[Bugfix][KV Offloading] Defer stores until block IDs are tracked

[yatesdr]

[Bugfix][KV Offloading] Defer stores until block IDs are tracked

Purpose

Fix a fatal OffloadingConnector._build_store_jobs assertion when KV pressure
causes eviction/preemption while tiered offloading and async scheduling are
active.

The store path currently derives num_chunks from token progress, then assumes
that two independently maintained collections have reached the same boundary:

• content-hash-derived offload_keys; and
• scheduler-propagated physical GPU block_ids.

Eviction/preemption can expose a transient state where the token-derived store
boundary is ahead of one of those collections. The resulting slices have
different lengths, and this assertion terminates EngineCore:

assert len(offload_keys) == len(offload_block_ids)

This change computes a ready boundary independently for every KV group:

min(
    num_storable_chunks,
    len(group_state.offload_keys),
    len(group_state.block_ids) // blocks_per_chunk,
)

Only that common ready prefix is offered to the offload manager. Store progress
also advances only through the capped boundary, so a lagging chunk is retried
after its key and complete set of source block IDs are tracked. This is
important: merely removing the assertion and relying on zip() would avoid the
crash but could advance the cursor past an unstored chunk, silently losing it.

When all three state sources are synchronized, the ready boundary equals the
existing token-derived boundary and behavior is unchanged.

Production evidence

The failure was reproduced on a GLM-5.2 hybrid deployment using TP4/DCP4,
MTP3, async scheduling, TieringOffloadingSpec, and a 644,864-token GPU KV
pool:

• concurrency 16 x 50k context (~800k active tokens): fatal assertion during
  eviction;
• concurrency 32 sweep: same assertion;
• concurrency 16 x 16k and concurrency <= 8 x 50k: no assertion while active
  demand remained below the pool;
• BF16, E4M3, and block-INT8 collective modes all reproduced the failure,
  ruling out the collective wire codec.

The deployed downstream vLLM commit is 7ea567a2458a4800a6a0e3e0a6ba41fcbd00d146.
The complete patch applies cleanly to that commit as well as current upstream
main.

Test Plan

1. Reproduce the lag directly in the existing OffloadingConnector scheduler
   fixture: make three chunks storable while only two GPU block IDs are tracked.
2. Verify the first store job contains the two ready chunks and advances only
   to chunk two.
3. Add the third block ID and verify a second store job retries exactly chunk
   three.
4. Run the complete neighboring scheduler suite, including async/sync
   scheduling, request finish, preemption, reset, MTP/EAGLE, hybrid groups, and
   sliding-window cases.
5. Run all repository pre-commit hooks applicable to the changed files.
6. Before submission, repeat the original above-pool workload on the target
   deployment and verify zero assertion, zero EngineDeadError, no restart,
   and successful request completion.

Test Result

Completed locally:

pytest tests/v1/kv_connector/unit/offloading_connector/test_scheduler.py
92 passed, 15 warnings in 40.95s

pre-commit run --files <scheduler.py> <test_scheduler.py>
all applicable hooks passed

• git diff --check: passed.
• Patch applicability against deployed v19 commit 7ea567a: passed.
• The new regression passes and proves the deferred chunk is retried rather
  than skipped.

Target-box overflow reproduction:

• Patched image SHA-256 began with 41078d5f; the mounted scheduler SHA-256
  began with a68a28e5.
• Deployment: block-INT8 wire mode, offloading enabled with 64 GB host DRAM,
  480k maximum model length, 16 maximum sequences, 0.970 GPU memory
  utilization, TP4/DCP4/MTP3, and a 611,840-token boot KV pool.
• 16/16 unique-prefix requests completed successfully with
  finish_reason=stop; each carried approximately 49,800 prompt tokens.
• Active demand reached approximately 797k tokens against a 611,840-token GPU
  KV pool, forcing the eviction path that crashed the unpatched connector.
• Prefix-cache metrics recorded 797,218 queries and zero hits, confirming that
  the requests were freshly prefilled rather than cache-served.
• RestartCount remained 0 -> 0 under a one-second watcher; container identity
  and StartedAt remained unchanged throughout the 11-minute run.
• No _build_store_jobs assertion, EngineDeadError, OOM, HTTP 5xx, or request
  error occurred.
• A post-load liveness request returned the expected answer 4 from the same
  engine instance. The container ID, StartedAt, and RestartCount=0 remained
  unchanged.

All production acceptance criteria passed.

No model evaluation is required for this change because it does not alter model
weights, kernels, logits, sampling, or transferred KV values. It changes only
when scheduler metadata declares a store chunk ready. The target overflow gate
above remains required because it validates the production scheduler lifecycle.

Non-duplication

Open-PR searches and fetched git-history searches for the assertion, store
boundary, and preemption/block-ID terms found no existing fix as of 2026-07-19.

PR #48596 is related but not
duplicative: it fixes final-block storage and a block-reuse race at request
finish. Both the deployed commit and current upstream main include that change
while retaining the failing length assertion addressed here.

AI assistance disclosure

OpenAI Codex assisted with failure analysis, patch preparation, and regression
test development. The commit contains an Assisted-by trailer. The production
overflow validation remains a merge gate and its results will be added here.

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[yatesdr]

Feedback please on no merge close?

[voipmonitor]

Closing this PR without a published replacement was a process mistake.

What happened:

• [Bugfix][KV Offloading] Defer stores until block IDs are tracked #133 targeted fix/gg-rebased-cutlass453-sm120-20260718, a transient integration branch, rather than the canonical dev/gilded-gnosis branch.
• During the later PR-topology cleanup, branches that were stacked on obsolete integration bases were closed so that fixes could be represented as direct-GG PRs.
• An exact local port onto GG was prepared, but it was not pushed and no replacement PR was opened before [Bugfix][KV Offloading] Defer stores until block IDs are tracked #133 was closed. The existence of a local branch was incorrectly treated as completion of the migration.

Resolution:

• Replacement: [GG] fix(kv-offload): defer stores until metadata is ready #153
• [GG] fix(kv-offload): defer stores until metadata is ready #153 is directly based on current dev/gilded-gnosis.
• The fix was re-audited against upstream vLLM [Bugfix][KV Offloading] Handle queued request aborts without allocated KV blocks vllm-project/vllm#49146 and adjusted to keep the readiness invariant inside storable_chunks().
• It covers both independently lagging metadata sources (offload_keys and complete block_ids chunks), retains the consistency assertion, and adds queued-abort coverage.
• The full offloading-connector unit suite passes: 142 tests.

Prevention for future PR consolidation:

1. Verify base.ref and merge-base against the canonical branch before publishing.
2. Open, validate, and link the replacement PR before closing the old PR.
3. Track every migration as old PR -> replacement PR -> tested commit; a local branch is not a published replacement.
4. Never close a superseded fix without a comment linking its replacement or explicitly stating that the patch was intentionally dropped.

No implementation from #133 is intentionally being discarded; #153 is its cleaner and more thoroughly validated successor.