PYTHON-4703 MongoClient should default to connect=False on FaaS environments

doc/changelog.rst

@@ -42,13 +42,19 @@ PyMongo 4.9 brings a number of improvements including:
 - Fixed a bug where PyMongo would raise ``InvalidBSON: date value out of range``
   when using :attr:`~bson.codec_options.DatetimeConversion.DATETIME_CLAMP` or
   :attr:`~bson.codec_options.DatetimeConversion.DATETIME_AUTO` with a non-UTC timezone.
+- The default value for ``connect`` in ``MongoClient`` is changed to ``False`` when running on
+  Function-as-a-service (FaaS).  On some FaaS systems, there is a ``fork()`` operation at function
+  startup.  By delaying the connection to the first operation, we avoid a deadlock.  See
+  `Is PyMongo Fork-Safe`_ for more information.
+
 
 Issues Resolved
 ...............
 
 See the `PyMongo 4.9 release notes in JIRA`_ for the list of resolved issues
 in this release.
 
+.. _Is PyMongo Fork-Safe : https://www.mongodb.com/docs/languages/python/pymongo-driver/current/faq/#is-pymongo-fork-safe-
 .. _PyMongo 4.9 release notes in JIRA: https://jira.mongodb.org/secure/ReleaseNote.jspa?projectId=10004&version=39940
 
 

pymongo/asynchronous/mongo_client.py

@@ -803,7 +803,10 @@ def __init__(
         if tz_aware is None:
             tz_aware = opts.get("tz_aware", False)
         if connect is None:
-            connect = opts.get("connect", True)
+            # Default to connect=True unless on a FaaS system, which might use fork.
+            from pymongo.pool_options import _is_faas
+
+            connect = opts.get("connect", not _is_faas())
         keyword_opts["tz_aware"] = tz_aware
         keyword_opts["connect"] = connect
 

pymongo/synchronous/mongo_client.py

@@ -802,7 +802,10 @@ def __init__(
         if tz_aware is None:
             tz_aware = opts.get("tz_aware", False)
         if connect is None:
-            connect = opts.get("connect", True)
+            # Default to connect=True unless on a FaaS system, which might use fork.
+            from pymongo.pool_options import _is_faas
+
+            connect = opts.get("connect", not _is_faas())
         keyword_opts["tz_aware"] = tz_aware
         keyword_opts["connect"] = connect
 

test/lambda/mongodb/app.py

@@ -8,6 +8,7 @@
 
 import json
 import os
+import warnings
 
 from bson import has_c as has_bson_c
 from pymongo import MongoClient
@@ -18,6 +19,9 @@
     ServerHeartbeatListener,
 )
 
+# Ensure there are no warnings raised in normal operation.
+warnings.simplefilter("error")
+
 open_connections = 0
 heartbeat_count = 0
 streaming_heartbeat_count = 0