[Bugfix] Don't crash engine on malformed output-socket frame (#44486)

[KKothuri]

Purpose

Fixes #44486. Also addresses #42398 (same root cause, reported as a feature request).

An external enterprise security scanner periodically probes the server's ports with junk/attack traffic. Two internal ZMQ decode sites treat such a frame as fatal:

1. Engine output socket (core_client.py) — a garbage frame makes msgspec raise a ValidationError (Expected array, got int) inside process_outputs_socket. It's caught by the broad except Exception and forwarded to outputs_queue, where get_output[_async] re-raises it, bubbling up to AsyncLLM.output_handler and killing the engine.
2. DP coordinator (coordinator.py) — process_input_socket decodes engine messages with decoder.decode(buffer) (and a raw msgspec.msgpack.decode on the front-end socket). run_coordinator only catches KeyboardInterrupt, so a ValidationError/DecodeError from a malformed frame propagates uncaught and kills the DP coordinator process (relevant when DP spans nodes and these sockets are TCP-bound).

This PR hardens both decode boundaries: a deserialization failure on these sockets is treated as a drop-this-frame event, not a fatal one.

Changes

• core_client.py — guard the decoder.decode(frames) call in both the sync (SyncMPClient) and async (AsyncMPClient) process_outputs_socket loops.
• coordinator.py — guard both decode sites in process_input_socket (the engine output_back socket and the front-end socket).
• In all cases: catch (msgspec.ValidationError, msgspec.DecodeError), log a warning, and drop the frame. The catch is deliberately narrow (ValidationError is a subclass of DecodeError) so a genuine internal protocol bug still surfaces. The coordinator uses continue, matching the loop's existing convention.

Note: the coordinator's validate_alive/sentinel-byte path (coordinator.py:295) was already non-fatal (logs and continues) and is unrelated to the crash.

Not a duplicate

• gh pr list --search "44486 in:body" / "42398 in:body" → none
• gh pr list --search "process_outputs_socket decode" → none
• gh pr list --search "engine output socket malformed" → only unrelated PRs

Issues #44486 and #42398 are open with no assignee and no linked PR.

Test

Two GPU-free regression tests in tests/v1/test_serial_utils.py, each driving the real loop over in-process ZMQ sockets:

• test_output_socket_drops_undecodable_frame — drives AsyncMPClient._ensure_output_queue_task; sends a garbage frame (an int, reproducing Expected array, got int) then a valid EngineCoreOutputs, and asserts the valid one is delivered.
• test_dp_coordinator_drops_undecodable_frame — drives DPCoordinatorProc.process_input_socket; a fake engine subscribes, a garbage frame is pushed to the engine output socket, then a valid stats frame, and asserts the coordinator survives and publishes the valid stats to the front-end.

Verified each fails without its fix and passes with it:

# core_client, WITHOUT fix
FAILED test_output_socket_drops_undecodable_frame
  AssertionError: ValidationError('Expected `array`, got `int`')

# coordinator, WITHOUT fix
FAILED test_dp_coordinator_drops_undecodable_frame
  AssertionError: coordinator crashed: ValidationError: Expected `array`, got `int`

# WITH both fixes
12 passed

Full file + lint:

$ python -m pytest tests/v1/test_serial_utils.py -q
12 passed

$ pre-commit run --files vllm/v1/engine/core_client.py vllm/v1/engine/coordinator.py tests/v1/test_serial_utils.py
ruff check ... Passed
ruff format ... Passed
Run mypy locally for lowest supported Python version ... Passed

AI assistance

AI assistance (Claude) was used for the analysis and implementation of this change. The submitter has reviewed every changed line and run the tests above.

[claude]

Claude Code Review

This pull request is from a fork — automated review is disabled. A repository maintainer can comment @claude review to run a one-time review.

[mergify]

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[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @KKothuri.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork

[localityv-hash]

Thanks for this fix. We hit #44486 in our own deployment and have been running this PR, backported onto a recent main, on a multi-node DP deployment with hybrid load balancing since 2026-10-08. Two cases still killed our DP Coordinator or left it stuck, and both are small additions to what is here.

1. Untyped decoding raises more than msgspec's own errors on junk.

try_decode_frame catches msgspec.ValidationError and msgspec.DecodeError. The front-end socket decodes untyped (msgspec.msgpack.decode(buffer)), and on random input that also raises:

• UnicodeDecodeError, when a msgpack str holds invalid UTF-8. That is roughly one random frame in seven. One of our Coordinators died of exactly this, from a stray frame on its front-end socket.
• TypeError, when a map is keyed by a map (the key is unhashable). msgspec 0.19 through 0.21 raise TypeError here; 0.22 raises DecodeError.

Reproduction:

import msgspec
msgspec.msgpack.decode(b"\xa3\xe0\x80\x80")  # UnicodeDecodeError
msgspec.msgpack.decode(b"\x81\x80\x01")      # TypeError on msgspec <= 0.21

We ran 200,000 random frames through both of the Coordinator's decoders. The typed decoder (MsgpackDecoder(EngineCoreOutputs)) only ever raised msgspec errors. The untyped one raised DecodeError, UnicodeDecodeError and TypeError. msgspec.ValidationError is a subclass of DecodeError, and UnicodeDecodeError is a subclass of ValueError.

There is also a third: input nested past the recursion limit (about 10,000 levels of arrays or maps on Python 3.12, e.g. b"\x91" * 20000 + b"\x01") raises RecursionError, which is a RuntimeError, on the untyped path (the typed decoder reports a ValidationError first). A scanner will not send that by accident, but a single 20 KB frame does it on purpose. So the catch becomes:

-    except (msgspec.ValidationError, msgspec.DecodeError) as e:
+    except (msgspec.DecodeError, ValueError, TypeError, RecursionError) as e:

The docstring could add: "Untyped decoding also raises UnicodeDecodeError (a str holding invalid UTF-8), TypeError (a map keyed by a map) and RecursionError (input nested too deep), so those are dropped too."

2. A stray message while engines subscribe makes the Coordinator return.

Before the main loop, the Coordinator waits for every engine to subscribe on publish_back:

for _ in self.engines:
    if publish_back.recv() != b"\x01":
        logger.error("DP Coordinator received unexpected message while waiting for engines to subscribe")
        return

publish_back is an XPUB bound to a routable TCP port in multi-node DP. Any other peer that subscribes to a topic sends a message that is not exactly b"\x01" (b"\x01" followed by the topic), and a scanner can do the same. The Coordinator then returns, and every engine waits for READY until the startup timeout. We hit this when we replayed the frames our deployment had received. Skipping the message keeps startup going:

-            # Wait until all engines subscribe.
-            for _ in self.engines:
-                if publish_back.recv() != b"\x01":
-                    logger.error(
-                        "DP Coordinator received unexpected message while "
-                        "waiting for engines to subscribe"
-                    )
-                    return
+            # Wait until all engines subscribe. In multi-node DP this socket
+            # is bound to a routable TCP endpoint, so anything else arriving
+            # meanwhile (e.g. a port scanner's probe) is skipped: returning
+            # here would leave every engine waiting for READY.
+            subscribed = 0
+            while subscribed < len(self.engines):
+                if publish_back.recv() == b"\x01":
+                    subscribed += 1
+                else:
+                    logger.warning(
+                        "DP Coordinator ignored a message that is not an "
+                        "engine subscription while waiting for engines to "
+                        "subscribe"
+                    )

Tests. Two tests cover these cases. The first:

def test_try_decode_frame_drops_what_untyped_decoding_raises():
    from vllm.v1.serial_utils import DROP_FRAME, try_decode_frame

    invalid_utf8_str = b"\xa3\xe0\x80\x80"
    map_keyed_map = b"\x81\x80\x01"
    for junk in (invalid_utf8_str, map_keyed_map):
        assert try_decode_frame(msgspec.msgpack.decode, junk, "test") is DROP_FRAME

The second starts a real DPCoordinatorProc on IPC sockets, subscribes a stray SUB to topic b"probe" before the engine subscribes, and checks that the engine still receives READY:

test_dp_coordinator_skips_a_stray_message_while_engines_subscribe

def test_dp_coordinator_skips_a_stray_message_while_engines_subscribe():
    """A non-subscription message before the engines subscribe is not fatal.

    The coordinator used to log and return on it, leaving every engine
    waiting for READY. A peer subscribing to a topic sends exactly such a
    message (``b"\\x01"`` followed by the topic).
    """
    import threading

    from vllm.utils.network_utils import get_open_zmq_ipc_path
    from vllm.v1.engine.coordinator import DPCoordinatorProc

    front_addr = get_open_zmq_ipc_path()
    back_output_addr = get_open_zmq_ipc_path()
    back_publish_addr = get_open_zmq_ipc_path()

    coord = DPCoordinatorProc(engine_count=1, enable_wave_coordination=True)
    captured: dict = {}

    def run():
        try:
            coord.process_input_socket(
                front_addr, back_output_addr, back_publish_addr, None
            )
            captured["exc"] = "returned"
        except BaseException as e:  # noqa: BLE001
            captured["exc"] = f"{type(e).__name__}: {e}"

    thread = threading.Thread(target=run, daemon=True)
    thread.start()

    ctx = coord.ctx
    try:
        stray = ctx.socket(zmq.SUB)
        stray.setsockopt(zmq.SUBSCRIBE, b"probe")
        stray.connect(back_publish_addr)
        time.sleep(0.2)  # let the stray subscription reach the XPUB first

        engine = ctx.socket(zmq.SUB)
        engine.setsockopt(zmq.SUBSCRIBE, b"")
        engine.connect(back_publish_addr)
        engine.RCVTIMEO = 5000
        assert engine.recv() == b"READY"
        assert "exc" not in captured, f"coordinator stopped: {captured.get('exc')}"
        assert thread.is_alive(), "coordinator stopped on the stray message"
    finally:
        with contextlib.suppress(Exception):
            ctx.destroy(linger=0)
        thread.join(timeout=5)

A small fix to the existing test. test_dp_coordinator_drops_undecodable_frame compares the published stats against [[5, 3]]. Each engine's entry is [waiting, running, kv_cache_usage], so with a recent main the comparison needs [engine[:2] for engine in ...] == [[5, 3]].

Two notes from running it.

• We left the core_client.py change out, because with hybrid load balancing that socket is IPC. We don't object to it.
• The PR currently has a merge conflict. If it would help, I'm happy to send a rebased version with these additions and you as co-author.

Our evidence, in short: we replayed the frames our deployment had received against a real Coordinator and NIXL listener bound to TCP. Without these fixes, 7 of 8 checks failed; with this PR plus the additions above, all 8 passed.

(AI assistance was used to investigate this and draft this comment; I have checked the findings and the suggested diff myself.)