fix: enable SQLite WAL mode to prevent database locking errors (#4048)

Fixes race condition causing "database is locked" errors during
concurrent writes to SQLite, particularly in streaming responses with
guardrails where multiple inference calls write simultaneously.

Enable Write-Ahead Logging (WAL) mode for SQLite which allows multiple
concurrent readers and one writer without blocking. Set busy_timeout to
5s so SQLite retries instead of failing immediately. Remove the logic
that disabled write queues for SQLite since WAL mode eliminates the
locking issues that prompted disabling them.

Fixes: test_output_safety_guardrails_safe_content[stream=True] flake
(cherry picked from commit 2381714904)
Signed-off-by: Charlie Doern <cdoern@redhat.com>

llama_stack/providers/utils/responses/responses_store.py

@@ -3,6 +3,7 @@
 #
 # This source code is licensed under the terms described in the LICENSE file in
 # the root directory of this source tree.
+import asyncio
 
 from llama_stack.apis.agents import (
     Order,
@@ -17,12 +18,12 @@ from llama_stack.apis.agents.openai_responses import (
 )
 from llama_stack.apis.inference import OpenAIMessageParam
 from llama_stack.core.datatypes import AccessRule
-from llama_stack.core.storage.datatypes import ResponsesStoreReference, SqlStoreReference
+from llama_stack.core.storage.datatypes import ResponsesStoreReference, SqlStoreReference, StorageBackendType
 from llama_stack.log import get_logger
 
 from ..sqlstore.api import ColumnDefinition, ColumnType
 from ..sqlstore.authorized_sqlstore import AuthorizedSqlStore
-from ..sqlstore.sqlstore import sqlstore_impl
+from ..sqlstore.sqlstore import _SQLSTORE_BACKENDS, sqlstore_impl
 
 logger = get_logger(name=__name__, category="openai_responses")
 
@@ -59,6 +60,13 @@ class ResponsesStore:
         base_store = sqlstore_impl(self.reference)
         self.sql_store = AuthorizedSqlStore(base_store, self.policy)
 
+        # Disable write queue for SQLite since WAL mode handles concurrency
+        # Keep it enabled for other backends (like Postgres) for performance
+        backend_config = _SQLSTORE_BACKENDS.get(self.reference.backend)
+        if backend_config and backend_config.type == StorageBackendType.SQL_SQLITE:
+            self.enable_write_queue = False
+            logger.debug("Write queue disabled for SQLite (WAL mode handles concurrency)")
+
         await self.sql_store.create_table(
             "openai_responses",
             {
@@ -77,6 +85,14 @@ class ResponsesStore:
             },
         )
 
+        if self.enable_write_queue:
+            self._queue = asyncio.Queue(maxsize=self._max_write_queue_size)
+            for _ in range(self._num_writers):
+                self._worker_tasks.append(asyncio.create_task(self._worker_loop()))
+            logger.debug(
+                f"Responses store write queue enabled with {self._num_writers} writers, max queue size {self._max_write_queue_size}"
+            )
+
     async def shutdown(self) -> None:
         return