PYTHON-5672 Fix SDAM error handling when pool checkout raises in __aenter__

When _PoolCheckout.__aenter__() raises (e.g. connect() fails), Python does
not call _ClientCheckout.__aexit__(), so handle() was never invoked and the
topology never learned about the failure. This caused test_5_check_out_fails_
connection_error to fail (missing PoolClearedEvent) and broke failover tests.

Fix: wrap both the pool checkout call and the post-checkout setup in
try/except BaseException blocks inside _ClientCheckout.__aenter__(), calling
handle() and (for post-checkout failures) checking the connection back in
before re-raising.

Author: blink1073

pymongo/asynchronous/mongo_client.py

@@ -2717,26 +2717,40 @@ async def __aenter__(self) -> AsyncConnection:
             self.contribute_socket(session._pinned_connection)
             return session._pinned_connection
         pool_checkout = _PoolCheckout(server.pool, self)
-        conn = await pool_checkout.__aenter__()
+        try:
+            conn = await pool_checkout.__aenter__()
+        except BaseException as exc:
+            # __aenter__ raised — pool already cleaned up internally.
+            # Run SDAM error handling so the topology learns about the failure.
+            await self.handle(type(exc), exc)
+            raise
         self._pool_checkout = pool_checkout
-        # Pin this session to the selected server or connection.
-        if (
-            in_txn
-            and session
-            and server.description.server_type
-            in (
-                SERVER_TYPE.Mongos,
-                SERVER_TYPE.LoadBalancer,
-            )
-        ):
-            session._pin(server, conn)
-        self.contribute_socket(conn)
-        if (
-            self.client._encrypter
-            and not self.client._encrypter._bypass_auto_encryption
-            and conn.max_wire_version < 8
-        ):
-            raise ConfigurationError("Auto-encryption requires a minimum MongoDB version of 4.2")
+        try:
+            # Pin this session to the selected server or connection.
+            if (
+                in_txn
+                and session
+                and server.description.server_type
+                in (
+                    SERVER_TYPE.Mongos,
+                    SERVER_TYPE.LoadBalancer,
+                )
+            ):
+                session._pin(server, conn)
+            self.contribute_socket(conn)
+            if (
+                self.client._encrypter
+                and not self.client._encrypter._bypass_auto_encryption
+                and conn.max_wire_version < 8
+            ):
+                raise ConfigurationError(
+                    "Auto-encryption requires a minimum MongoDB version of 4.2"
+                )
+        except BaseException as exc:
+            await self.handle(type(exc), exc)
+            await pool_checkout.__aexit__(type(exc), exc, None)
+            self._pool_checkout = None
+            raise
         return conn
 
     async def __aexit__(

pymongo/synchronous/mongo_client.py

@@ -2706,26 +2706,40 @@ def __enter__(self) -> Connection:
             self.contribute_socket(session._pinned_connection)
             return session._pinned_connection
         pool_checkout = _PoolCheckout(server.pool, self)
-        conn = pool_checkout.__enter__()
+        try:
+            conn = pool_checkout.__enter__()
+        except BaseException as exc:
+            # __aenter__ raised — pool already cleaned up internally.
+            # Run SDAM error handling so the topology learns about the failure.
+            self.handle(type(exc), exc)
+            raise
         self._pool_checkout = pool_checkout
-        # Pin this session to the selected server or connection.
-        if (
-            in_txn
-            and session
-            and server.description.server_type
-            in (
-                SERVER_TYPE.Mongos,
-                SERVER_TYPE.LoadBalancer,
-            )
-        ):
-            session._pin(server, conn)
-        self.contribute_socket(conn)
-        if (
-            self.client._encrypter
-            and not self.client._encrypter._bypass_auto_encryption
-            and conn.max_wire_version < 8
-        ):
-            raise ConfigurationError("Auto-encryption requires a minimum MongoDB version of 4.2")
+        try:
+            # Pin this session to the selected server or connection.
+            if (
+                in_txn
+                and session
+                and server.description.server_type
+                in (
+                    SERVER_TYPE.Mongos,
+                    SERVER_TYPE.LoadBalancer,
+                )
+            ):
+                session._pin(server, conn)
+            self.contribute_socket(conn)
+            if (
+                self.client._encrypter
+                and not self.client._encrypter._bypass_auto_encryption
+                and conn.max_wire_version < 8
+            ):
+                raise ConfigurationError(
+                    "Auto-encryption requires a minimum MongoDB version of 4.2"
+                )
+        except BaseException as exc:
+            self.handle(type(exc), exc)
+            pool_checkout.__exit__(type(exc), exc, None)
+            self._pool_checkout = None
+            raise
         return conn
 
     def __exit__(