PYTHON-5504 Start backoff on first retry not second

Author: ShaneHarvey

pymongo/asynchronous/client_session.py

@@ -706,9 +706,9 @@ async def callback(session, custom_arg, custom_kwarg=None):
         start_time = time.monotonic()
         retry = 0
         while True:
-            if retry > 1:  # Implement exponential backoff after the first retry.
+            if retry:  # Implement exponential backoff on retry.
                 jitter = random.random()  # noqa: S311
-                backoff = jitter * min(_BACKOFF_INITIAL * (2 ** (retry - 1)), _BACKOFF_MAX)
+                backoff = jitter * min(_BACKOFF_INITIAL * (2**retry), _BACKOFF_MAX)
                 await asyncio.sleep(backoff)
             retry += 1
             await self.start_transaction(

pymongo/synchronous/client_session.py

@@ -704,9 +704,9 @@ def callback(session, custom_arg, custom_kwarg=None):
         start_time = time.monotonic()
         retry = 0
         while True:
-            if retry > 1:  # Implement exponential backoff after the first retry.
+            if retry:  # Implement exponential backoff on retry.
                 jitter = random.random()  # noqa: S311
-                backoff = jitter * min(_BACKOFF_INITIAL * (2 ** (retry - 1)), _BACKOFF_MAX)
+                backoff = jitter * min(_BACKOFF_INITIAL * (2**retry), _BACKOFF_MAX)
                 time.sleep(backoff)
             retry += 1
             self.start_transaction(read_concern, write_concern, read_preference, max_commit_time_ms)