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/inference/inference_store.py

@@ -35,6 +35,7 @@ class InferenceStore:
         self.reference = reference
         self.sql_store = None
         self.policy = policy
+        self.enable_write_queue = True
 
         # Async write queue and worker control
         self._queue: asyncio.Queue[tuple[OpenAIChatCompletion, list[OpenAIMessageParam]]] | None = None
@@ -47,14 +48,13 @@ class InferenceStore:
         base_store = sqlstore_impl(self.reference)
         self.sql_store = AuthorizedSqlStore(base_store, self.policy)
 
-        # Disable write queue for SQLite to avoid concurrency issues
-        backend_name = self.reference.backend
-        backend_config = _SQLSTORE_BACKENDS.get(backend_name)
-        if backend_config is None:
-            raise ValueError(
-                f"Unregistered SQL backend '{backend_name}'. Registered backends: {sorted(_SQLSTORE_BACKENDS)}"
-            )
-        self.enable_write_queue = backend_config.type != StorageBackendType.SQL_SQLITE
+        # 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(
             "chat_completions",
             {
@@ -66,6 +66,14 @@ class InferenceStore:
             },
         )
 
+        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"Inference store write queue enabled with {self._num_writers} writers, max queue size {self._max_write_queue_size}"
+            )
+
     async def shutdown(self) -> None:
         if not self._worker_tasks:
             return