Make sure that max_retries=0 is treated differently than None (#1080)

* Make sure that max_retries=0 is treated differently than None

When max_retries=0, it should not retry at all instead of retrying infinitely.

* retry_always should use max_retries=None instead of 0.

* Added test for zero retries

* Removed Offending spaces from blank line

kombu/utils/functional.py

@@ -342,7 +342,7 @@ def retry_over_time(fun, catch, args=None, kwargs=None, errback=None,
         try:
             return fun(*args, **kwargs)
         except catch as exc:
-            if max_retries and retries >= max_retries:
+            if max_retries is not None and retries >= max_retries:
                 raise
             if end and time() > end:
                 raise

t/unit/utils/test_functional.py

@@ -226,6 +226,21 @@ class test_retry_over_time:
                 errback=None, timeout=1,
             )
 
+    @mock.sleepdeprived(module=utils)
+    def test_retry_zero(self):
+        with pytest.raises(self.Predicate):
+            retry_over_time(
+                self.myfun, self.Predicate,
+                max_retries=0, errback=self.errback, interval_max=14,
+            )
+        assert self.index == 0
+        # no errback
+        with pytest.raises(self.Predicate):
+            retry_over_time(
+                self.myfun, self.Predicate,
+                max_retries=0, errback=None, interval_max=14,
+            )
+
     @mock.sleepdeprived(module=utils)
     def test_retry_once(self):
         with pytest.raises(self.Predicate):
@@ -261,7 +276,7 @@ class test_retry_over_time:
 
         assert retry_over_time(
             fun, self.Predicate,
-            max_retries=0, errback=None, interval_max=14) == 42
+            max_retries=None, errback=None, interval_max=14) == 42
         assert fun.calls == 11