.now(datetime.timezone.utc)
                     if now > self._next_iteration:
                         self._next_iteration = now
                 except self._valid_exception as exc:
+                    self._last_iteration_failed = True
                     if not self.reconnect:
                         raise
                     await asyncio.sleep(backoff.delay())