fix: enable SQLite WAL mode to prevent database locking errors (backport #4048) (#4226)

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<hr>This is an automatic backport of pull request #4048 done by
[Mergify](https://mergify.com).

Signed-off-by: Charlie Doern <cdoern@redhat.com>
Co-authored-by: Ashwin Bharambe <ashwin.bharambe@gmail.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
+        # Disable write queue for SQLite since WAL mode handles concurrency
-        backend_name = self.reference.backend
+        # Keep it enabled for other backends (like Postgres) for performance
-        backend_config = _SQLSTORE_BACKENDS.get(backend_name)
+        backend_config = _SQLSTORE_BACKENDS.get(self.reference.backend)
-        if backend_config is None:
+        if backend_config and backend_config.type == StorageBackendType.SQL_SQLITE:
-            raise ValueError(
+            self.enable_write_queue = False
-                f"Unregistered SQL backend '{backend_name}'. Registered backends: {sorted(_SQLSTORE_BACKENDS)}"
+            logger.debug("Write queue disabled for SQLite (WAL mode handles concurrency)")
-            )
+
-        self.enable_write_queue = backend_config.type != StorageBackendType.SQL_SQLITE
         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

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
 

llama_stack/providers/utils/sqlstore/sqlalchemy_sqlstore.py

@@ -17,6 +17,7 @@ from sqlalchemy import (
     String,
     Table,
     Text,
+    event,
     inspect,
     select,
     text,
@@ -75,7 +76,36 @@ class SqlAlchemySqlStoreImpl(SqlStore):
         self.metadata = MetaData()
 
     def create_engine(self) -> AsyncEngine:
-        return create_async_engine(self.config.engine_str, pool_pre_ping=True)
+        # Configure connection args for better concurrency support
+        connect_args = {}
+        if "sqlite" in self.config.engine_str:
+            # SQLite-specific optimizations for concurrent access
+            # With WAL mode, most locks resolve in milliseconds, but allow up to 5s for edge cases
+            connect_args["timeout"] = 5.0
+            connect_args["check_same_thread"] = False  # Allow usage across asyncio tasks
+
+        engine = create_async_engine(
+            self.config.engine_str,
+            pool_pre_ping=True,
+            connect_args=connect_args,
+        )
+
+        # Enable WAL mode for SQLite to support concurrent readers and writers
+        if "sqlite" in self.config.engine_str:
+
+            @event.listens_for(engine.sync_engine, "connect")
+            def set_sqlite_pragma(dbapi_conn, connection_record):
+                cursor = dbapi_conn.cursor()
+                # Enable Write-Ahead Logging for better concurrency
+                cursor.execute("PRAGMA journal_mode=WAL")
+                # Set busy timeout to 5 seconds (retry instead of immediate failure)
+                # With WAL mode, locks should be brief; if we hit 5s there's a bigger issue
+                cursor.execute("PRAGMA busy_timeout=5000")
+                # Use NORMAL synchronous mode for better performance (still safe with WAL)
+                cursor.execute("PRAGMA synchronous=NORMAL")
+                cursor.close()
+
+        return engine
 
     async def create_table(
         self,