[BugFix] Fix engine hanging after KV cache initialization failure (#35478)

Signed-off-by: Shiyan Deng <dsy842974287@meta.com>
Co-authored-by: Lu Fang <30275821+houseroad@users.noreply.github.com>

vllm/v1/engine/core.py

 # SPDX-License-Identifier: Apache-2.0
 # SPDX-FileCopyrightText: Copyright contributors to the vLLM project
+import contextlib
 import os
 import queue
 import signal
@@ -117,9 +118,17 @@ class EngineCore:
             self._eep_scale_up_before_kv_init()
 
         # Setup KV Caches and update CacheConfig after profiling.
-        num_gpu_blocks, num_cpu_blocks, kv_cache_config = self._initialize_kv_caches(
-            vllm_config
+        try:
+            num_gpu_blocks, num_cpu_blocks, kv_cache_config = (
+                self._initialize_kv_caches(vllm_config)
+            )
+        except Exception:
+            logger.exception(
+                "EngineCore failed during KV cache initialization; "
+                "shutting down executor."
             )
+            self.model_executor.shutdown()
+            raise
 
         vllm_config.cache_config.num_gpu_blocks = num_gpu_blocks
         vllm_config.cache_config.num_cpu_blocks = num_cpu_blocks
@@ -958,13 +967,33 @@ class EngineCoreProc(EngineCore):
             addresses = self.startup_handshake(
                 handshake_socket, local_client, headless, parallel_config_to_update
             )
+            exc_during_init = False
+            try:
                 yield addresses
-
+            except Exception:
+                exc_during_init = True
+                raise
+            finally:
+                if exc_during_init:
+                    # Send FAILED status so the front-end detects init
+                    # failure immediately via ZMQ instead of waiting for
+                    # process sentinel (which may be delayed by cleanup).
+                    with contextlib.suppress(Exception):
+                        handshake_socket.send(
+                            msgspec.msgpack.encode(
+                                {
+                                    "status": "FAILED",
+                                    "local": local_client,
+                                    "headless": headless,
+                                }
+                            )
+                        )
+                else:
                     # Send ready message.
                     num_gpu_blocks = vllm_config.cache_config.num_gpu_blocks
-            # We pass back the coordinator stats update address here for the
-            # external LB case for our colocated front-end to use (coordinator
-            # only runs with rank 0).
+                    # We pass back the coordinator stats update address
+                    # here for the external LB case for our colocated
+                    # front-end to use (coordinator only runs with rank 0).
                     dp_stats_address = self.frontend_stats_publish_address
 
                     # Include config hash for DP configuration validation

vllm/v1/engine/utils.py

@@ -1101,6 +1101,11 @@ def wait_for_engine_startup(
 
             start_pending[0 if local else 1] -= 1
             engine.state = CoreEngineState.READY
+        elif status == "FAILED":
+            raise RuntimeError(
+                f"Engine core {eng_index} reported initialization failure. "
+                "See root cause above."
+            )
         else:
             raise RuntimeError(
                 f"Unexpected {status} message for "