fix: Enable pool_pre_ping by default for non-SQLite database engines

This change sets `pool_pre_ping=True` in SQLAlchemy engine kwargs for database backends other than SQLite. This helps ensure that connections from the pool are still valid before being used, preventing issues with stale or disconnected connections. Tests are added to verify the default behavior and that explicit overrides are respected

Close #4211

Co-authored-by: George Weale <gweale@google.com>
PiperOrigin-RevId: 864886767

Author: GWeale

src/google/adk/sessions/database_session_service.py

@@ -113,6 +113,8 @@ def __init__(self, db_url: str, **kwargs: Any):
         connect_args = dict(engine_kwargs.get("connect_args", {}))
         connect_args.setdefault("check_same_thread", False)
         engine_kwargs["connect_args"] = connect_args
+      elif url.get_backend_name() != "sqlite":
+        engine_kwargs.setdefault("pool_pre_ping", True)
 
       db_engine = create_async_engine(db_url, **engine_kwargs)
       if db_engine.dialect.name == "sqlite":

tests/unittests/sessions/test_session_service.py

@@ -16,10 +16,12 @@
 from datetime import timezone
 import enum
 import sqlite3
+from unittest import mock
 
 from google.adk.errors.already_exists_error import AlreadyExistsError
 from google.adk.events.event import Event
 from google.adk.events.event_actions import EventActions
+from google.adk.sessions import database_session_service
 from google.adk.sessions.base_session_service import GetSessionConfig
 from google.adk.sessions.database_session_service import DatabaseSessionService
 from google.adk.sessions.in_memory_session_service import InMemorySessionService
@@ -61,6 +63,51 @@ async def session_service(request, tmp_path):
     await service.close()
 
 
+def test_database_session_service_enables_pool_pre_ping_by_default():
+  captured_kwargs = {}
+
+  def fake_create_async_engine(_db_url: str, **kwargs):
+    captured_kwargs.update(kwargs)
+    fake_engine = mock.Mock()
+    fake_engine.dialect.name = 'postgresql'
+    fake_engine.sync_engine = mock.Mock()
+    return fake_engine
+
+  with mock.patch.object(
+      database_session_service,
+      'create_async_engine',
+      side_effect=fake_create_async_engine,
+  ):
+    database_session_service.DatabaseSessionService(
+        'postgresql+psycopg2://user:pass@localhost:5432/db'
+    )
+
+  assert captured_kwargs.get('pool_pre_ping') is True
+
+
+def test_database_session_service_respects_pool_pre_ping_override():
+  captured_kwargs = {}
+
+  def fake_create_async_engine(_db_url: str, **kwargs):
+    captured_kwargs.update(kwargs)
+    fake_engine = mock.Mock()
+    fake_engine.dialect.name = 'postgresql'
+    fake_engine.sync_engine = mock.Mock()
+    return fake_engine
+
+  with mock.patch.object(
+      database_session_service,
+      'create_async_engine',
+      side_effect=fake_create_async_engine,
+  ):
+    database_session_service.DatabaseSessionService(
+        'postgresql+psycopg2://user:pass@localhost:5432/db',
+        pool_pre_ping=False,
+    )
+
+  assert captured_kwargs.get('pool_pre_ping') is False
+
+
 @pytest.mark.asyncio
 async def test_sqlite_session_service_accepts_sqlite_urls(
     tmp_path, monkeypatch