fix: Fix race condition in PostgresSaver (#2494)

Signed-off-by: Tyler Ball <tyleraball@gmail.com>
Co-authored-by: Phoenix Logan <plogan@chanzuckerberg.com>
Co-authored-by: Tyler Ball <2481463+tyler-ball@users.noreply.github.com>

libs/checkpoint-postgres/langgraph/checkpoint/postgres/_internal.py

@@ -0,0 +1,21 @@
+"""Shared utility functions for the Postgres checkpoint & storage classes."""
+
+from contextlib import contextmanager
+from typing import Iterator, Union
+
+from psycopg import Connection
+from psycopg.rows import DictRow
+from psycopg_pool import ConnectionPool
+
+Conn = Union[Connection[DictRow], ConnectionPool[Connection[DictRow]]]
+
+
+@contextmanager
+def get_connection(conn: Conn) -> Iterator[Connection[DictRow]]:
+    if isinstance(conn, Connection):
+        yield conn
+    elif isinstance(conn, ConnectionPool):
+        with conn.connection() as conn:
+            yield conn
+    else:
+        raise TypeError(f"Invalid connection type: {type(conn)}")