PYTHON-5808 Drop support for MongoDB 4.2

Bump minimum supported server version to MongoDB 4.4 (wire version 9).
Remove dead code paths that only applied to wire version < 8 servers,
including the legacy OP_GET_MORE protocol, the use_command dispatch
mechanism, and the from_command response field.

Author: blink1073

.evergreen/generated_configs/tasks.yml

@@ -491,15 +491,6 @@ buildvariants:
       - rhel87-small
 
   # Server version tests
-  - name: mongodb-v4.2
-    tasks:
-      - name: .server-version
-    display_name: "* MongoDB v4.2"
-    run_on:
-      - rhel87-small
-    expansions:
-      VERSION: "4.2"
-    tags: [coverage_tag]
   - name: mongodb-v4.4
     tasks:
       - name: .server-version

.evergreen/generated_configs/variants.yml

@@ -21,7 +21,7 @@
 # Globals
 ##############
 
-ALL_VERSIONS = ["4.2", "4.4", "5.0", "6.0", "7.0", "8.0", "rapid", "latest"]
+ALL_VERSIONS = ["4.4", "5.0", "6.0", "7.0", "8.0", "rapid", "latest"]
 CPYTHONS = ["3.10", "3.11", "3.12", "3.13", "3.14t", "3.14"]
 PYPYS = ["pypy3.11"]
 MIN_SUPPORT_VERSIONS = ["3.9", "pypy3.9", "pypy3.10"]

.evergreen/scripts/generate_config_utils.py

@@ -15,7 +15,7 @@ a native Python driver for MongoDB, offering both synchronous and asynchronous A
 [gridfs](https://github.com/mongodb/specifications/blob/master/source/gridfs/gridfs-spec.md/)
 implementation on top of `pymongo`.
 
-PyMongo supports MongoDB 4.0, 4.2, 4.4, 5.0, 6.0, 7.0, and 8.0. PyMongo follows [semantic versioning](https://semver.org/spec/v2.0.0.html) for its releases.
+PyMongo supports MongoDB 4.4, 5.0, 6.0, 7.0, and 8.0. PyMongo follows [semantic versioning](https://semver.org/spec/v2.0.0.html) for its releases.
 
 ## Documentation
 

README.md

@@ -4,6 +4,7 @@ Changelog
 Changes in Version 4.18.0
 -------------------------
 
+- Dropped support for MongoDB 4.2. PyMongo now requires MongoDB 4.4 or later.
 - Improved TLS connection performance by reusing TLS sessions across connections
   to the same server, avoiding a full handshake on each new connection.
   Session resumption is supported on all Python versions for synchronous clients

doc/changelog.rst

@@ -140,13 +140,9 @@ async def get_cursor(
         cmd = {"aggregate": self._aggregation_target, "pipeline": self._pipeline}
         cmd.update(self._options)
 
-        # Apply this target's read concern if:
-        # readConcern has not been specified as a kwarg and either
-        # - server version is >= 4.2 or
-        # - server version is >= 3.2 and pipeline doesn't use $out
-        if ("readConcern" not in cmd) and (
-            not self._performs_write or (conn.max_wire_version >= 8)
-        ):
+        # Apply this target's read concern if readConcern has not been specified as a kwarg.
+        # $out/$merge pipelines also support readConcern on all supported server versions (4.4+).
+        if "readConcern" not in cmd:
             read_concern = self._target.read_concern
         else:
             read_concern = None

pymongo/asynchronous/aggregation.py

@@ -576,10 +576,6 @@ async def execute_no_results(
             raise ConfigurationError(
                 "Must be connected to MongoDB 4.4+ to use hint on unacknowledged delete commands."
             )
-        if unack and self.uses_hint_update and conn.max_wire_version < 8:
-            raise ConfigurationError(
-                "Must be connected to MongoDB 4.2+ to use hint on unacknowledged update commands."
-            )
         if unack and self.uses_sort and conn.max_wire_version < 25:
             raise ConfigurationError(
                 "Must be connected to MongoDB 8.0+ to use sort on unacknowledged update commands."

pymongo/asynchronous/bulk.py

@@ -1010,10 +1010,6 @@ async def _update(
             else:
                 update_doc["arrayFilters"] = array_filters
         if hint is not None:
-            if not acknowledged and conn.max_wire_version < 8:
-                raise ConfigurationError(
-                    "Must be connected to MongoDB 4.2+ to use hint on unacknowledged update commands."
-                )
             if not isinstance(hint, str):
                 hint = helpers_shared._index_document(hint)
             update_doc["hint"] = hint
@@ -3295,14 +3291,6 @@ async def _find_and_modify_helper(
                     )
                 cmd["arrayFilters"] = list(array_filters)
             if hint is not None:
-                if conn.max_wire_version < 8:
-                    raise ConfigurationError(
-                        "Must be connected to MongoDB 4.2+ to use hint on find and modify commands."
-                    )
-                elif not acknowledged and conn.max_wire_version < 9:
-                    raise ConfigurationError(
-                        "Must be connected to MongoDB 4.4+ to use hint on unacknowledged find and modify commands."
-                    )
                 cmd["hint"] = hint
             out = await self._command(
                 conn,

pymongo/asynchronous/collection.py

@@ -1804,14 +1804,6 @@ async def _checkout(
                 ):
                     session._pin(server, conn)
                 err_handler.contribute_socket(conn)
-                if (
-                    self._encrypter
-                    and not self._encrypter._bypass_auto_encryption
-                    and conn.max_wire_version < 8
-                ):
-                    raise ConfigurationError(
-                        "Auto-encryption requires a minimum MongoDB version of 4.2"
-                    )
                 yield conn
 
     async def _select_server(

pymongo/asynchronous/mongo_client.py

@@ -157,18 +157,18 @@ async def run_operation(
         assert listeners is not None
         start = datetime.now()
 
-        use_cmd = operation.use_command(conn)
+        conn.validate_session(operation.client, operation.session)  # type: ignore[arg-type]
         more_to_come = bool(operation.conn_mgr and operation.conn_mgr.more_to_come)
-        cmd, dbn = await self.operation_to_command(operation, conn, use_cmd)
+        cmd, dbn = await self.operation_to_command(operation, conn, True)
         if more_to_come:
             request_id = 0
             data = b""
             max_doc_size = 0
         else:
-            message = operation.get_message(read_preference, conn, use_cmd)
+            message = operation.get_message(read_preference, conn)
             request_id, data, max_doc_size = self._split_message(message)
 
-        user_fields = _CURSOR_DOC_FIELDS if use_cmd else None
+        user_fields = _CURSOR_DOC_FIELDS
 
         docs, reply, duration = await run_cursor_command(
             conn,
@@ -211,7 +211,6 @@ async def run_operation(
                 conn=conn,
                 duration=duration,
                 request_id=request_id,
-                from_command=use_cmd,
                 docs=docs,  # type: ignore[arg-type]
                 more_to_come=more_to_come,
             )
@@ -221,7 +220,6 @@ async def run_operation(
                 address=self._description.address,
                 duration=duration,
                 request_id=request_id,
-                from_command=use_cmd,
                 docs=docs,  # type: ignore[arg-type]
             )