Follow-up to #96634 (stall-fallback retry, #78981) addressing
donovan-yohan's post-merge adversarial review. The stall path detaches a
timed-out primary worker (fence cancel wins; future stays on the pool)
and immediately runs the fallback against the SAME ContextCompressor,
creating two verified races:
1. Late-primary snapshot restore: the detached primary's unwind called
_restore_compressor_attempt_state with the PRIMARY's pre-attempt
snapshot. Landing after the fallback's commit it rolled
_previous_summary/cooldown/provenance/telemetry back to pre-primary
values, silently discarding fallback-owned state.
2. Shared _compression_cancelled_check: the late primary's `finally`
cleared the callback the fallback had just installed, so the
fallback's F4 cancellation consult read None.
Fix: a monotonic per-compressor attempt generation claimed under one
module lock (_claim_compressor_attempt). Snapshot restores carry their
claiming generation and no-op when stale; the cancelled-check set/clear
moves into owner-stamped helpers (_install_compression_cancelled_check /
_clear_compression_cancelled_check_if_owner) so only the installing
attempt can clear it. The commit fence keeps owning COMMIT admission;
the generation owns compressor-ATTRIBUTE writes — two boundaries.
Legacy callers (attempt_generation=None) and slotted third-party
compressors (generation 0) keep the historical unconditional behavior.
Secondary review items:
- Lean chunk digests during a stall-fallback retry now follow the
summary onto the pinned healthy route: take_pinned_summary_route()
echoes the consumed route into a context-local
_SUMMARY_ROUTE_CONSUMED, and _build_chunk_digests passes
attempt_summary_route_kwargs() (non-consuming) to call_llm. The pin's
single-use contract for the SUMMARY call is unchanged — the
main-model retry still never re-issues the pinned route.
- Worker re-run repeating pre-compression callbacks: documented as an
accepted limitation on _retry_compression_on_fallback_chain
(built-ins idempotent; resuming mid-pipeline would couple the retry
to host callback ordering).
Tests (tests/agent/test_compression_attempt_ownership.py, 10 cases):
deterministic interleavings for both races (late-primary restore
no-ops + preserves fallback state; stale finally cannot clear the
fallback's callback), legacy/slotted compatibility, digest route
follow + context-locality of the consumed echo. Mutation-checked:
reverting only the two prod files to origin/main fails the suite;
restored stack green (21 passed incl. the original #78981 suite).
The one red in the wider sweep
(test_silence_cannot_approach_double_idle_timeout) is pre-existing on
clean origin/main — verified independently.
kshitijk4poor
61cd299c6e