[PD][NixlPush][Bugfix] Notify D of NIXL writes that failed to post so it stops waiting

[YannikHinteregger]

Summary

Fixes row C2 of #48633.

Problem: In push mode the prefill node writes KV straight into the decode node's memory. The decode node knows it has everything by counting the notifications that come in. If the prefill node cannot start one of those transfers it just logs and moves on, so the decode node counts toward a number it never reaches. The request hangs until its lease expires while holding blocks over a half-written cache.

Solution: This PR introduces a new NIXL message that the prefill node sends when it cannot start a transfer. The decode node counts it like a normal notification, so it still reaches the number it expects. Only then does it fail the request, which guarantees no other transfer is still writing into its blocks. From there the existing KV load failure policy applies.

Tested by

.venv/bin/python -m pytest tests/v1/kv_connector/unit/test_nixl_push_connector.py -q
# 100 passed

.venv/bin/python -m pytest \
  tests/v1/kv_connector/unit/test_nixl_connector.py \
  tests/v1/kv_connector/unit/test_nixl_connector_hma.py \
  tests/v1/kv_connector/unit/test_nixl_heartbeat.py \
  tests/v1/kv_connector/unit/test_multi_connector.py -q
# 273 passed, 1 skipped, 4 failed

The 4 failing tests are unrelated and also fail on main.

Also tested on a 3 GPU machine: a prefill instance with 2 ranks (TP=2) pushing to a decode instance with 1 rank. Ran these scenarios, 10 times each:

• Both P ranks succeed: D completes the request normally.
• One P rank fails first, the other succeeds after: D waits for the successful transfer, then fails the request.
• One P rank succeeds first, the other fails after: D waits for the failure message, then fails the request.

After every failure the next request succeeded with the same output as before. On main, D is never told about the failed transfer and the request hangs.

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[claude]

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[mergify]

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

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

[NickLucche]

Sorry for the delay, will get to this when I'm back 😅

[YannikHinteregger]

No worry haha enjoy your time off

[mergify]

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

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

[YannikHinteregger]

Technically this adds a new message type, so one could make an argument to bump the version.
However, the change is backward compatible: an older decode node reads the new message as an unknown request and ignores it, so it behaves exactly like today. Bumping would instead make mixed versions refuse to connect.

Should I bump it or keep it like that?

[YannikHinteregger]

Hi @NickLucche @njhill, could I ask you for a review? I iterated over it and made the code a bit simpler and tested it on a 3 GPU machine to make sure the fix works. I also left an inline comment where I'd like your input.
Thanks!

[mergify]

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

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

[YannikHinteregger]

@liuzijing2014 Just resolved a conflict after you merged #58875.
Since you know this code well, could I ask you for a review? Thank you!