[v0ropaev]

What?

nixl_xfer_handle and nixl_prepped_dlist_handle now get the owning nixl_agent rather than the pybind nixlBind.nixlAgent, which is what the parameter is named and documented as, and the two release calls in each go through self._agent.agent.

Why?

The deferred release in nixl_xfer_handle.__del__ never happens. The class docstring says what should:

__del__ calls release() and if it fails, it logs the failure and defers release by queuing the handle in leaked xfer handles list, which will be re-released during agent destruction

_leaked_xfer_handles is set up in nixl_agent.__init__ (_api.py:232) and drained in nixl_agent.__del__ (:285-296). But the handle is built with self.agent at :610 and :649, the pybind object, so self._agent._leaked_xfer_handles.append(...) looks the attribute up on that, raises AttributeError, and the inner except Exception: pass swallows it. py::class_<nixlAgent> in src/bindings/python/nixl_bindings.cpp:569 has no py::dynamic_attr(), so there is no instance dictionary for the name to come from either.

So the one case the deferral exists for, a releaseXferReq that fails because the transfer is still in flight and cannot be aborted, logs "keeping handle alive in agent leak list" and then drops the handle. Nothing retries it at agent destruction, and the memory the request holds stays pinned for the life of the process.

The drain side was the same mistake, self.releaseXferReq instead of self.agent.releaseXferReq, and was fixed in #1960. It has had an empty list to drain ever since.

Measured. I cannot build NIXL here (macOS, and the PyPI nixl package is a shim over the linux platform wheels), so I ran the real nixl_agent and nixl_xfer_handle out of _api.py with nixl._bindings stubbed by a class with __slots__ = (), which is the attribute behaviour pybind11 gives a class without dynamic_attr, and a releaseXferReq that raises:

shipped -> _leaked_xfer_handles = []
fixed   -> _leaked_xfer_handles = ['0xdead']

The log line appears in both runs.

How?

Passing the wrapper rather than the pybind agent makes _agent mean what the docstring says, which keeps the finalizer from having to reach for two different objects. The release calls grow one attribute hop.

nixl_prepped_dlist_handle never touched the leak list, so it works today; it is changed only so that the two classes mean the same thing by the same argument. If you would rather leave it alone I will drop that half.

Both classes are exported in __all__, so the constructor argument is technically public. They are produced by the agent and documented as opaque, and nothing in the repo constructs one outside _api.py, but it is a signature change in spirit and worth knowing about.

An alternative is to keep _agent as the pybind agent and give the handle a second reference for the leak list. That is one more slot in a finalizer for the same effect, which is why I did not.

Test

test_leaked_xfer_handle_is_queued_on_the_agent in test/python/test_nixl_api.py, using the existing one_empty_agent fixture, with monkeypatch putting a releaseXferReq that raises in place of agent.agent so the finalizer takes its failure path.

I could not run it here, since it needs a built NIXL for the fixture. The body is what the run above exercises against the real classes, so what is untested locally is the fixture, which every other test in that file already uses.

black 24.10.0, isort 5.12.0, flake8 7.1.2 with the hook's own arguments, and mypy 1.14.1 are all clean on both files, and mypy reports the same on a clean main.

Summary by CodeRabbit

• Bug Fixes
  • Improved cleanup reliability when releasing transfer resources fails. Affected resources remain tracked for follow-up cleanup instead of being overlooked.
  • Applied consistent failure handling across prepared transfers, transfer initialization, and descriptor lists, reducing the risk of resources being lost during cleanup.

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📝 Walkthrough

The Python handle wrappers now retain the owning nixl_agent and use its binding for release calls. A test checks that a transfer handle is queued on the agent when release fails during finalization.

Changes: Python handle ownership

Layer / File(s)	Summary
Wrapper ownership and cleanup src/api/python/_api.py, test/python/test_nixl_api.py	Handle creation passes the nixl_agent wrapper to prepared descriptor-list and transfer handles. Release calls use the wrapper’s binding. A test checks that a failed transfer release queues the handle on the owning agent.

Priority: ⬇️ Low

Estimated code review effort: 2 (Simple) | ~10 minutes

Change: Bug fix

Suggested reviewers: ovidiusm

Merge Risk: ⚪ Minimal · up to 20649

The ownership change routes handle releases through their owning agent, and the regression test clears its fabricated handle before teardown. No actionable merge risk remains in the reviewed changes.

Pre-merge checks: 4 · 1

Failed checks: 1 warning

Check name	Status	Explanation	Resolution
Docstring Coverage		Docstring coverage is 46.15% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 13 functions across 2 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

Passed checks: 4 passed

Check name	Status	Explanation
Linked Issues check		Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check		Check skipped because no linked issues were found for this pull request.
Title check		The title clearly and concisely describes the primary change: queueing failed transfer handles on their owning agent.
Description check		The description includes complete What, Why, and How sections. It explains the bug, the implementation, the test coverage, and validation results.

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[lluki]

Thanks for your contribution, this looks good to me.

The naming is unfortunate, having both agent and _agent and having the classes accessing private members is not great, but this problem is outside the scope of this PR.

[ColinNV]

/build

[svc-nixl]

🤖 CI Triage Agent — nixl-ci-dl-gpu · commit 3cbcc6aa

TL;DR: The new test test_leaked_xfer_handle_is_queued_on_the_agent parks a fake handle 0xDEAD in nixl_agent._leaked_xfer_handles and never clears it, so when the monkeypatch is undone and the real agent is finalized, nixl_agent.__del__ passes 0xDEAD to the real C++ releaseXferReq() and pytest dies with SIGSEGV (exit 139). Clear the leak list at the end of the test (or keep the failing stub installed for the agent's lifetime).

Full analysis

Summary: Stage "Run DL Python tests" (#219) failed — pytest -s test/python crashed with Fatal Python error: Segmentation fault in nixl_agent.__del__, and srun reported gb-nvl-082-compute01: task 0: Exited with exit code 139.

Root cause: Code defect introduced by this PR's leak-list change, not infrastructure. Sequence visible in the log and source:

1. Last test output before the crash is the new leak-list path: NIXL ERROR _api.py:122 nixl_xfer_handle finalization failed for 0xdead; keeping handle alive in agent leak list.
2. test_leaked_xfer_handle_is_queued_on_the_agent (test/python/test_nixl_api.py:563-580) monkeypatches one_empty_agent.agent with FailingRelease, creates nixl_xfer_handle(one_empty_agent, 0xDEAD), deletes it so nixl_xfer_handle.__del__ (src/api/python/_api.py:116-131) appends the bogus integer 0xDEAD to self._agent._leaked_xfer_handles, and asserts the list contents — but leaves 0xDEAD in the list.
3. At teardown, monkeypatch restores the real pybind nixlAgent. When one_empty_agent is finalized, nixl_agent.__del__ drains the list and calls self.agent.releaseXferReq(0xDEAD) — src/api/python/_api.py:288, which is exactly the frame in the fatal traceback (_api.py", line 288 in __del__). The C++ side treats the value as a nixlXferReqH* and dereferences address 0xdead → SIGSEGV. The except Exception wrapper around it cannot catch a segfault.

So the test leaves a poison value that is guaranteed to be dereferenced by the very destructor the test is documenting.

Implicated commit: a6b99c8 "Queue a leaked xfer handle on the agent that owns the list" — Dmitry Voropaev (merged into the branch by 3cbcc6a, Colin Hirsch)

File: test/python/test_nixl_api.py:563-580 (poison handle never cleared); crash site src/api/python/_api.py:288 (nixl_agent.__del__ → releaseXferReq)

Suggested fix: Make the test not hand a fabricated pointer to the real agent. Minimal change — clear the list after asserting, inside a try/finally so it happens even on assertion failure:

    handle = nixl_xfer_handle(one_empty_agent, 0xDEAD)
    del handle
    gc.collect()

    try:
        assert one_empty_agent._leaked_xfer_handles == [0xDEAD]
    finally:
        # Don't let the agent destructor hand this fake pointer to the real
        # C++ releaseXferReq() after monkeypatch restores `agent` (segfaults).
        one_empty_agent._leaked_xfer_handles.clear()

Two hardening follow-ups worth doing in the same PR, since the crash is reachable from real use too:

• In nixl_agent.__del__, wrap the drain loop in except BaseException and drain into a local copy, so a failure finalizing one handle can't leave the list half-processed. (This does not protect against an invalid pointer — only C++ validation can.)
• Preferably have releaseXferReq on the C++/pybind side reject handles it doesn't own (lookup in the agent's live-request set, raise nixlNotFoundError) rather than blindly dereferencing, so a stale/bogus handle from the Python finalizer degrades to an exception instead of a segfault.

Note the test is also the last thing running at crash time, so the remaining tests in the 46-item session never ran; expect additional failures to surface once this is fixed.

Related: PR #2381 "Queue a leaked xfer handle on the agent that owns the list" (the PR under test); related earlier work: #1960 "fix(python): release deferred transfer handles through agent"

[svc-nixl]

🤖 CI Triage Agent — nixl-ci-gpu-vr · commit 3cbcc6aa

TL;DR: test_leaked_xfer_handle_is_queued_on_the_agent seeds the bogus handle 0xDEAD into the agent's leak list while agent is monkeypatched; once pytest reverts the monkeypatch at teardown, nixl_agent.__del__ drains that list against the real pybind agent, which dereferences 0xdead and segfaults (exit 139). Fix: have the test clear _leaked_xfer_handles (or keep a non-C++ stub installed) before teardown, and harden the drain loop.

Full analysis

Summary: Stage "Run Python tests" (node 219) failed — pytest -s test/python died with Fatal Python error: Segmentation fault (core dumped), srun task 0 on vr-nvl72-prd-l11-165-c11 exited 139.

Root cause: A use of a fabricated pointer value as a real NIXL transfer handle. The new test does:

monkeypatch.setattr(one_empty_agent, "agent", FailingRelease())
handle = nixl_xfer_handle(one_empty_agent, 0xDEAD)
del handle; gc.collect()
assert one_empty_agent._leaked_xfer_handles == [0xDEAD]

nixl_xfer_handle.__del__ sees releaseXferReq raise, logs the message visible in the log (nixl_xfer_handle finalization failed for 0xdead; keeping handle alive in agent leak list, _api.py:122) and appends 0xDEAD to one_empty_agent._leaked_xfer_handles. The test then passes — but pytest undoes the monkeypatch at teardown, restoring the real nixlBind.nixlAgent. When the fixture's nixl_agent is finalized moments later, nixl_agent.__del__ drains the leak list and calls the genuine self.agent.releaseXferReq(0xDEAD), which reinterprets 0xdead as a nixlXferReqH* and dereferences it. The crash traceback points exactly there:

File ".../nixl_cu13/_api.py", line 288 in __del__

Line 288 is self.agent.releaseXferReq(h) inside nixl_agent.__del__. The surrounding try/except Exception cannot catch SIGSEGV, so the whole pytest process dies and takes the remaining tests with it. This is deterministic, not a flake, and not infra-related — the UCX Connection refused and UVX plugin messages earlier in the log are from unrelated negative tests that passed.

Implicated commit: a6b99c8e — "Queue a leaked xfer handle on the agent that owns the list", Dmitry Voropaev (reaching CI via merge 3cbcc6aa, Colin Hirsch, branch fix/xfer-handle-leak-list)

File: test/python/test_nixl_api.py:563-580 (the leaking test); crash site src/api/python/_api.py:288 in nixl_agent.__del__

Suggested fix: Two changes, the first being the actual bug:

1. Make the test not leave a poisoned handle behind for the real agent to free. Either drain the list inside the test, or replace the agent permanently so teardown never restores the C++ object:

    assert one_empty_agent._leaked_xfer_handles == [0xDEAD]
    # Don't let the agent destructor hand 0xDEAD to the real C++ agent:
    # monkeypatch is reverted at teardown, so clear the list here.
    one_empty_agent._leaked_xfer_handles.clear()

A cleaner variant is to assert on a copy and clear in a finally, or install the FailingRelease stub via direct attribute assignment on a locally-constructed agent whose lifetime ends inside the test.

2. Harden the drain loop so a bad entry can never take down the interpreter — __del__ should not pass unvalidated integers to C++. Add handle validation in releaseXferReq on the C++/pybind side (reject handles not present in the agent's live-request registry, returning NIXL_ERR_INVALID_PARAM) so a stale or bogus handle raises a Python exception instead of segfaulting. This also protects real users whose handles outlive their agent.

Related: PR #2381 (this change); PR #2244 "core: keep transfer pollable after failed release for cancellation" and PR #2252 "posix: refuse to release a transfer with outstanding I/O" both touch the failed-release path and are worth coordinating with for the validation work.

[svc-nixl]

🤖 CI Triage Agent — nixl-ci-non-gpu · commit 3cbcc6aa

TL;DR: All six "Test Python" matrix legs segfault deterministically in nixl_agent.__del__ (src/api/python/_api.py:288) because the new leak-list drain re-calls releaseXferReq() on the bogus handle 0xDEAD that test_leaked_xfer_handle_is_queued_on_the_agent plants — by then monkeypatch has restored the real pybind agent, so the C++ layer dereferences address 0xdead. Fix: don't re-attempt release in the agent destructor (log only), and have the test drain/clear _leaked_xfer_handles before teardown.

Full analysis

Summary: pytest -s test/python dies with Fatal Python error: Segmentation fault (core dumped, exit via SIGSEGV at .gitlab/test_python.sh: line 71) in every one of the six parallel "Test Python" stages; the "Build" and "Test CPP" stages all passed.

Root cause: A use-after-garbage pointer introduced by this PR's leak-list feature, triggered by its own unit test.

The test at test/python/test_nixl_api.py:563-580 monkeypatches the agent's pybind object with a Python stub whose releaseXferReq always raises, then creates nixl_xfer_handle(one_empty_agent, 0xDEAD) and deletes it. The finalizer at _api.py:116-132 catches the exception, logs it, and queues the raw integer on the owning agent:

2026-10-12 18:48:22 NIXL ERROR _api.py:122 nixl_xfer_handle finalization failed for 0xdead; keeping handle alive in agent leak list
.Fatal Python error: Segmentation fault

The assertion itself passes (note the . immediately after the error line). The crash happens during teardown: pytest's monkeypatch fixture undoes the patch and restores the real nixlBind.nixlAgent, then the agent is collected and nixl_agent.__del__ (_api.py:283-296) iterates the leak list and calls self.agent.releaseXferReq(0xDEAD) — now on the genuine C++ agent. 0xdead is not a valid nixlXferReqH*, so the C++ side dereferences it and the process dies. The traceback pins it exactly:

Current thread ...
  File ".../nixl_cu13/_api.py", line 288 in __del__
  File ".../_pytest/runner.py", line 150 in runtestprotocol

Line 288 is self.agent.releaseXferReq(h). The except Exception guard there cannot help — a segfault is not a Python exception. Two design problems compound: the destructor retries a release that is already known to have failed, and it does so without any guarantee the handle or the owning C++ agent is still the one the handle came from.

Implicated commit: a6b99c8e "Queue a leaked xfer handle on the agent that owns the list" (Dmitry Voropaev, 2026-10-11), as merged into this branch at 3cbcc6aa (Colin Hirsch) — the head commit under test.

File: src/api/python/_api.py:288 (the retry in nixl_agent.__del__); trigger at test/python/test_nixl_api.py:574-578

Suggested fix: Pick the first; the second and third are hardening worth doing anyway.

1. Make the drain non-retrying. A handle that already failed to release is not going to release successfully later, and the destructor has no way to prove the pointer is still valid. Replace the loop body at _api.py:286-296 with diagnostics only:

   def __del__(self):
       if getattr(self, "_leaked_xfer_handles", None):
           for h in self._leaked_xfer_handles:
               logger.error("Leaked nixl_xfer_handle 0x%x was never released", h)
           self._leaked_xfer_handles.clear()

   If reclaiming the memory is genuinely required, the retry must go through a C++ entry point that validates the handle against the agent's live request set rather than trusting a raw integer.

2. Make the test self-cleaning. Even with (1), the test shouldn't leave a poisoned value where a future change could dereference it. Add one_empty_agent._leaked_xfer_handles.clear() after the assert on line 580, or keep the FailingRelease stub installed for the agent's whole lifetime instead of letting monkeypatch restore the real agent while the list is still populated.

3. Validate in C++. releaseXferReq should reject a handle it doesn't recognise with an exception rather than dereferencing it — this is the same class of bug as issue releaseXferReq use-after-free (SegFault) + nixlBasicDesc match key excludes metaInfo #2067 ("releaseXferReq use-after-free (SegFault)"), which suggests the C++ side still has no handle validation.

Related: PR #2381 (this PR — "Queue a leaked xfer handle on the agent that owns the list"); issue #2067 ("releaseXferReq use-after-free (SegFault) + nixlBasicDesc match key excludes metaInfo") is the same missing-handle-validation weakness on the C++ side.

[svc-nixl]

🤖 CI Triage Agent — nixl-ci-gpu · commit 3cbcc6aa

TL;DR: The new test test_leaked_xfer_handle_is_queued_on_the_agent leaves the bogus handle 0xDEAD in nixl_agent._leaked_xfer_handles, and when monkeypatch restores the real pybind agent at teardown, nixl_agent.__del__ passes 0xDEAD to the real releaseXferReq(), which casts it to nixlXferReqH* and dereferences it → segfault (exit 139). Fix: clear the leak list at the end of that test, and make the releaseXferReq binding reject handles the agent doesn't own.

Full analysis

Summary: Run Python tests stage of nixl-ci-gpu #4054 crashed — pytest -s test/python died with Fatal Python error: Segmentation fault (core dumped, srun task exit code 139 on mizu04) right after test_leaked_xfer_handle_is_queued_on_the_agent.

Root cause: A use of a fabricated pointer, not an infra problem. The log shows the sequence precisely:

NIXL ERROR _api.py:122 nixl_xfer_handle finalization failed for 0xdead; keeping handle alive in agent leak list
Fatal Python error: Segmentation fault
Current thread ...:
  File ".../nixl_cu13/_api.py", line 288 in __del__
  File ".../_pytest/runner.py", line 150 in runtestprotocol

test_leaked_xfer_handle_is_queued_on_the_agent (test/python/test_nixl_api.py:563-580) monkeypatches one_empty_agent.agent with a FailingRelease stub, then constructs nixl_xfer_handle(one_empty_agent, 0xDEAD) and deletes it. nixl_xfer_handle.__del__ (src/api/python/_api.py:116-132) catches the stub's RuntimeError and appends the raw integer 0xDEAD to one_empty_agent._leaked_xfer_handles — which is exactly what the test asserts.

The test never cleans that up. At test teardown pytest's monkeypatch restores the real pybind nixlAgent onto the instance, and then the fixture's agent is garbage-collected. nixl_agent.__del__ drains the leak list through the now-real agent:

for h in list(self._leaked_xfer_handles):
    try:
        self.agent.releaseXferReq(h)        # _api.py:288  <- crash frame

The binding does no validation — src/bindings/python/nixl_bindings.cpp:878-883 casts the integer straight to a pointer: agent.releaseXferReq((nixlXferReqH *)reqh). Dereferencing address 0xdead segfaults. The except Exception guard in __del__ cannot catch SIGSEGV, so the whole pytest process dies and the stage fails. Note the stack frame is runtestprotocol, i.e. teardown of that very test, confirming the ordering.

Implicated commit: a6b99c8e — "Queue a leaked xfer handle on the agent that owns the list", Dmitry Voropaev (2026-10-11), which introduced both nixl_agent._leaked_xfer_handles/the draining __del__ and the new test; brought onto this branch via merge 3cbcc6aa (Colin Hirsch).

File: test/python/test_nixl_api.py:563-580 (test leaves 0xDEAD queued); crash site src/api/python/_api.py:288; missing validation at src/bindings/python/nixl_bindings.cpp:878-883.

Suggested fix:

1. Make the test clean up after itself so no fake pointer survives into the real agent's destructor — e.g. after the assertion add one_empty_agent._leaked_xfer_handles.clear(), or better, assert on a copy and clear inside the same block:

   assert one_empty_agent._leaked_xfer_handles == [0xDEAD]
   one_empty_agent._leaked_xfer_handles.clear()

   (Cleanest is an explicit teardown/try: ... finally: so the list is emptied even if the assert fails.)
2. Harden the API so a bad handle can never crash the interpreter: have releaseXferReq in nixl_bindings.cpp validate that the pointer is a handle the agent actually owns and return NIXL_ERR_NOT_FOUND (→ nixlNotFoundError) instead of blind-casting a user-supplied integer. That turns this class of bug into a catchable exception, which nixl_agent.__del__'s existing except Exception already handles.
3. Optionally, have nixl_agent.__del__ drain the leak list only through the same agent object that queued the handle (store (agent_obj, handle) pairs), so a swapped-out agent attribute can't be handed foreign handles.

Related: PR #2381 (this branch, fix/xfer-handle-leak-list); predecessor PR #1960 "fix(python): release deferred transfer handles through agent". Also relevant to handle-release hardening: #2244, #2252.

[ColinNV]

/build

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⚠️ Outside diff range comments (1)

🟠 Major · Clear the fabricated handle before agent teardown. · test_nixl_api.py:576

test/python/test_nixl_api.py:576
🩺 Stability & Availability | 🟠 Major | ⚡ Quick win

Clear the fabricated handle before agent teardown.

Line 576 creates a handle with 0xDEAD. When release fails, its finalizer queues that value in one_empty_agent._leaked_xfer_handles. The test leaves the value there. During teardown, nixl_agent.__del__ passes it to the restored pybind binding, which can crash the process. Remove the queued test value in a finally block before pytest restores the real binding.

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Review comment at @test/python/test_nixl_api.py at line 576:
Update the test around the fabricated handle created with 0xDEAD to remove that
value from one_empty_agent._leaked_xfer_handles in a finally block, ensuring
cleanup runs before pytest restores the real binding and agent teardown.

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Outside diff comments:
Review comments at @test/python/test_nixl_api.py:
- Line 576: Update the test around the fabricated handle created with 0xDEAD to
remove that value from one_empty_agent._leaked_xfer_handles in a finally block,
ensuring cleanup runs before pytest restores the real binding and agent
teardown.

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• src/api/python/_api.py
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[svc-nixl]

🤖 CI Triage Agent — nixl-ci-dl-gpu · commit 6db56b9e

TL;DR: The new test test_leaked_xfer_handle_is_queued_on_the_agent parks a fabricated handle 0xDEAD in the real agent's leak list, and when nixl_agent.__del__ later drains that list through the real C++ releaseXferReq, the bogus pointer is dereferenced and the pytest process segfaults (exit 139). Fix the test to clear _leaked_xfer_handles before teardown (or keep the failing-release stub installed for the agent's lifetime).

Full analysis

Summary: Run DL Python tests stage failed — pytest -s test/python died with Fatal Python error: Segmentation fault (core dumped, srun task exit code 139) on node gb-nvl-147-compute05.

Root cause: The log pins the crash precisely. The last test line before the fault is:

NIXL ERROR _api.py:122 nixl_xfer_handle finalization failed for 0xdead; keeping handle alive in agent leak list
Fatal Python error: Segmentation fault
Current thread ...:
  File ".../nixl_cu13/_api.py", line 288 in __del__

Line 288 is self.agent.releaseXferReq(h) inside nixl_agent.__del__, the loop that drains _leaked_xfer_handles.

The sequence is:

1. test_leaked_xfer_handle_is_queued_on_the_agent (test/python/test_nixl_api.py:563-580) monkeypatches one_empty_agent.agent with a FailingRelease stub that raises, then constructs nixl_xfer_handle(one_empty_agent, 0xDEAD) — a completely fabricated handle value — and deletes it.
2. nixl_xfer_handle.__del__ (src/api/python/_api.py:116-132) catches the stub's RuntimeError, logs the message seen in the log, and appends 0xDEAD to one_empty_agent._leaked_xfer_handles (line 129). The assert passes, test reports ..
3. At teardown, monkeypatch reverts agent.agent back to the real pybind agent, but 0xDEAD is still sitting in the leak list. When the fixture's agent is garbage-collected, nixl_agent.__del__ (line 283-296) iterates the list and hands 0xDEAD to the real nixlAgent::releaseXferReq.
4. nixlAgent::releaseXferReq (src/core/nixl_agent.cpp:1334-1359) does zero validation — its first act is if (req_hndl->status == NIXL_IN_PROG), dereferencing address 0xdead, then delete req_hndl. That is the segfault.

The try/except Exception around line 288 cannot help: a bad-pointer dereference in C++ is not a catchable Python exception. The test passed its own assertion; the process died afterwards during teardown, so pytest never got to report a summary.

Implicated commit: a6b99c8e — "Queue a leaked xfer handle on the agent that owns the list", Dmitry Voropaev (PR #2381, branch fix/xfer-handle-leak-list; merged into the tested tree via 3cbcc6aa). This commit introduced both the agent-owned leak list and the test that seeds it with a sentinel value.

File: test/python/test_nixl_api.py:563-580 (the test that leaves 0xDEAD queued) and src/api/python/_api.py:283-296 (the unguarded drain loop that dereferences it)

Suggested fix: Two changes, the first being sufficient to green the build:

1. Make the test clean up after itself so the sentinel never reaches the real agent. Add after the assertion:

   assert one_empty_agent._leaked_xfer_handles == [0xDEAD]
   # Drop the fabricated handle: the real C++ releaseXferReq would
   # dereference 0xDEAD during agent teardown and segfault.
   one_empty_agent._leaked_xfer_handles.clear()

   Even cleaner is to make the stub permanent for the agent's lifetime (set one_empty_agent.agent = FailingRelease() directly rather than via monkeypatch, so teardown cannot restore the real agent), which also tests the __del__ error-logging path at line 291.

2. Harden the drain loop defensively — nixl_agent.__del__ currently trusts arbitrary ints. Consider storing the nixl_xfer_handle objects (or a (handle, owning_agent) pair validated against handles the agent actually issued) instead of raw ints, so a handle the agent never created is skipped rather than dereferenced. Separately, nixlAgent::releaseXferReq accepting a raw pointer with no sanity check makes any stale/garbage handle from Python an instant process kill; a null check at minimum would be worthwhile.

Related: PR #2381 — #2381 (the PR under test; the failure is self-inflicted by its own new test). Prior work on this path: c8758860 "fix(python): release deferred transfer handles through agent" (#1960).

[svc-nixl]

🤖 CI Triage Agent — nixl-ci-gpu-vr · commit 6db56b9e

TL;DR: pytest crashed with a SIGSEGV (exit 139) in nixl_agent.__del__ because the new leak-list drain added by PR #2381 calls releaseXferReq() on the raw sentinel handle 0xDEAD injected by test_leaked_xfer_handle_is_queued_on_the_agent, and the C++ side dereferences that integer as a nixlXferReqH*. Fix is to not re-attempt a raw-pointer release in the agent destructor (store/validate handle objects, or clear the leak list in the test teardown).

Full analysis

Summary: Stage "Run Python tests" (node 219) failed — pytest -s test/python died with Fatal Python error: Segmentation fault (core dumped), and srun reported vr-nvl72-prd-l11-165-c15: task 0: Exited with exit code 139.

Root cause: A genuine code defect in the PR under test, not infra. The log shows the exact sequence:

NIXL ERROR _api.py:122 nixl_xfer_handle finalization failed for 0xdead; keeping handle alive in agent leak list
Fatal Python error: Segmentation fault
Current thread ...:
  File ".../nixl_cu13/_api.py", line 288 in __del__

• test_leaked_xfer_handle_is_queued_on_the_agent (test/python/test_nixl_api.py:563) monkeypatches one_empty_agent.agent with a FailingRelease stub and constructs nixl_xfer_handle(one_empty_agent, 0xDEAD) — a fabricated, non-existent handle.
• nixl_xfer_handle.__del__ (src/api/python/_api.py:116-131) catches the stub's RuntimeError, logs the message seen above, and appends the raw int 0xDEAD to agent._leaked_xfer_handles — which is exactly what the test asserts, so the test body itself passes.
• The crash comes afterwards. During pytest's teardown, monkeypatch restores the real pybind agent, then the one_empty_agent fixture is collected, and nixl_agent.__del__ (src/api/python/_api.py:283-296) drains the leak list: self.agent.releaseXferReq(h) at line 288 — the exact frame in the fatal traceback. This now hands 0xDEAD to the real C++ releaseXferReq, which casts it to nixlXferReqH* and dereferences it with no validity check, segfaulting the interpreter.

The try/except Exception around line 288 cannot save it: a wild-pointer dereference in C++ is a signal, not a Python exception. The UCX "Connection refused" line at 06:44:50 and the UVX plugin errors are unrelated expected negative-test noise; there are no large timestamp gaps (whole run is ~45 s of continuous output), so this is not a hang or timeout.

Implicated commit: a6b99c8e "Queue a leaked xfer handle on the agent that owns the list" — Dmitry Voropaev (PR #2381, branch fix/xfer-handle-leak-list, merged with main at 3cbcc6aa; build head 6db56b9e)

File: src/api/python/_api.py:288 (drain in nixl_agent.__del__), triggered by src/api/python/_api.py:129 and test/python/test_nixl_api.py:563-580

Suggested fix: Pick one, preferably both:

1. Don't re-release raw ints in the agent destructor. A handle whose release already failed once is not safely re-releasable, and _leaked_xfer_handles carries no provenance. Either store the nixl_xfer_handle objects and drain through a path that goes via the same pybind agent that originally owned them, or drop the drain in __del__ entirely and keep the list purely diagnostic (log the leak at exit rather than retrying a free). The current retry converts a recoverable leak into a hard crash.
2. Make the test self-contained. Keep the failing stub installed for the agent's whole lifetime, or clear the list before teardown so the destructor never sees the sentinel:

   assert one_empty_agent._leaked_xfer_handles == [0xDEAD]
   one_empty_agent._leaked_xfer_handles.clear()  # never hand 0xDEAD to the real agent

3. Defensive hardening (follow-up): give releaseXferReq the same stale/unknown-handle guard that postXferReq/estimateXferCost got in PR Guard postXferReq/estimateXferCost against stale-generation xfer handles #2027, so a bogus handle returns an error instead of dereferencing wild memory.

Related: PR #2381 (the PR under test), issue #2067 "releaseXferReq use-after-free (SegFault)" — same unvalidated-handle class of bug, PR #2027 "Guard postXferReq/estimateXferCost against stale-generation xfer handles" — the precedent for fix #3, PR #1960 "release deferred transfer handles through agent" — earlier work on this same code path

[svc-nixl]

🤖 CI Triage Agent — nixl-ci-non-gpu · commit 6db56b9e

TL;DR: The six "Test Python" stages all segfaulted in nixl_agent.__del__ because PR #2381's new leaked-handle retry re-calls releaseXferReq() on a handle whose release already failed — including the deliberately bogus 0xdead handle from the negative test, which the C++ layer dereferences and crashes on. Stop re-releasing handles that already failed to release (or only queue handles that failed with a retryable status), and mark them released.

Full analysis

Summary: pytest -s test/python crashed with Fatal Python error: Segmentation fault (core dumped, exit via .gitlab/test_python.sh: line 71) in all six parallel "Test Python" stages of build #3508.

Root cause: The test suite exercises a negative path with a fabricated transfer handle 0xdead. The log shows the sequence exactly:

NIXL ERROR _api.py:122 nixl_xfer_handle finalization failed for 0xdead; keeping handle alive in agent leak list
.Fatal Python error: Segmentation fault
Current thread ...:
  File ".../nixl_cu12/_api.py", line 288 in __del__

The new code in this branch does two things that combine badly:

1. nixl_xfer_handle.__del__ (src/api/python/_api.py:116-132) catches the exception from releaseXferReq(0xdead), logs it, and appends the raw integer to self._agent._leaked_xfer_handles. It never sets self._released = True, so the handle is recorded as "retry me later".
2. nixl_agent.__del__ (src/api/python/_api.py:283-296) then iterates that list and calls self.agent.releaseXferReq(h) again at line 288 — the exact frame in the fault traceback.

A release that failed once will fail the same way again; worse, for a handle that was never a valid object (0xdead), the C++ releaseXferReq dereferences a garbage pointer. That is a SIGSEGV, not a Python exception, so the except Exception wrapper around line 288 cannot contain it and the interpreter dies — taking the whole pytest session down after the test itself had already passed (note the . before Fatal Python error).

This is a code defect introduced by the branch under test, not flaky infrastructure: the failure is deterministic and identical across all six independently-built variants, while every "Build" and "Test CPP" stage passed.

Implicated commit: [REDACTED:Hex High Entropy String] on branch fix/xfer-handle-leak-list (PR #2381, "Queue a leaked xfer handle on the agent that owns the list")

File: src/api/python/_api.py:288 (the re-release loop in nixl_agent.__del__), with the queuing site at src/api/python/_api.py:129

Suggested fix: Don't blindly retry a release that already failed. Concretely:

• Only queue a handle for deferred release when the failure is genuinely retryable — e.g. the transfer is still in progress (NIXL_IN_PROG) — and never when the backend reported an invalid/unknown handle. That alone keeps 0xdead out of the leak list.
• Set self._released = True in nixl_xfer_handle.__del__ once ownership moves to the agent's leak list, so the handle can't be released twice by two different paths.
• In nixl_agent.__del__, guard the retry: it currently relies on except Exception, which cannot catch a segfault. Validate the handle on the C++ side so releaseXferReq returns an error status for an unknown handle instead of dereferencing it — that makes the Python-level try/except actually meaningful and hardens the API against any caller passing a stale handle.
• Add a regression test asserting that an agent holding a bogus leaked handle can be garbage-collected without crashing the interpreter.

As a smaller alternative, dropping the leak-list retry entirely restores the previous behaviour (log and move on), which the passing history of this stage shows was stable.

Related: #2381

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[svc-nixl]

🤖 CI Triage Agent — nixl-ci-gpu · commit 6db56b9e

TL;DR: The Python test stage segfaulted in nixl_agent.__del__ because the new leak-list test queues a fake handle (0xDEAD) and the agent destructor later passes it to the real C++ releaseXferReq, dereferencing a bogus pointer. Fix: have the test drain one_empty_agent._leaked_xfer_handles after asserting (and/or validate handles in releaseXferReq).

Full analysis

Summary: Stage "Run Python tests" (node 234) failed — pytest -s test/python died with Fatal Python error: Segmentation fault (exit 139 on node mizu01) right after test_leaked_xfer_handle_is_queued_on_the_agent.

Root cause: The log pinpoints the crash precisely:

NIXL ERROR _api.py:122 nixl_xfer_handle finalization failed for 0xdead; keeping handle alive in agent leak list
Fatal Python error: Segmentation fault
Current thread ...:
  File ".../nixl_cu13/_api.py", line 288 in __del__
  File ".../_pytest/runner.py", line 150 in runtestprotocol

_api.py:288 is self.agent.releaseXferReq(h) inside nixl_agent.__del__, which drains self._leaked_xfer_handles.

The new test test_leaked_xfer_handle_is_queued_on_the_agent (test/python/test_nixl_api.py:563-580) monkeypatches one_empty_agent.agent with a FailingRelease stub, constructs nixl_xfer_handle(one_empty_agent, 0xDEAD), and deletes it. The finalizer at _api.py:116-132 catches the stub's RuntimeError and appends the synthetic value 0xDEAD to _leaked_xfer_handles — the assertion then passes.

The problem is what happens after the test: pytest's monkeypatch teardown restores the real pybind nixlAgent onto one_empty_agent.agent, and the leak list still contains 0xDEAD. When the fixture's agent is garbage-collected during runtestprotocol teardown, nixl_agent.__del__ (src/api/python/_api.py:283-296) iterates the list and calls the real releaseXferReq(0xDEAD). 0xDEAD was never a valid nixlXferReqH*, so C++ dereferences a wild pointer → SIGSEGV. The try/except Exception around it cannot catch a native segfault.

This is deterministic for this branch, not flaky infra: the crash occurs in the very test the PR adds, on the test's own fixture teardown.

Implicated commit: a6b99c8e — "Queue a leaked xfer handle on the agent that owns the list", Dmitry Voropaev (on branch fix/xfer-handle-leak-list, merged forward in 3cbcc6aa by Colin Hirsch)

File: test/python/test_nixl_api.py:563-580 (poisons the leak list); crash site src/api/python/_api.py:288 in nixl_agent.__del__

Suggested fix:

1. Make the test clean up after itself so the sentinel never reaches the real agent:

    assert one_empty_agent._leaked_xfer_handles == [0xDEAD]
    # Don't let the agent destructor hand a synthetic handle to the real
    # C++ releaseXferReq -- monkeypatch is undone before __del__ runs.
    one_empty_agent._leaked_xfer_handles.clear()

Using try/finally (or an autouse cleanup) is safer, so the list is cleared even if the assertion fails.
2. Harden the production path too, since a Python-level except Exception gives no protection against an invalid handle: validate the pointer in the C++ releaseXferReq binding (return NIXL_ERR_INVALID_PARAM for a handle not in the agent's live-request set) rather than dereferencing it. Otherwise any genuinely stale handle in the leak list will crash the interpreter at agent teardown — which defeats the purpose of the leak list.
3. Consider having the finalizer store the nixl_xfer_handle object (or validate on insert) instead of a raw int, so only handles the agent actually issued can ever be re-released.

Related: PR #2381 — "Queue a leaked xfer handle on the agent that owns the list" (the PR under test)

[svc-nixl]

🤖 CI Triage Agent — nixl-ci-build-wheel · commit 6db56b9e

TL;DR: The build's wheels compiled and pushed fine; both Allocate Environment stages failed because salloc on the gb200nvl72_ci partition never got a node within its one-hour --immediate=3600 window and exited with "Unable to allocate resources: Connection timed out". This is a cluster-capacity/queueing failure, not a defect in PR #2381 — re-run the job, and reduce the request from an exclusive whole GB200 node to the GPUs actually needed.

Full analysis

Summary: nixl-ci-build-wheel #2178 failed in the two parallel Allocate Environment stages (node IDs 562 build_helper_vllm and 579 build_helper_sglang); every build/packaging stage before them succeeded.

Root cause: Slurm could not satisfy the allocation request on partition gb200nvl72_ci. Both stages ran:

salloc -N 1 -p gb200nvl72_ci --job-name=nixl-ci-build-wheel-{vllm,sglang}-2178 \
       --immediate=3600 --time=02:00:00 --no-shell --account=oberon-gb-ci

and both logged salloc: lua: Setting job to exclusive mode as no gpu count was specified, then Pending job allocation 2499757 / 2499800, queued and waiting for resources, and finally salloc: error: Unable to allocate resources: Connection timed out.

The timing confirms this is queue starvation rather than a hang in nixl code: the vllm salloc was issued at 06:48:31 and errored at 07:48:40 — exactly the 3600 s --immediate deadline; the sglang one at 06:50:22 → 07:50:31, likewise exactly 3600 s. salloc itself was alive and waiting the whole time; it gave up on its own deadline because no node in the partition became free. Nothing in the PR under test (fix/xfer-handle-leak-list, 6db56b9) is involved — the aarch64 wheel nixl_cu13-1.6.0-cp312-cp312-manylinux_2_28_aarch64.whl built, was repaired by auditwheel, and both vllm-nixl:2178 and sglang-nixl:2178 images were pushed successfully before allocation was even attempted.

A contributing factor is visible in the request itself: because no --gpus/--gres count is given, the site Lua plugin promotes the job to exclusive whole-node mode, so each of the two parallel branches must wait for an entire GB200 NVL72 node — two whole nodes for one build, on a partition whose scarcity is exactly what the hour-long wait demonstrates.

Implicated commit: none — no code or CI commit caused this. The nearest relevant precedent is [REDACTED:Hex High Entropy String] (NirWolfer, "CI: request 2 GPUs instead of a whole node for nixl-ci-gpu-vr"), which fixed this same whole-node-request pattern for another pipeline.

File: the slurm.allocation call for the build_helper_vllm / build_helper_sglang branches of the nixl-ci-build-wheel pipeline (args logged verbatim as [partition:gb200nvl72_ci, headNode:dlcluster.nvidia.com, headUser:svc-nixl, nodes:1, jobTimeout:02:00:00, immediateTimeout:3600, ...]); the allocation helper itself lives in the external swx-jenkins-lib shared library, not in this repo.

Suggested fix:

1. Immediate: retrigger build [GPUNetIO] Clean up lifecycle ownership #2178. The failure is a transient capacity shortage; nothing needs changing in the PR. Note allocations 2499757 (vllm) and 2499800 (sglang) if an admin needs to inspect the queue history.
2. Durable: add an explicit GPU count to the slurm.allocation call for these two branches (e.g. extraArgs: ['--account=oberon-gb-ci', '--gpus=2']) so the Lua plugin stops escalating to exclusive whole-node. This mirrors what [REDACTED:Hex High Entropy String] did for nixl-ci-gpu-vr and should cut queue wait substantially on a contended GB200 partition.
3. Also worth doing: the two sanity branches each grab their own full node for what is only a wheel-install smoke test — consider sharing one allocation between the vllm and sglang checks, or moving them to a less contended partition. And since --immediate failure is a routine capacity condition rather than a code fault, wrapping the allocation in a bounded retry (or reporting it as UNSTABLE rather than FAILURE) would stop capacity blips from showing up as PR failures.

Related: PR #2237 ("ci: make wheel builds DR-safe") touches the same wheel pipeline and may be relevant to allocation robustness; precedent fix for the whole-node request pattern is #2352 / commit [REDACTED:Hex High Entropy String].

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[svc-nixl]

🤖 CI Triage Agent — nixl-ci-dl-gpu-ep · commit 6db56b9e

TL;DR: The build didn't fail on code — the "Allocate DL EP Environment" stage's salloc sat queued on the gb200nvl72_cx8 partition for the full --immediate=3600 window and died with "Unable to allocate resources: Connection timed out"; the fix is to stop requesting a whole exclusive GB200 node (pass an explicit --gpus/--gres) and/or retry the allocation instead of failing the build.

Full analysis

Summary: Stage "Allocate DL EP Environment" (node 176) failed after 3,610 s when the Slurm salloc for job nixl-ci-dl-gpu-ep-1676 could not get a node on partition gb200nvl72_cx8.

Root cause: Pure resource-availability failure in the CI's Slurm allocation step, not a defect from commit 6db56b9 / PR #2381. The log shows:

salloc: lua: Setting job to exclusive mode as no gpu count was specified
salloc: Pending job allocation 2499821
salloc: job 2499821 queued and waiting for resources
salloc: error: Unable to allocate resources: Connection timed out

The invocation is salloc -N 1 -p gb200nvl72_cx8 --immediate=3600 --time=02:30:00 --no-shell --account=blackwell. The single gap in the log (06:52:53Z → 07:53:01Z, ~60 min) is exactly the --immediate=3600 wait, and the stage duration (3,610,088 ms) matches it to within 10 s — i.e. the job was queued, never started, and Slurm returned the timeout at the deadline. Nothing in the pipeline hung: the preceding stage "Compiling NIXL EP Docker Image for DL" (node 161) completed cleanly, built and pushed nixl-ci-dl-gpu-ep-test:1676, and the only error there is a cosmetic Doxygen HTML_EXTRA_FILES complaint about missing docs/doxygen/*.png.

A contributing factor is visible in the log: because no GPU count is requested, the site's Lua job submit plugin promotes the job to exclusive whole-node mode, so the build has to wait for an entire idle GB200 NVL72 node rather than a couple of GPUs — which makes queue starvation on a scarce partition much likelier.

Implicated commit: unknown — no code commit implicated. The failing commit 6db56b9 (branch fix/xfer-handle-leak-list, PR #2381) is unrelated; the failure is in the Jenkins/Slurm allocation step.

File: Jenkins shared-library call site for slurm.allocation in the nixl-ci-dl-gpu-ep job definition under .ci/jenkins/ (args logged as partition:gb200nvl72_cx8, nodes:1, jobTimeout:02:30:00, immediateTimeout:3600, jobName:nixl-ci-dl-gpu-ep-1676) — exact file not resolvable via read_source_file; .ci/jenkins/pipeline/* lookups returned not-found.

Suggested fix:

1. Request GPUs explicitly rather than inheriting exclusive whole-node mode — add --gpus=<N> (or --gres=gpu:<N>) to the extraArgs for this job, mirroring what was already done for the Vera Rubin pipeline in commit [REDACTED:Hex High Entropy String] ("CI: request 2 GPUs instead of a whole node for nixl-ci-gpu-vr", PR CI: request 2 GPUs instead of a whole node for nixl-ci-gpu-vr #2352). This alone should dramatically cut queue time on gb200nvl72_cx8.
2. Make the allocation step resilient: wrap salloc in a bounded retry loop, or mark the stage UNSTABLE/ABORTED rather than FAILURE when the error is Unable to allocate resources, so a saturated partition doesn't report as a PR regression and re-trigger triage.
3. Re-run build Ofer/nixlbench scenarios #1676 as-is to confirm — no code change on PR Queue a leaked xfer handle on the agent that owns the list #2381 is needed.

Do not raise jobTimeout (02:30:00); the job never started, so the test time limit is irrelevant here — the relevant knob is immediateTimeout/queue admission, and raising that just lengthens the stall.

Related: PR #2352 (commit [REDACTED:Hex High Entropy String]) — the same whole-node-vs-GPU-count allocation fix applied to nixl-ci-gpu-vr; PR #1f87bd7b/#2339 and a9fb70a/#2311 show prior work hardening these CI allocation paths.

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[iyastreb]

/build

[coderabbitai]

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⚠️ Outside diff range comments (1)

🟠 Major · Remove the fabricated handle before agent teardown. · test_nixl_api.py:576

test/python/test_nixl_api.py:576
🩺 Stability & Availability | 🟠 Major | ⚡ Quick win

Remove the fabricated handle before agent teardown.

This test queues 0xDEAD on the fixture agent. When the agent is finalized, nixl_agent.__del__ passes queued values to the real releaseXferReq; CI reports that this causes a segmentation fault. Remove the test value in a finally block after checking the queue, or use an isolated fake owner.

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Review comment at @test/python/test_nixl_api.py at line 576:
Update the test using one_empty_agent and nixl_xfer_handle so the fabricated
0xDEAD handle is removed in a finally block after the queue assertion, before
the fixture agent is torn down; alternatively, use an isolated fake owner for
the queued value.

---

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Outside diff comments:
Review comments at @test/python/test_nixl_api.py:
- Line 576: Update the test using one_empty_agent and nixl_xfer_handle so the
fabricated 0xDEAD handle is removed in a finally block after the queue
assertion, before the fixture agent is torn down; alternatively, use an isolated
fake owner for the queued value.

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• src/api/python/_api.py
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[svc-nixl]

🤖 CI Triage Agent — nixl-ci-dl-gpu · commit 2250639b

TL;DR: The DL python test job crashed with SIGSEGV (exit 139) because the new test test_leaked_xfer_handle_is_queued_on_the_agent leaves the fake handle 0xDEAD in nixl_agent._leaked_xfer_handles, and after monkeypatch restores the real pybind agent, nixl_agent.__del__ passes that bogus pointer to the real C++ releaseXferReq(). Fix: drain/clear the leak list at the end of the test (or assert on a copy and then clear it) so the agent destructor never dereferences the sentinel.

Full analysis

Summary: pytest -s test/python died with Fatal Python error: Segmentation fault in nixl/_api.py:288 (nixl_agent.__del__), making .gitlab/test_python.sh return 139 and failing stage "Run DL Python tests".

Root cause: test_leaked_xfer_handle_is_queued_on_the_agent (test/python/test_nixl_api.py:563) monkeypatches one_empty_agent.agent with a FailingRelease stub, creates nixl_xfer_handle(one_empty_agent, 0xDEAD) and deletes it. nixl_xfer_handle.__del__ catches the stub's exception and appends the raw integer 0xDEAD to agent._leaked_xfer_handles (src/api/python/_api.py:129). The test asserts the list contains 0xDEAD and ends — but never clears it. At teardown monkeypatch restores the real pybind nixlAgent, and when the fixture's agent is finalized, nixl_agent.__del__ iterates the leak list and calls self.agent.releaseXferReq(0xDEAD) on the real C++ agent (src/api/python/_api.py:286-288). 0xdead is not a valid nixlXferReqH*, so the C++ side dereferences it and the process faults — this is not catchable by the surrounding except Exception. The log confirms the exact ordering: NIXL ERROR _api.py:122 nixl_xfer_handle finalization failed for 0xdead; keeping handle alive in agent leak list immediately followed by the segfault with _api.py", line 288 in __del__ at the top of the stack.

Implicated commit: a6b99c8 — "Queue a leaked xfer handle on the agent that owns the list", Dmitry Voropaev (branch fix/xfer-handle-leak-list, PR #2381; merged with main as 3cbcc6a)

File: test/python/test_nixl_api.py:563-580 (sentinel left behind) triggering src/api/python/_api.py:288 (self.agent.releaseXferReq(h) in nixl_agent.__del__)

Suggested fix:

1. Primary — make the test clean up after itself so the real agent never sees the sentinel:

    handle = nixl_xfer_handle(one_empty_agent, 0xDEAD)
    del handle
    gc.collect()

    assert one_empty_agent._leaked_xfer_handles == [0xDEAD]
    # Don't let the agent destructor hand a bogus pointer to the real C++ agent
    one_empty_agent._leaked_xfer_handles.clear()

(equivalently, keep FailingRelease installed for the agent's lifetime, e.g. by deleting the agent inside the monkeypatch scope, or use try/finally to clear the list.)
2. Defensive hardening — nixlAgent::releaseXferReq should validate the handle (generation/registry lookup) and raise a Python exception instead of faulting on an arbitrary integer; that would also make nixl_agent.__del__'s except Exception guard actually effective. See the prior work in issue #2067 / PR #2027 on stale-handle guarding.

Secondary observation (not the build-breaking failure, worth a separate look): test_prep_mem_view also reported F in this run, preceded by UCX invalid memh for md_index=6/7 → ucx_backend.cpp:817 Failed to prepare local memory view: Failed to create device memory list(local): Invalid parameter on gb200-nvl4-ts2-93. This is unrelated to the leak-list change and looks like a UCX GPU device-API / memh registration issue.

Related: PR #2381 (the change under test), issue #2067 (releaseXferReq use-after-free SegFault), PR #2027 (guard xfer handles against stale generations), PR #1960 (release deferred transfer handles through agent)

[svc-nixl]

🤖 CI Triage Agent — nixl-ci-non-gpu · commit 2250639b

TL;DR: All six "Test Python" matrix legs segfault in nixl_agent.__del__ (_api.py:288) because the new test test_leaked_xfer_handle_is_queued_on_the_agent leaves the fake handle 0xDEAD in the agent's leak list; after monkeypatch restores the real pybind agent, the agent destructor passes 0xDEAD to C++ releaseXferReq() and dereferences it. Fix: clear the leak list at the end of the test (and/or validate the handle before releasing).

Full analysis

Summary: pytest -s test/python crashed with Fatal Python error: Segmentation fault (core dumped, exit via SIGSEGV from .gitlab/test_python.sh: line 71) in every Test Python stage of build #3522; the C++ build and Test CPP stages all passed.

Root cause: The log shows the crash sequence precisely and identically in all legs:

NIXL ERROR _api.py:122 nixl_xfer_handle finalization failed for 0xdead; keeping handle alive in agent leak list
.Fatal Python error: Segmentation fault
Current thread ...:
  File ".../nixl_cu12/_api.py", line 288 in __del__

The last test to run, test_leaked_xfer_handle_is_queued_on_the_agent (test/python/test_nixl_api.py:563), monkeypatches one_empty_agent.agent with a FailingRelease stub, constructs nixl_xfer_handle(one_empty_agent, 0xDEAD) and deletes it. nixl_xfer_handle.__del__ (src/api/python/_api.py:116-131) catches the stub's RuntimeError, logs the message seen above, and appends the raw integer 0xDEAD to one_empty_agent._leaked_xfer_handles. The test then asserts and returns — leaving 0xDEAD in the list.

At teardown, monkeypatch reverts one_empty_agent.agent back to the real pybind nixlAgent, and the fixture's agent is garbage-collected. nixl_agent.__del__ drains the leak list and calls self.agent.releaseXferReq(h) at src/api/python/_api.py:288 with h == 0xDEAD. That is a bogus pointer handed to C++, which dereferences it → SIGSEGV. The except Exception guard around it cannot catch a segfault, so the whole pytest process dies and the stage fails (the test itself had already "passed", which is why a . precedes the fatal error).

Implicated commit: a6b99c8 — "Queue a leaked xfer handle on the agent that owns the list", Dmitry Voropaev (PR #2381, branch fix/xfer-handle-leak-list; merged with main in 3cbcc6a by Colin Hirsch)

File: test/python/test_nixl_api.py:563-580 (sentinel left in the leak list) crashing at src/api/python/_api.py:288

Suggested fix:

1. Make the test not leave a poison value behind — after the assertion, drain the list before the fixture's agent is finalized:

    assert one_empty_agent._leaked_xfer_handles == [0xDEAD]
    one_empty_agent._leaked_xfer_handles.clear()   # don't hand 0xDEAD to the real C++ agent

Alternatively keep FailingRelease installed for the agent's whole lifetime (e.g. patch the fixture's agent permanently rather than via monkeypatch, or use a locally-constructed agent whose agent attribute is never restored).
2. Harden the production path so a stale/invalid handle cannot crash the interpreter during interpreter shutdown: validate the handle in nixlAgent::releaseXferReq (bindings) and return NIXL_ERR_INVALID_PARAM for handles not in the agent's live request set, instead of dereferencing. nixl_agent.__del__ relies on except Exception, which is useless against a segfault, so the safety has to live on the C++ side.

Related: PR #2381 "Queue a leaked xfer handle on the agent that owns the list" — #2381

[svc-nixl]

🤖 CI Triage Agent — nixl-ci-gpu · commit 2250639b

TL;DR: The Python test suite segfaulted (exit 139) in nixl_agent.__del__ because the new leak-list test leaves a bogus handle (0xDEAD) in agent._leaked_xfer_handles, and after monkeypatch restores the real pybind agent, the agent destructor passes that fake pointer to releaseXferReq(). Fix: have the test clear the leak list before teardown (and make the drain in __del__/releaseXferReq tolerate invalid handles).

Full analysis

Summary: Stage "Run Python tests" (node 234) failed — pytest -s test/python crashed with Fatal Python error: Segmentation fault at _api.py:288 in __del__; srun: error: mizu01: task 0: Exited with exit code 139.

Root cause: The new test test_leaked_xfer_handle_is_queued_on_the_agent (added in this PR) monkeypatches one_empty_agent.agent with a FailingRelease stub, then finalizes nixl_xfer_handle(one_empty_agent, 0xDEAD). The finalizer's except branch logs and appends the raw integer to the agent's leak list — visible in the log immediately before the crash:

NIXL ERROR _api.py:122 nixl_xfer_handle finalization failed for 0xdead; keeping handle alive in agent leak list

The test asserts _leaked_xfer_handles == [0xDEAD] and ends, but never drains the list. pytest's monkeypatch teardown then restores the real pybind nixlAgent onto self.agent, and when the fixture's agent object is collected, nixl_agent.__del__ loops over the leftover entries and calls self.agent.releaseXferReq(0xDEAD) on the real C++ agent. 0xDEAD is not a valid nixlXferReqH*, so the C++ side dereferences garbage and the process dies with SIGSEGV. The try/except Exception around the call cannot catch a segfault, so the whole pytest process is lost (the final . and summary are never printed). Note this is a crash after the assertion passed — the test logic is right, the teardown is not. The C++ tests stage passed, so this is specific to the new Python finalizer path, not an infra or node problem.

Implicated commit: a6b99c8 "Queue a leaked xfer handle on the agent that owns the list" — Dmitry Voropaev (branch fix/xfer-handle-leak-list, merged forward in 3cbcc6a by Colin Hirsch)

File: test/python/test_nixl_api.py:563-580 (fake handle never drained) crashing at src/api/python/_api.py:288 (self.agent.releaseXferReq(h) in nixl_agent.__del__); handle enqueued at src/api/python/_api.py:129

Suggested fix:

1. Minimal, unblocks CI — make the test clean up the sentinel so the agent destructor never sees it:

    assert one_empty_agent._leaked_xfer_handles == [0xDEAD]
    one_empty_agent._leaked_xfer_handles.clear()

Better still, use try/finally (or a fixture-scoped finalizer) so the list is cleared even if the assert fails, and consider asserting against a real handle obtained from initialize_xfer rather than a fabricated address.
2. Hardening (recommended alongside) — the drain loop in nixl_agent.__del__ is a best-effort path that currently trusts raw integers across an agent-identity change. Record the owning pybind agent together with the handle when queuing, and skip entries whose owner is no longer the current self.agent; and/or have releaseXferReq on the C++ side validate the handle against the agent's live request set and raise nixlNotFoundError instead of dereferencing it. Without that, any genuinely stale handle at interpreter shutdown can segfault user processes, not just CI.

Related: PR #2381 (this change); possibly interacting with #2244 "core: keep transfer pollable after failed release for cancellation" and #2252 "posix: refuse to release a transfer with outstanding I/O", which both affect when releaseXferReq fails and thus how often this leak path is taken.

[svc-nixl]

🤖 CI Triage Agent — nixl-ci-gpu-vr · commit 2250639b

TL;DR: pytest test/python/test_nixl_api.py segfaulted (exit 139) in nixl_agent.__del__ because the new test test_leaked_xfer_handle_is_queued_on_the_agent leaves the fabricated handle 0xDEAD in _leaked_xfer_handles, and monkeypatch restores the real pybind agent before finalization, so the real C++ releaseXferReq(0xDEAD) dereferences a bogus pointer. Fix: clear the leak list at the end of the test (and/or validate the handle in __del__/the binding).

Full analysis

Summary: Stage "Run Python tests" (node 219) failed — pytest -s test/python died with Fatal Python error: Segmentation fault (core dumped), srun: task 0: Exited with exit code 139 on vr-nvl72-ts4b-l11-130-c06.

Root cause: A genuine code/test defect, not infra, and not a timeout — the crash happened ~1 s after the last test line, with no stalls anywhere in the log.

The log shows the sequence exactly:

NIXL ERROR _api.py:122 nixl_xfer_handle finalization failed for 0xdead; keeping handle alive in agent leak list
Fatal Python error: Segmentation fault
Current thread ...:
  File ".../nixl_cu13/_api.py", line 288 in __del__
  File ".../_pytest/runner.py", line 150 in runtestprotocol

test_leaked_xfer_handle_is_queued_on_the_agent (test/python/test_nixl_api.py:563) deliberately drives the new deferral path: it monkeypatch.setattrs the agent's pybind object to a FailingRelease stub, constructs nixl_xfer_handle(one_empty_agent, 0xDEAD), deletes it, and asserts _leaked_xfer_handles == [0xDEAD]. It never drains that list.

At test teardown monkeypatch reverts one_empty_agent.agent back to the real nixlBind.nixlAgent, while 0xDEAD is still queued. When the fixture's agent is then finalized, nixl_agent.__del__ (src/api/python/_api.py:283-296) iterates the leak list and calls the real self.agent.releaseXferReq(0xDEAD) at line 288. The C++ side treats the integer as a nixlXferReqH* and dereferences it — segfault. The try/except Exception wrapper around it cannot catch a SIGSEGV, so the whole pytest process dies and takes the stage with it.

The traceback frame being runtestprotocol (teardown), not a test body, confirms the finalizer ran after the patch was undone.

Implicated commit: a6b99c8e — "Queue a leaked xfer handle on the agent that owns the list", Dmitry Voropaev (PR #2381, branch fix/xfer-handle-leak-list, head 2250639). This commit added both nixl_agent._leaked_xfer_handles draining in __del__ and the test that seeds it with a fake handle.

File: src/api/python/_api.py:288 (crash site, __del__); test/python/test_nixl_api.py:563-580 (test that seeds the bad handle)

Suggested fix: Make the test self-cleaning — the fabricated handle must never reach the real binding. Add to the end of test_leaked_xfer_handle_is_queued_on_the_agent:

    assert one_empty_agent._leaked_xfer_handles == [0xDEAD]
    # Don't let the sentinel reach the real releaseXferReq: monkeypatch restores
    # the pybind agent at teardown, and the agent destructor drains this list.
    one_empty_agent._leaked_xfer_handles.clear()

Defence in depth, both worth doing independently of the test fix:

• In nixl_agent.__del__, wrap the drain in except BaseException and skip handles that are not known-live; better, track leaked handles as the original nixl_xfer_handle objects bound to the agent instance that produced them, so a handle can never be released through a different (restored) agent object.
• Validate the pointer in the pybind releaseXferReq wrapper and raise nixlInvalidParamError for a handle the agent does not own, rather than dereferencing blind. A Python-visible exception is recoverable; a segfault in a finalizer kills the entire test session and masks all remaining tests.

Related: PR #2381 — "Queue a leaked xfer handle on the agent that owns the list" (the PR under test here); prior work on this path: c8758860 "fix(python): release deferred transfer handles through agent (#1960)".

[ColinNV]

@v0ropaev The code is good, can you please fix the test?

Could the test create the handle through an agent factory, with native calls mocked? Direct construction bypasses the ownership mismatch, so the assertion also passes before the fix.

It may also be worth clearing the fake leak entry in finally, before monkeypatch restores the real agent; otherwise, agent destruction could pass 0xDEAD into native cleanup.

[2026-10-13T12:46:07.058Z] .2026-10-13 12:46:06 NIXL INFO _api.py:281 Initialized NIXL agent: 9e646740-2e46-4d72-9f30-49bef78b066b
[2026-10-13T12:46:07.058Z] 2026-10-13 12:46:06 NIXL ERROR _api.py:122 nixl_xfer_handle finalization failed for 0xdead; keeping handle alive in agent leak list
[2026-10-13T12:46:07.058Z] .Fatal Python error: Segmentation fault
[2026-10-13T12:46:07.058Z]
[2026-10-13T12:46:07.058Z] Current thread 0x00007f0244b74540 (most recent call first):
[2026-10-13T12:46:07.058Z] File "/usr/local/lib/python3.12/dist-packages/nixl_cu12/_api.py", line 288 in del
[2026-10-13T12:46:07.058Z] File "/usr/local/lib/python3.12/dist-packages/_pytest/runner.py", line 150 in runtestprotocol
[2026-10-13T12:46:07.058Z] File "/usr/local/lib/python3.12/dist-packages/_pytest/runner.py", line 118 in pytest_runtest_protocol
[2026-10-13T12:46:07.058Z] File "/usr/local/lib/python3.12/dist-packages/pluggy/_callers.py", line 121 in _multicall
[2026-10-13T12:46:07.058Z] File "/usr/local/lib/python3.12/dist-packages/pluggy/_manager.py", line 120 in _hookexec
[2026-10-13T12:46:07.058Z] File "/usr/local/lib/python3.12/dist-packages/pluggy/_hooks.py", line 512 in call
[2026-10-13T12:46:07.058Z] File "/usr/local/lib/python3.12/dist-packages/_pytest/main.py", line 408 in pytest_runtestloop
[2026-10-13T12:46:07.058Z] File "/usr/local/lib/python3.12/dist-packages/pluggy/_callers.py", line 121 in _multicall
[2026-10-13T12:46:07.058Z] File "/usr/local/lib/python3.12/dist-packages/pluggy/_manager.py", line 120 in _hookexec
[2026-10-13T12:46:07.058Z] File "/usr/local/lib/python3.12/dist-packages/pluggy/_hooks.py", line 512 in call
[2026-10-13T12:46:07.058Z] File "/usr/local/lib/python3.12/dist-packages/_pytest/main.py", line 384 in _main
[2026-10-13T12:46:07.058Z] File "/usr/local/lib/python3.12/dist-packages/_pytest/main.py", line 330 in wrap_session
[2026-10-13T12:46:07.058Z] File "/usr/local/lib/python3.12/dist-packages/_pytest/main.py", line 377 in pytest_cmdline_main
[2026-10-13T12:46:07.058Z] File "/usr/local/lib/python3.12/dist-packages/pluggy/_callers.py", line 121 in _multicall
[2026-10-13T12:46:07.058Z] File "/usr/local/lib/python3.12/dist-packages/pluggy/_manager.py", line 120 in _hookexec
[2026-10-13T12:46:07.058Z] File "/usr/local/lib/python3.12/dist-packages/pluggy/_hooks.py", line 512 in call
[2026-10-13T12:46:07.058Z] File "/usr/local/lib/python3.12/dist-packages/_pytest/config/init.py", line 229 in _main
[2026-10-13T12:46:07.058Z] File "/usr/local/lib/python3.12/dist-packages/_pytest/config/init.py", line 253 in _console_main
[2026-10-13T12:46:07.058Z] File "/usr/local/bin/pytest", line 8 in
[2026-10-13T12:46:07.058Z]
[2026-10-13T12:46:07.058Z] Extension modules: numpy._core._multiarray_umath, numpy.linalg._umath_linalg, torch._C, torch._C._dynamo.autograd_compiler, torch._C._dynamo.eval_frame, torch._C._dynamo.guards, torch._C._dynamo.utils, torch._C._fft, torch._C._linalg, torch._C._nested, torch._C._nn, torch._C._sparse, torch._C._special, cuda.bindings._bindings.cydriver, cuda.bindings.cydriver, cuda.bindings.driver, cuda.bindings._bindings.cyruntime_ptds, cuda.bindings._bindings.cyruntime, cuda.bindings.cyruntime, cuda.bindings.runtime (total: 20)
[2026-10-13T12:46:08.428Z] .gitlab/test_python.sh: line 71: 24226 Segmentation fault (core dumped) pytest -s test/python

[v0ropaev]

You're right on both, thanks. The old code ended up in the same leak list through an AttributeError when the handle got the nixl_agent directly, so the test couldn't fail. It now gets the handle from initialize_xfer with a fake native agent that returns 0xDEAD and refuses the release. I ran that body against the old and new _api.py with the bindings stubbed out, and the leak list comes back empty before the fix and [0xdead] after. The fake entry is cleared in finally, so the destructor won't pass it to the native release anymore, which is what segfaulted in the gitlab run.

[ColinNV]

/build