feat: `--config` file support in sglang (#4272)

components/src/dynamo/sglang/args.py

 # SPDX-FileCopyrightText: Copyright (c) 2025 NVIDIA CORPORATION & AFFILIATES. All rights reserved.
 # SPDX-License-Identifier: Apache-2.0
-
 import argparse
 import contextlib
 import logging
 import os
 import socket
 import sys
+import tempfile
 from argparse import Namespace
 from dataclasses import dataclass
 from enum import Enum
+from pathlib import Path
 from typing import Any, Dict, Generator, List, Optional
 
+import yaml
 from sglang.srt.server_args import ServerArgs
+from sglang.srt.server_args_config_parser import ConfigArgumentMerger
 
 from dynamo._core import get_reasoning_parser_names, get_tool_parser_names
 from dynamo.common.config_dump import register_encoder
@@ -211,6 +214,70 @@ def _set_parser(
         return dynamo_str
 
 
+def _extract_config_section(
+    args: List[str], config_path: str, config_key: str
+) -> tuple[List[str], str]:
+    """
+    Extract a section from nested YAML and create temp flat file.
+
+    Args:
+        args: CLI arguments list
+        config_path: Path to the YAML config file
+        config_key: Key to extract from nested YAML
+
+    Returns:
+        tuple: (modified args with temp file path, temp file path for cleanup)
+
+    Raises:
+        ValueError: If config file not found, key missing, or invalid format
+    """
+    logging.info(f"Extracting config section '{config_key}' from {config_path}")
+
+    path = Path(config_path)
+    if not path.exists():
+        raise ValueError(f"Config file not found: {config_path}")
+
+    with open(config_path, "r") as f:
+        config_data = yaml.safe_load(f)
+
+    if not isinstance(config_data, dict):
+        raise ValueError(
+            f"Config file must contain a dictionary, got {type(config_data).__name__}"
+        )
+
+    available_keys = list(config_data.keys())
+    logging.info(f"Available config keys in {config_path}: {available_keys}")
+
+    if config_key not in config_data:
+        raise ValueError(
+            f"Config key '{config_key}' not found in {config_path}. "
+            f"Available keys: {available_keys}"
+        )
+
+    section_data = config_data[config_key]
+
+    if not isinstance(section_data, dict):
+        raise ValueError(
+            f"Config section '{config_key}' must be a dictionary, got {type(section_data).__name__}"
+        )
+
+    temp_fd, temp_path = tempfile.mkstemp(suffix=".yaml", prefix="dynamo_config_")
+
+    try:
+        with os.fdopen(temp_fd, "w") as f:
+            yaml.dump(section_data, f)
+        logging.info(f"Successfully wrote config section '{config_key}' to temp file")
+    except Exception:
+        os.unlink(temp_path)
+        raise
+
+    config_index = args.index("--config")
+    args = list(args)
+    args[config_index + 1] = temp_path
+
+    return args, temp_path
+
+
 async def parse_args(args: list[str]) -> Config:
     """Parse CLI arguments and return combined configuration.
     Download the model if necessary.
@@ -245,12 +312,56 @@ async def parse_args(args: list[str]) -> Config:
 
         parser.add_argument(*info["flags"], **kwargs)
 
+    # Config key argument (for nested configs)
+    parser.add_argument(
+        "--config-key",
+        type=str,
+        default=None,
+        help="Key to select from nested config file (e.g., 'prefill', 'decode')",
+    )
+
     # SGLang args
     bootstrap_port = _reserve_disaggregation_bootstrap_port()
     ServerArgs.add_cli_args(parser)
 
+    # Handle config file if present
+    temp_config_file = None  # Track temp file for cleanup
+    if "--config" in args:
+        # Check if --config-key is also present
+        if "--config-key" in args:
+            key_index = args.index("--config-key")
+            config_key = args[key_index + 1]
+            config_index = args.index("--config")
+            config_path = args[config_index + 1]
+
+            # Extract nested section to temp file
+            args, temp_config_file = _extract_config_section(
+                args, config_path, config_key
+            )
+
+            # Remove --config-key from args (not recognized by SGLang)
+            args = args[:key_index] + args[key_index + 2 :]
+
+        # Extract boolean actions from the parser to handle them correctly in YAML
+        boolean_actions = []
+        for action in parser._actions:
+            if hasattr(action, "dest") and hasattr(action, "action"):
+                if action.action in ["store_true", "store_false"]:
+                    boolean_actions.append(action.dest)
+
+        # Merge config file arguments with CLI arguments
+        config_merger = ConfigArgumentMerger(boolean_actions=boolean_actions)
+        args = config_merger.merge_config_with_args(args)
+
     parsed_args = parser.parse_args(args)
 
+    # Clean up temp file if created
+    if temp_config_file and os.path.exists(temp_config_file):
+        try:
+            os.unlink(temp_config_file)
+        except Exception:
+            logging.warning(f"Failed to clean up temp config file: {temp_config_file}")
+
     # Auto-set bootstrap port if not provided
     if not any(arg.startswith("--disaggregation-bootstrap-port") for arg in args):
         args_dict = vars(parsed_args)