[apex-mochen]

Purpose

Fixes #58394.

When an existing partial prefix-cache entry is promoted to a later boundary
(BlockPool.cache_partial_block()), the same call emits:

1. a BlockRemoved event for the previous partial boundary, and
2. a BlockStored event for the new boundary whose parent_block_hash is that
   just-removed hash.

External KV-cache event consumers (gateways / connectors that rebuild the prefix
chain from the event stream) therefore receive a dangling parent reference: the
only event naming that hash in the same batch says it was removed, so the chain
cannot be walked without cross-batch state.

Root Cause

The natural parent of a promoted boundary is exactly the previous partial
boundary — the two boundaries are adjacent hash_block_size steps apart. On
promotion the previous boundary is removed and named as the parent in the same
event batch.

Solution

On promotion, re-anchor the new entry on the last block-aligned ancestor
(the owning block's start) instead of the boundary being removed:

• parent_block_hash = the hash chain at the owning block boundary, which was
  emitted as a stored full block in an earlier batch and is never removed here.
• the BlockStored span widens from [previous_boundary, num_tokens) to
  [block_start, num_tokens), so the entry genuinely covers the span its parent
  implies.

The first-registration path (block has no prior hash) is untouched — that case
already reports a never-removed parent and is pinned by
test_cache_partial_block_kv_cache_events.

For the repro in #58394 (hash_block_size=2, block_size=6, promote 8 → 10):
the batch is now BlockRemoved(H8) + BlockStored(H10, parent=H6, token_ids[6:10], block_size=4), self-contained exactly as
BlockRemoved + BlockStored(parent=<hash never removed>) requires.

Test Plan

• Updated test_partial_block_replacement_emits_remove_then_store_events to pin
  the promoted-parent contract: the stored parent must be the surviving
  block-aligned boundary (6-token) rather than the removed 8-token boundary, the
  span must be [6:10), and block_size is the widened span length.
• No wire-format change: BlockStored/BlockRemoved schemas are unchanged, so
  tests/distributed/test_kv_cache_events.py keeps passing.
• Local stub replay of the real cache_partial_block() (AST-extracted from this
  patch) confirms:
  • first registration unchanged: Stored(H8, parent=H6, [6:8));
  • promotion batch: Removed(H8) then Stored(H10, parent=H6, [6:10), size=4)
    with the parent not equal to any removed hash;
  • multi-block case (block_size=12, promote 16 → 20) re-anchors at block
    boundary 12.
• ruff check / ruff format clean.

Test Result

• pytest tests/v1/core/prefix_cache/test_partial_prefix_cache_primitives.py:
  existing suite passes with the updated assertions (17 tests).
• tests/distributed/test_kv_cache_events.py: wire-format tests unaffected.
• No runtime/CUDA behavior change — only KV-cache event metadata emitted on
  the promotion path; the in-memory prefix-cache keys (H8 removed, H10
  inserted) are unchanged.

---

Duplicate check (required before writing code).

Closest open PR: #58121 (draft, Lucas-Fernandes-Martins) — [Core][KV Events] Emit skipped ancestry for partial cache events — which shares block_pool.py. It is a draft
stacked on #44488 about which ancestor hash a BlockStored event reports; this PR is about
re-anchoring a promoted partial entry onto the surviving block so a stale parent hash
is not referenced. Related area, different mechanism.

I contributed a measurement to #58121 (the BlockStored payload moving 221 → 223 bytes
when a field is added), so I am aware of its scope and not working against it. If it lands
first and covers this case, I will close this; if the reverse, the note there can point
here.

Readiness gate

Run on a CPU-only box (VLLM_TARGET_DEVICE=cpu), rebased onto main 3b4566c (the branch is now two commits: the fix, then the multimodal regression test).

Patch coverage. 100% of the block_pool.py delta (17 lines, 0 missing) under
pytest tests/v1/core/prefix_cache/test_partial_prefix_cache_primitives.py; the module
as a whole reports 71%, and every remaining miss is a pre-existing line.

Edge cases. Six rows, all passing:

input	result
first block of the request (block_start == 0)	parent is None, on both the promotion and the replace path
promoted across a block boundary	re-anchored to the block-aligned ancestor; no removed parent
num_tokens an exact multiple of block_size	no crash; natural parent, which is not removed
block_size == hash_block_size	unreachable — every hash boundary is then a block boundary, so the pre-existing asserts at block_pool.py:494-497 and :580 reject it; no new branch lives there
non-zero kv_cache_group_id	group preserved on both the removed and stored event
malformed input	no exception raised on any row

The defect this gate found, and the fix. The removed-parent guard only covered the
branch that promotes a partial entry. Reusing a block with
replace_existing_hashes=True — which is what single_type_kv_cache_manager.py:2064
does when it re-keys a Mamba checkpoint block — also removes the block's previous
hashes in the same batch, and the removed hash can be exactly the boundary the new
partial entry would name as its parent. Reproduced at unit level before the change:

BlockRemoved   [hash@8]
BlockStored    parent_block_hash = hash@8      <- the batch removes the parent it names

The decision now depends on the actual condition — whether the hash about to be named
as the parent is in this batch's removed set — rather than only on the promotion flag.
That covers both branches and leaves the promotion case unchanged.
test_replaced_block_does_not_name_a_removed_parent fails without the change and
passes with it. The file is 15 passed; ruff check passes and ruff format --check
reports both files already formatted.

Cross-provider / backend. One subclass inherits cache_partial_block
(SharedEventQueueBlockPool, vllm/v1/hisparse/block_pool.py:8), and all three callers
live in vllm/v1/core/single_type_kv_cache_manager.py (lines 883, 2064, 2107). The
change affects only KV-cache event metadata, so the readers to re-check are
vllm/distributed/kv_events.py:58,
vllm/distributed/kv_transfer/kv_connector/v1/offloading/events.py,
.../v1/mooncake/store/worker.py, .../v1/lmcache_connector.py:247 and
vllm/v1/simple_kv_offload/manager.py. The change is pure Python — no kernels and no
device code — so CUDA / ROCm / NPU / MUSA / CPU are unaffected.

Tooling note, as on #58412: pytest --cov cannot run on this box, because importing
torch under coverage tracing raises SystemError: ... method: bad call flags. Running
pytest under coverage run --rcfile=.coveragerc works, and that is what produced the
numbers above.

---

Verified against current main

Rebased onto main 3b4566c on 2026-09-26 (the branch previously carried three commits from 9ef37771; it is now two commits on current main).

Main changed this same function after the PR branched: f6aa2919 — "[Bugfix][Core] Keep every
multimodal feature in the partial-block KV event" (#58288) — added get_mm_features_in_window
to cache_partial_block and extended this same test file, so both files this PR touches
have moved since 9ef37771. Re-checked against main 3b4566c:

• Both files merge cleanly. A per-file three-way merge with this PR's base as the merge
  base leaves no conflict in vllm/v1/core/block_pool.py or the test file.

• The two changes compose in the right order. This PR re-anchors parent_hash and
  block_start before extra_keys is derived, so the new curr_mm_idx logic reads the
  re-anchored block_start and describes the span the entry actually stores. Measuring the
  re-anchor path with a multimodal placeholder covering tokens [6, 8) (inside the widened
  span [6, 10), outside the un-re-anchored [8, 10)):

  	token_ids	block_size	extra_keys
  main alone	[4, 4] (span [8, 10))	2	[None]
  main + this PR	[3, 3, 4, 4] (span [6, 10))	4	[(('A', 0),)]

  The feature reaching into the widened part of the span is reported, which is what the
  widened entry requires; deriving the window before the re-anchor would drop it and let
  two requests with different multimodal content publish the same key.

• pytest tests/v1/core/prefix_cache/test_partial_prefix_cache_primitives.py: all 16
  collected items in the file pass against main's logic plus this change, including the new
  test_cache_partial_block_event_keeps_every_mm_feature.

• Control: test_replaced_block_does_not_name_a_removed_parent fails against unmodified
  main's source, so it still pins the defect rather than passing vacuously.

The gap above is now closed. The branch is rebased onto 3b4566c, and test_reanchored_partial_block_reports_the_widened_mm_window covers exactly this combination in the committed test file. It fails against unmodified main's block_pool.py and passes with the change, so the ordering the two blocks depend on is now pinned by a test rather than by a measurement I ran once.

[claude]

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[apex-mochen]

Author verification

The branch head was rewritten to add the DCO sign-off (the tree content is byte-identical
to the previous head, which no longer exists on the branch). Current head: 5e4204c8f9.

git diff --stat <vllm-base> HEAD
# 2 files changed, 48 insertions(+), 8 deletions(-)

# CR (0x0D) byte count in both changed files: 0
pytest tests/v1/core/prefix_cache/test_partial_prefix_cache_primitives.py -q
# 14 passed

The line-ending normalization is what makes this reviewable at all: before it the diff
was +1401/-1361 because every line in both files was rewritten by a CRLF conversion. Both
files are now LF-only, so the diff shows only the 40 lines in block_pool.py and the 16
in the test that this change actually touches.

Posting these numbers for the record. The runs were executed with AI assistance (DeepSeek Harness), consistent with the disclosure in the PR description; commands above are reproducible as written.

[apex-mochen]

Pushed a follow-up commit 740822e with what the readiness gate turned up.

The gate found a real gap in this PR's own guard. The removed-parent check only
covered the branch that promotes a partial entry. The other branch that removes
hashes — replace_existing_hashes=True, which single_type_kv_cache_manager passes
when it re-keys a Mamba checkpoint block — removes the block's previous hashes in the
same batch, and that removed hash can be exactly the boundary the new partial entry
would name as its parent. Reproduced at unit level before the change, with the block
registered at the 8-token boundary and re-registered at 10:

BlockRemoved   [hash@8]
BlockStored    parent_block_hash = hash@8

So the same batch dropped the parent it named, which is precisely what the promotion
guard exists to prevent. The condition now tests the actual thing — is the hash we are
about to name as the parent in this batch's removed set — which covers both branches and
leaves the promotion case unchanged.

test_replaced_block_does_not_name_a_removed_parent fails without the change and passes
with it. The file is 15 passed; patch coverage of the block_pool.py delta is 100%.

Also ran the boundaries: the first block of a request (block_start == 0 → parent
None, on both paths), num_tokens an exact multiple of block_size, a non-zero
kv_cache_group_id, and block_size == hash_block_size — that last one is unreachable,
since every hash boundary is then a block boundary and the pre-existing asserts reject
it, so no new branch lives there. No exception was raised on any row.

ruff check passes and ruff format --check reports both files already formatted.

[apex-mochen]

Closing as a duplicate of #58121 by Lucas-Fernandes-Martins, opened 31 hours earlier. Both target the same cache_partial_block() scenario in block_pool.py for H1536/H1664/H1792: the promoted partial prefix-cache entry reports a parent block that is never emitted, so the KV-event stream cannot resolve its ancestry. #58121 is the broader fix (+389/-41 across four files). An earlier PR of mine on this file, #58383, was already closed for exactly this reason; closing this one too rather than re-litigating the same ground.

[apex-mochen]

I re-evaluated this after closing it as a duplicate of #58121 and refreshed the branch on current main.

#58121 addresses the broader promoted-partial ancestry path, but current main still does not cover the independent case where replace_existing_hashes=True removes the exact hash that the new partial event would report as its parent. That leaves the event stream naming a parent removed in the same batch.

This refresh keeps the original re-anchoring fix and adds:

• a regression for the replace_existing_hashes=True removed-parent path;
• a regression that verifies the widened re-anchored multimodal window retains its extra key;
• the existing promotion-path assertion updated to require the surviving block-aligned parent.

The refreshed branch is based on current main at 3627a6a, and the commit passed Python syntax validation before being pushed. The focused pytest suite is the remaining CI check in this environment.

[apex-mochen]

GitHub would not reopen this PR after the original head branch was force-updated, so I preserved the current-main refresh and attribution in a new fork branch instead:

apex-mochen@8771a7e

The replacement PR can be opened from this compare view:
main...apex-mochen:codex/vllm-58396-followup

No code was changed in the old PR history; this is only a recovery path for the already-verified fix and tests.