chore: refactor server.main (#3462)

# What does this PR do?
As shown in #3421, we can scale stack to handle more RPS with k8s
replicas. This PR enables multi process stack with uvicorn --workers so
that we can achieve the same scaling without being in k8s.

To achieve that we refactor main to split out the app construction
logic. This method needs to be non-async. We created a new `Stack` class
to house impls and have a `start()` method to be called in lifespan to
start background tasks instead of starting them in the old
`construct_stack`. This way we avoid having to manage an event loop
manually.


## Test Plan
CI

> uv run --with llama-stack python -m llama_stack.core.server.server
benchmarking/k8s-benchmark/stack_run_config.yaml

works.

> LLAMA_STACK_CONFIG=benchmarking/k8s-benchmark/stack_run_config.yaml uv
run uvicorn llama_stack.core.server.server:create_app --port 8321
--workers 4

works.

llama_stack/core/server/server.py

@@ -6,6 +6,7 @@
 
 import argparse
 import asyncio
+import concurrent.futures
 import functools
 import inspect
 import json
@@ -50,17 +51,15 @@ from llama_stack.core.request_headers import (
     request_provider_data_context,
     user_from_scope,
 )
-from llama_stack.core.resolver import InvalidProviderError
 from llama_stack.core.server.routes import (
     find_matching_route,
     get_all_api_routes,
     initialize_route_impls,
 )
 from llama_stack.core.stack import (
+    Stack,
     cast_image_name_to_string,
-    construct_stack,
     replace_env_vars,
-    shutdown_stack,
     validate_env_pair,
 )
 from llama_stack.core.utils.config import redact_sensitive_fields
@@ -156,21 +155,34 @@ def translate_exception(exc: Exception) -> HTTPException | RequestValidationErro
         )
 
 
-async def shutdown(app):
-    """Initiate a graceful shutdown of the application.
-
-    Handled by the lifespan context manager. The shutdown process involves
-    shutting down all implementations registered in the application.
+class StackApp(FastAPI):
     """
-    await shutdown_stack(app.__llama_stack_impls__)
+    A wrapper around the FastAPI application to hold a reference to the Stack instance so that we can
+    start background tasks (e.g. refresh model registry periodically) from the lifespan context manager.
+    """
+
+    def __init__(self, config: StackRunConfig, *args, **kwargs):
+        super().__init__(*args, **kwargs)
+        self.stack: Stack = Stack(config)
+
+        # This code is called from a running event loop managed by uvicorn so we cannot simply call
+        # asyncio.run() to initialize the stack. We cannot await either since this is not an async
+        # function.
+        # As a workaround, we use a thread pool executor to run the initialize() method
+        # in a separate thread.
+        with concurrent.futures.ThreadPoolExecutor() as executor:
+            future = executor.submit(asyncio.run, self.stack.initialize())
+            future.result()
 
 
 @asynccontextmanager
-async def lifespan(app: FastAPI):
+async def lifespan(app: StackApp):
     logger.info("Starting up")
+    assert app.stack is not None
+    app.stack.create_registry_refresh_task()
     yield
     logger.info("Shutting down")
-    await shutdown(app)
+    await app.stack.shutdown()
 
 
 def is_streaming_request(func_name: str, request: Request, **kwargs):
@@ -386,73 +398,61 @@ class ClientVersionMiddleware:
         return await self.app(scope, receive, send)
 
 
-def main(args: argparse.Namespace | None = None):
-    """Start the LlamaStack server."""
-    parser = argparse.ArgumentParser(description="Start the LlamaStack server.")
+def create_app(
+    config_file: str | None = None,
+    env_vars: list[str] | None = None,
+) -> StackApp:
+    """Create and configure the FastAPI application.
 
-    add_config_distro_args(parser)
-    parser.add_argument(
-        "--port",
-        type=int,
-        default=int(os.getenv("LLAMA_STACK_PORT", 8321)),
-        help="Port to listen on",
-    )
-    parser.add_argument(
-        "--env",
-        action="append",
-        help="Environment variables in KEY=value format. Can be specified multiple times.",
-    )
+    Args:
+        config_file: Path to config file. If None, uses LLAMA_STACK_CONFIG env var or default resolution.
+        env_vars: List of environment variables in KEY=value format.
+        disable_version_check: Whether to disable version checking. If None, uses LLAMA_STACK_DISABLE_VERSION_CHECK env var.
 
-    # Determine whether the server args are being passed by the "run" command, if this is the case
-    # the args will be passed as a Namespace object to the main function, otherwise they will be
-    # parsed from the command line
-    if args is None:
-        args = parser.parse_args()
+    Returns:
+        Configured StackApp instance.
+    """
+    config_file = config_file or os.getenv("LLAMA_STACK_CONFIG")
+    if config_file is None:
+        raise ValueError("No config file provided and LLAMA_STACK_CONFIG env var is not set")
 
-    config_or_distro = get_config_from_args(args)
-    config_file = resolve_config_or_distro(config_or_distro, Mode.RUN)
+    config_file = resolve_config_or_distro(config_file, Mode.RUN)
 
+    # Load and process configuration
     logger_config = None
     with open(config_file) as fp:
         config_contents = yaml.safe_load(fp)
         if isinstance(config_contents, dict) and (cfg := config_contents.get("logging_config")):
             logger_config = LoggingConfig(**cfg)
         logger = get_logger(name=__name__, category="core::server", config=logger_config)
-        if args.env:
-            for env_pair in args.env:
+
+        if env_vars:
+            for env_pair in env_vars:
                 try:
                     key, value = validate_env_pair(env_pair)
-                    logger.info(f"Setting CLI environment variable {key} => {value}")
+                    logger.info(f"Setting environment variable {key} => {value}")
                     os.environ[key] = value
                 except ValueError as e:
                     logger.error(f"Error: {str(e)}")
-                    sys.exit(1)
+                    raise ValueError(f"Invalid environment variable format: {env_pair}") from e
+
         config = replace_env_vars(config_contents)
         config = StackRunConfig(**cast_image_name_to_string(config))
 
     _log_run_config(run_config=config)
 
-    app = FastAPI(
+    app = StackApp(
         lifespan=lifespan,
         docs_url="/docs",
         redoc_url="/redoc",
         openapi_url="/openapi.json",
+        config=config,
     )
 
     if not os.environ.get("LLAMA_STACK_DISABLE_VERSION_CHECK"):
         app.add_middleware(ClientVersionMiddleware)
 
-    try:
-        # Create and set the event loop that will be used for both construction and server runtime
-        loop = asyncio.new_event_loop()
-        asyncio.set_event_loop(loop)
-
-        # Construct the stack in the persistent event loop
-        impls = loop.run_until_complete(construct_stack(config))
-
-    except InvalidProviderError as e:
-        logger.error(f"Error: {str(e)}")
-        sys.exit(1)
+    impls = app.stack.impls
 
     if config.server.auth:
         logger.info(f"Enabling authentication with provider: {config.server.auth.provider_config.type.value}")
@@ -553,9 +553,54 @@ def main(args: argparse.Namespace | None = None):
     app.exception_handler(RequestValidationError)(global_exception_handler)
     app.exception_handler(Exception)(global_exception_handler)
 
-    app.__llama_stack_impls__ = impls
     app.add_middleware(TracingMiddleware, impls=impls, external_apis=external_apis)
 
+    return app
+
+
+def main(args: argparse.Namespace | None = None):
+    """Start the LlamaStack server."""
+    parser = argparse.ArgumentParser(description="Start the LlamaStack server.")
+
+    add_config_distro_args(parser)
+    parser.add_argument(
+        "--port",
+        type=int,
+        default=int(os.getenv("LLAMA_STACK_PORT", 8321)),
+        help="Port to listen on",
+    )
+    parser.add_argument(
+        "--env",
+        action="append",
+        help="Environment variables in KEY=value format. Can be specified multiple times.",
+    )
+
+    # Determine whether the server args are being passed by the "run" command, if this is the case
+    # the args will be passed as a Namespace object to the main function, otherwise they will be
+    # parsed from the command line
+    if args is None:
+        args = parser.parse_args()
+
+    config_or_distro = get_config_from_args(args)
+
+    try:
+        app = create_app(
+            config_file=config_or_distro,
+            env_vars=args.env,
+        )
+    except Exception as e:
+        logger.error(f"Error creating app: {str(e)}")
+        sys.exit(1)
+
+    config_file = resolve_config_or_distro(config_or_distro, Mode.RUN)
+    with open(config_file) as fp:
+        config_contents = yaml.safe_load(fp)
+        if isinstance(config_contents, dict) and (cfg := config_contents.get("logging_config")):
+            logger_config = LoggingConfig(**cfg)
+        else:
+            logger_config = None
+        config = StackRunConfig(**cast_image_name_to_string(replace_env_vars(config_contents)))
+
     import uvicorn
 
     # Configure SSL if certificates are provided
@@ -593,7 +638,6 @@ def main(args: argparse.Namespace | None = None):
     if ssl_config:
         uvicorn_config.update(ssl_config)
 
-    # Run uvicorn in the existing event loop to preserve background tasks
     # We need to catch KeyboardInterrupt because uvicorn's signal handling
     # re-raises SIGINT signals using signal.raise_signal(), which Python
     # converts to KeyboardInterrupt. Without this catch, we'd get a confusing
@@ -604,13 +648,9 @@ def main(args: argparse.Namespace | None = None):
     # Another approach would be to ignore SIGINT entirely - let uvicorn handle it through its own
     # signal handling but this is quite intrusive and not worth the effort.
     try:
-        loop.run_until_complete(uvicorn.Server(uvicorn.Config(**uvicorn_config)).serve())
+        asyncio.run(uvicorn.Server(uvicorn.Config(**uvicorn_config)).serve())
     except (KeyboardInterrupt, SystemExit):
         logger.info("Received interrupt signal, shutting down gracefully...")
-    finally:
-        if not loop.is_closed():
-            logger.debug("Closing event loop")
-            loop.close()
 
 
 def _log_run_config(run_config: StackRunConfig):