PYTHON-4019 Infinite loop in generic transactional provider due to dup keys (#2438)

Author: blink1073

doc/conf.py

@@ -86,6 +86,7 @@
 # sourceforge.net is giving a 403 error, but is still accessible from the browser.
 linkcheck_ignore = [
     "https://github.com/mongodb/specifications/blob/master/source/server-discovery-and-monitoring/server-monitoring.md#requesting-an-immediate-check",
+    "https://github.com/mongodb/specifications/blob/master/source/transactions-convenient-api/transactions-convenient-api.md#handling-errors-inside-the-callback",
     "https://github.com/mongodb/libmongocrypt/blob/master/bindings/python/README.rst#installing-from-source",
     r"https://wiki.centos.org/[\w/]*",
     r"https://sourceforge.net/",

pymongo/asynchronous/client_session.py

@@ -660,6 +660,12 @@ async def callback(session, custom_arg, custom_kwarg=None):
         ``with_transaction`` starts a new transaction and re-executes
         the ``callback``.
 
+        The ``callback`` MUST NOT silently handle command errors
+        without allowing such errors to propagate. Command errors may abort the
+        transaction on the server, and an attempt to commit the transaction will
+        be rejected with a ``NoSuchTransaction`` error.  For more information see
+        the `transactions specification`_.
+
         When :meth:`~AsyncClientSession.commit_transaction` raises an exception with
         the ``"UnknownTransactionCommitResult"`` error label,
         ``with_transaction`` retries the commit until the result of the
@@ -689,6 +695,9 @@ async def callback(session, custom_arg, custom_kwarg=None):
         :return: The return value of the ``callback``.
 
         .. versionadded:: 3.9
+
+        .. _transactions specification:
+            https://github.com/mongodb/specifications/blob/master/source/transactions-convenient-api/transactions-convenient-api.md#handling-errors-inside-the-callback
         """
         start_time = time.monotonic()
         while True:

pymongo/synchronous/client_session.py

@@ -659,6 +659,12 @@ def callback(session, custom_arg, custom_kwarg=None):
         ``with_transaction`` starts a new transaction and re-executes
         the ``callback``.
 
+        The ``callback`` MUST NOT silently handle command errors
+        without allowing such errors to propagate. Command errors may abort the
+        transaction on the server, and an attempt to commit the transaction will
+        be rejected with a ``NoSuchTransaction`` error.  For more information see
+        the `transactions specification`_.
+
         When :meth:`~ClientSession.commit_transaction` raises an exception with
         the ``"UnknownTransactionCommitResult"`` error label,
         ``with_transaction`` retries the commit until the result of the
@@ -688,6 +694,9 @@ def callback(session, custom_arg, custom_kwarg=None):
         :return: The return value of the ``callback``.
 
         .. versionadded:: 3.9
+
+        .. _transactions specification:
+            https://github.com/mongodb/specifications/blob/master/source/transactions-convenient-api/transactions-convenient-api.md#handling-errors-inside-the-callback
         """
         start_time = time.monotonic()
         while True: