fix: Replace per-call Lock creation with shared lock in SQLiteSession

The lock pattern 'with self._lock if self._is_memory_db else threading.Lock()'
creates a new Lock object for each operation when using file-based databases,
providing no thread safety for concurrent access.

This fix uses the shared self._lock for both memory and file databases,
ensuring proper synchronization for concurrent operations.

Changes:
- get_items() (line 123)
- add_items() (line 177)
- pop_item() (line 218)
- clear_session() (line 255)

Testing:
- Existing test test_sqlite_session_concurrent_access passes
- Manual verification shows proper lock reuse
- No breaking changes to API or behavior

Author: gn00295120

src/agents/memory/sqlite_session.py

@@ -4,6 +4,7 @@
 import json
 import sqlite3
 import threading
+from contextlib import nullcontext
 from pathlib import Path
 
 from ..items import TResponseInputItem
@@ -55,6 +56,10 @@ def __init__(
             self._init_db_for_connection(init_conn)
             init_conn.close()
 
+    def _lock_context(self):
+        """Return a context manager guarding writes for in-memory databases."""
+        return self._lock if self._is_memory_db else nullcontext()
+
     def _get_connection(self) -> sqlite3.Connection:
         """Get a database connection."""
         if self._is_memory_db:
@@ -120,7 +125,7 @@ async def get_items(self, limit: int | None = None) -> list[TResponseInputItem]:
 
         def _get_items_sync():
             conn = self._get_connection()
-            with self._lock if self._is_memory_db else threading.Lock():
+            with self._lock_context():
                 if limit is None:
                     # Fetch all items in chronological order
                     cursor = conn.execute(
@@ -174,7 +179,7 @@ async def add_items(self, items: list[TResponseInputItem]) -> None:
         def _add_items_sync():
             conn = self._get_connection()
 
-            with self._lock if self._is_memory_db else threading.Lock():
+            with self._lock_context():
                 # Ensure session exists
                 conn.execute(
                     f"""
@@ -215,7 +220,7 @@ async def pop_item(self) -> TResponseInputItem | None:
 
         def _pop_item_sync():
             conn = self._get_connection()
-            with self._lock if self._is_memory_db else threading.Lock():
+            with self._lock_context():
                 # Use DELETE with RETURNING to atomically delete and return the most recent item
                 cursor = conn.execute(
                     f"""
@@ -252,7 +257,7 @@ async def clear_session(self) -> None:
 
         def _clear_session_sync():
             conn = self._get_connection()
-            with self._lock if self._is_memory_db else threading.Lock():
+            with self._lock_context():
                 conn.execute(
                     f"DELETE FROM {self.messages_table} WHERE session_id = ?",
                     (self.session_id,),