[Bugfix][KV Offload][P2P] Fix EngineCore crash reconnecting to a reaped peer

[wsyjh8]

Purpose

Fixes #49809. With the OffloadingConnector P2P secondary tier, EngineCore dies with
AssertionError: ZmqConnection to <peer>:7777 already exists when a request reconnects to a peer
whose session was just reaped.

Relaxing that assertion is not sufficient. Three distinct defects sit on the same path, and fixing
any one of them alone converts the crash into a different crash.

Root cause

Defect 1 — a dead connection stays registered in the transport

P2PSession calls mark_dead() on its ControlConnection from session.poll(), which
P2PSecondaryTierManager._poll_once() runs after ZmqTransport.poll() has already swept dead
connections for that tick. _reap_dead_sessions() then drops the session from _sessions but
never touches ZmqTransport._connections, so the dead entry stays registered until the next
transport poll.

This has two consequences:

• An on_new_request() landing in that gap calls _control.connect(peer_id), trips
  assert peer_id not in self._connections, and the AssertionError escapes into the engine
  loop. This is the reported crash.
• poll() swept dead connections last, after _recv_router(). A reconnecting peer's first
  message was therefore enqueued into the still-registered dead connection and discarded when it
  was swept. P2PSession.attach_connection() announces itself exactly once, so that ConnectMsg
  was lost rather than retried.

Defect 2 — mark_dead() and close() shared one flag, so close() leaked both sockets

mark_dead() set self._closed, the same flag close() guards on. A close() following a
mark_dead() therefore returned early and never released the DEALER or its monitor socket. That
leaks two ZMQ sockets per peer death and contradicts the lifecycle documented in control/base.py,
where close() is specified as "Release all resources (sockets, monitors)". It also keeps the
never-closed DEALER owning inproc://p2p-monitor-<peer>, so the reconnect fails with EADDRINUSE.

Defect 3 — the monitor endpoint was derived from peer_id alone

libzmq unregisters an inproc endpoint on its reaper thread after zmq_close() has returned.
So even with a correct close(), an immediate reconnect to the same peer races that teardown and
intermittently fails with EADDRINUSE.

Reproduction

Measured in-tree against the branch base 26d725c, N = 50 fresh transports each doing
connect → mark_dead → connect:

variant	code under test	result
base as-is	26d725c unmodified	AssertionError 50 / 50
assertion relaxed only	26d725c + connect() pops and closes the stale entry instead of asserting; nothing else changed	ZMQError(EADDRINUSE) 50 / 50
this PR	this branch	clean reconnect 50 / 50

The middle row is a synthetic variant built for diagnosis — it is not the state of any branch,
and checking out 26d725c will not reproduce it. It exists only to show that removing the
assertion by itself does not fix anything; it just moves the failure onto defect 2.

Defect 3 in isolation — 5 rounds × 20 same-peer reconnects:

arm	code under test	EADDRINUSE per round (of 20)
peer-derived endpoint	this branch, with the monitor endpoint reverted to peer-derived	10, 9, 6, 5, 11 → 25–55 %
per-connection endpoint	this branch	0, 0, 0, 0, 0 → 0 %

Both arms carry the corrected close() from defect 2, so the monitor endpoint is the only
variable between them. The first arm is likewise a synthetic variant of this branch, not a state
of main. It is a race, so the range is what matters rather than any single figure.

Environment: Linux, Python 3.12.3, pyzmq 27.1.0, libzmq 4.3.5.

Changes

All in vllm/v1/kv_offload/tiering/p2p/control/zmq.py, plus two docstring clarifications in
control/base.py:

• Separate _dead from _closed; alive is not (_dead or _closed), mark_dead() only flips
  liveness, and close() always releases the sockets. send() now guards on not self.alive so
  it still raises after mark_dead(). (defect 2)
• connect() retires a registered dead connection before opening the replacement. A live
  registered connection is still a genuine duplicate and still asserts. (defect 1)
• The inproc monitor endpoint is unique per connection via a monotonic _monitor_seq, which
  never resets. (defect 3)
• poll() sweeps dead connections before routing traffic, extracted as
  _sweep_dead_connections() and also run after _check_monitors(). The old trailing sweep is
  gone, so no connection is left registered after being marked dead. (defect 1, second
  consequence)

Test plan

Extended the existing tests/v1/kv_offload/tiering/p2p/test_zmq_transport.py rather than adding a
new file: a new TestZmqReconnect class, plus one assertion added to
test_dead_connection_removed_on_poll. All CPU-only — the transport binds a loopback ROUTER and
opens DEALERs toward an unbound port, so no model, GPU, or second process is needed.

pytest -v tests/v1/kv_offload/tiering/p2p/test_zmq_transport.py

Each new case fails without the fix. With control/ reverted to the branch base and the new
tests kept in place:

$ git checkout 26d725c -- vllm/v1/kv_offload/tiering/p2p/control/
$ python -m pytest -q tests/v1/kv_offload/tiering/p2p/test_zmq_transport.py

______ TestZmqTransportConnectivity.test_dead_connection_removed_on_poll _______
>           assert new_conns[0]._sockets.dealer.closed
E           assert False
E            +  where False = <zmq.Socket(zmq.DEALER) at 0x7659751815c0>.closed
_________ TestZmqReconnect.test_close_after_mark_dead_releases_sockets _________
>           assert dealer.closed
E           assert False
E            +  where False = <zmq.Socket(zmq.DEALER) at 0x765975181080>.closed
____________ TestZmqReconnect.test_connect_retires_dead_connection _____________
>       assert peer_id not in self._connections, (
E       AssertionError: ZmqConnection to 127.0.0.1:60207 already exists
vllm/v1/kv_offload/tiering/p2p/control/zmq.py:131: AssertionError
____________ TestZmqReconnect.test_repeated_reconnect_to_same_peer _____________
>               assert conn._sockets.dealer.closed
E               assert False
_______ TestZmqReconnect.test_inbound_message_survives_dead_registration _______
>       raise AssertionError(f"no inbound connection within {deadline}s")
E       AssertionError: no inbound connection within 2.0s

5 failed, 11 passed in 6.72s

The four new cases map onto the defects above:

• test_close_after_mark_dead_releases_sockets — defect 2, the socket leak. Uses real sockets
  deliberately: a MagicMock reports .closed as truthy, so a mock-based version false-passes.
• test_connect_retires_dead_connection — defect 1, the reported AssertionError.
• test_repeated_reconnect_to_same_peer — defect 3; 10 reconnects catch the EADDRINUSE race.
• test_inbound_message_survives_dead_registration — defect 1's second consequence. Waits on a
  zmq.Poller over the ROUTER so the arrival is deterministic rather than timing-dependent.

With this PR, the full file is green, and stays green across 10 consecutive runs:

$ for i in $(seq 1 10); do python -m pytest -q tests/v1/kv_offload/tiering/p2p/test_zmq_transport.py; done

16 passed in 4.35s      16 passed in 4.80s
16 passed in 4.08s      16 passed in 4.95s
16 passed in 4.62s      16 passed in 4.49s
16 passed in 4.10s      16 passed in 4.99s
16 passed in 4.16s      16 passed in 3.80s

runs with nonzero exit: 0 / 10

The four pre-existing cases touched by the _dead/_closed split — test_mark_dead,
test_send_raises_when_closed, test_duplicate_connect_asserts and
test_dead_connection_removed_on_poll — are inside those 16. test_send_raises_when_closed still
matches the "closed connection" message, and test_duplicate_connect_asserts still gets an
AssertionError for a live duplicate, since that assertion was kept.

Committed tests vs. the manual stress runs

Two figures quoted in the issue thread
came from manual stress runs, not from committed tests. Separating them so the test file is not
read as doing more than it does:

Committed: test_repeated_reconnect_to_same_peer loops 10 reconnects. That is what CI runs,
and 10 is enough to catch the defect-3 race at the rate measured in the 5 × 20 table above.

Manual, not committed — "300 flapping reconnects": the patched transport driven 300 times
through connect(peer) → mark_dead() → teardown, once per teardown path, counting DEALERs left
open and entries left registered:

reap path: mark_dead + close(): 300/300 ok | leaked dealers 0 | residual _connections 1 | errors NONE
poll path: mark_dead + poll(): 300/300 ok | leaked dealers 0 | residual _connections 0 | errors NONE

residual _connections 1 on the reap path is expected — that arm never polls, so the final
connection is still registered when the loop ends.

Manual, not committed — "27/300 (~9%) EADDRINUSE": a standalone libzmq loop (a DEALER plus
monitor() on a fixed inproc address, 300 rounds), isolating defect 3 without the transport:

same addr, dealer closed   : 27/300 EADDRINUSE   iter33: Address in use
unique addr, dealer closed : 0/300 EADDRINUSE
same addr, dealer LEAKED   : 299/300 EADDRINUSE  iter1: Address in use

Both stress runs were done on a Windows host (CPython 3.13.11, pyzmq 27.1.0, libzmq 4.3.5) against
the real control/{zmq,base}.py with only vllm.logger and msgspec stubbed — a different
environment from the Linux in-tree figures earlier in this PR, which are the ones to rely on. The
in-tree equivalent of the second run is the 5 × 20 defect-3 table.

Lint:

$ pre-commit run --files vllm/v1/kv_offload/tiering/p2p/control/zmq.py \
    vllm/v1/kv_offload/tiering/p2p/control/base.py \
    tests/v1/kv_offload/tiering/p2p/test_zmq_transport.py
ruff check ..... Passed
ruff format .... Passed
typos .......... Passed
Run mypy for Python 3.10 ... Passed
(all other hooks Passed or Skipped — no files to check)
[pre-commit exit=0]   # and no hook rewrote anything

No model evaluation is included: this touches only control-plane connection lifecycle and cannot
affect model output, accuracy, or sampling.

Known remaining window

The sweep added to the top of poll() closes the window this bug actually occurs in: a session
calls mark_dead() during session.poll(), i.e. after ZmqTransport.poll() has already swept for
that tick, so the entry survives exactly until the next transport poll. That between-polls window
is where the reported crash lives, and it is now closed.

Two narrower windows remain open and are deliberately not addressed here:

• Death observed mid-poll. A peer that dies while poll() is running is not seen until the
  next _check_monitors(). Its connection reads as alive for the remainder of that tick, so a
  message can still be routed into a connection that is already doomed.
• Silent death. A peer that disappears without a TCP FIN — host loss, network partition — is
  not detected until the ZMQ heartbeat expires, up to HEARTBEAT_TIMEOUT = 10 s with the current
  settings. For that interval the connection is indistinguishable from a healthy one, and a
  connect() to that peer still hits the live-duplicate assertion.

Both are limits on how quickly death is detected, not defects in how a dead connection is
retired, and closing them would mean changing the detection mechanism rather than the lifecycle.
Neither is a regression: both behave exactly as they do on main today. The code comments in
poll() and in test_inbound_message_survives_dead_registration state this scope explicitly so
the next reader does not over-read the guarantee.

Relationship to #48021

Not a duplicate. As of 2026-07-25, gh pr list --state open --search "49809 in:body" returns
nothing, and no open PR touches p2p/control/. #48021 (open, unmerged, last updated 2026-07-24)
is the only adjacent work.

No file overlap with #48021, and this fix is orthogonal to any rewrite of
_reap_dead_sessions: the socket leak and the inproc endpoint reuse are transport-layer
problems, independent of manager-side cleanup ordering. #48021 touches p2p/manager.py and
p2p/session/* and does not modify control/ at all; it rewrites _reap_dead_sessions but does
not detach the connection from the control transport — it adds no _control. reference anywhere.

Worth noting for sequencing: #48021 adds a self._conn.mark_dead() call in the exception path of
P2PSession._do_send, so once it lands, any send failure produces this same dead-but-registered
state on an ordinary request path — widening this crash's window. The two changes compose cleanly
in either merge order.

This is also why the fix belongs in the transport rather than in _reap_dead_sessions: the reap is
not the only producer of a stale entry. _accept_new_peers() rejects a duplicate peer by calling
conn.close() on the fresh inbound connection while leaving it registered until the next poll,
which reaches the same assertion. Fixing connect() covers both; detaching from the manager would
cover only one, and would need a new ControlTransport abstract method.

Out of scope / follow-up

P2PSecondaryTierManager._poll_once() runs _accept_new_peers() before _reap_dead_sessions(),
and _accept_new_peers() rejects a peer that still has a session with duplicate connection from …. That ordering looks wrong, but I could not construct a reachable path: a session is always
reaped in the same tick its connection dies, so _accept_new_peers() never observes a stale one.
Left untouched deliberately — it is latent, and _poll_once/_reap_dead_sessions are being
rewritten in #48021, so changing the order here would collide for no present benefit.

AI assistance

AI assistance (Claude) was used for the investigation, the patch, and the tests.

I have reviewed every changed line myself and reproduced the failures before and after: the five
failing cases on the unpatched base, the 50/50 AssertionError → EADDRINUSE → clean progression
in the table above, the full file green across 10 consecutive runs, and pre-commit clean with no
hook rewrites. All figures quoted in this PR come from those in-tree runs on the branch as it
stands, rebased onto 26d725c.

[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.

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