[Bugfix] Account for GQA replication in NIXL handshake block_len validation

[waynehacking8]

Purpose

Fixes #45330.

_validate_remote_agent_handshake assumes block_len scales linearly with 1/tp_size. When tp_size > total_num_kv_heads, GQA replication caps the per-rank head count at max(1, heads // tp), so block_len stops scaling. With the reporter's matrix (8 KV heads): D_TP=16 ← P_TP=8 has one head per rank on both sides → identical block_lens, but the assertion expects local_block_len * tp_ratio (2×) and rejects the valid handshake.

Fix

Compute the expected remote block_len from the actual per-rank head ratio (max(1, heads // tp) on each side — the same quantity the topology already exposes as local_physical_heads) instead of the raw tp_ratio:

• Identical to the previous formula whenever neither side is capped → all currently passing configurations (tp2→tp4, tp4→tp8, plain hetero-TP) are bit-for-bit unaffected.
• The transfer path already supports the capped case (tp_mapping computes rank_offset_factor explicitly for tp_size > total_num_kv_heads); only the validation was wrong.
• The reporter's tp16→tp8 row (P-side replication with P_TP > D_TP) remains explicitly rejected by the existing unsupported-combination guard at the top of the validation — unchanged; this PR fixes the supported D-side-replication rows (tp8→tp16, tp4→tp16).

Test Plan

pytest tests/v1/kv_connector/unit/test_nixl_connector.py::TestNixlHandshake -q

New test_handshake_validates_gqa_replicated_block_len: local TP=16 with 8 total KV heads (capped at 1 head/rank), remote TP=8 (also 1 head/rank), identical block_lens — must validate cleanly.

Test Result

• New test: fails (AssertionError) without the fix, passes with it.
• TestNixlHandshake: 12/12 pass.
• Rest of test_nixl_connector.py: 8 pass, 1 pre-existing env failure (test_abort_timeout_on_prefiller[ray], "FlashInfer requires GPUs with sm75 or higher" inside the ray worker — verified identical on a clean tree in this cu128/SM120 env, unrelated).
• pre-commit (ruff check/format, mypy) passes on changed files.

Duplicate-work check

gh issue view 45330 --comments: 0 comments, 0 assignees (filed 2026-06-12 by kannakAWS). gh pr list --search "45330 in:body" / --search "nixl handshake block_len": no open PRs. No competing work as of this submission.

AI assistance disclosure

Developed with AI assistance (Claude Code). I reviewed every changed line and ran the tests above.

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🤖 Generated with Claude Code

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[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @waynehacking8.

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

[waynehacking8]

Rebased onto main to resolve the conflict with #44583 (per-region KV transfer classification).

The rebase is not mechanical, so summarizing what changed: #44583 rewrote this validation as a per-region REPLICATE/SPLIT loop, where _is_region_replicated flags per-region MLA and is_kv_replicated(remote) catches remote_tp > total_kv_heads. That covers the remote-replicated case, but the SPLIT branches still assert linear tp_ratio scaling — so the #45330 scenario (D_TP=16 ← P_TP=8 with 8 KV heads: local capped at 1 head/rank, remote at exactly the cap boundary) still takes the SPLIT branch and rejects the valid handshake.

Ported the fix onto the new structure: SPLIT regions now compare against the actual per-rank head ratio (local_len * remote_heads // local_heads), which is identical to tp_ratio scaling whenever GQA replication caps neither side, so all currently-passing configurations are unaffected. REPLICATE regions are untouched, and the capped-remote-with-negative-tp_ratio combination remains rejected by the existing top-of-function guard.

Tests after rebase:

.venv/bin/python -m pytest tests/v1/kv_connector/unit/test_nixl_connector.py -k "gqa_replicated or block_len or handshake" -q
22 passed

Full-file run has one failure, test_abort_timeout_on_prefiller[ray], which fails identically on origin/main on this machine (Ray actor GPU-capability detection in my env, unrelated to this change). pre-commit (ruff, ruff-format, mypy) passes on both changed files.