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SPEC-071-A: Fresh Pre-Create Dedup Re-check at Dispatch

Amends SPEC-064-A. Adds a fresh re-check immediately before the dispatcher opens a PR, in swarm/agent.sh.

1. Problem

ADR-064 evaluates its dedup predicates from a snapshot taken once at pass start (fetch_main_ref + dispatch_open_pr_goals). Across a pass — and in the gap before gh pr create — that snapshot goes stale: a sibling proof of the goal can merge, or a concurrent dispatcher can open a PR. The branch is then dispatched into a dead “already proved” duplicate. All observed post-ADR-064 duplicates were of this class.

2. goal_taken_fresh

goal_taken_fresh() {
  local goal="$1"
  fetch_main_ref || true                          # refresh origin/main
  goal_already_proved "$goal" && return 0         # merged since pass start?
  dispatch_open_pr_goals | grep -qxF "$goal"      # PR opened since pass start?
}

Returns 0 (taken → skip) if the goal is now proved on origin/main or now has an open prove PR. Uses git grep + one core-API gh pr list — never the 30/min search API. Any git/gh failure degrades to “not taken” (return non-zero), so dispatch is never blocked by infra health.

3. Placement in dispatch_queue

The check runs after the governor gate and immediately before dispatch_queued_proof_branch, so it only costs a fetch + list for the handful of branches actually being dispatched (bounded by UNSORRY_DISPATCH_LIMIT), not for every queued branch:

if ! submission_governor_allows; then ... fi
if goal_taken_fresh "$goal"; then           # ADR-071
  log "... goal $goal was taken during this pass (merged or already PR'd)"
  seen_goals="$seen_goals$goal "
  continue
fi
if dispatch_queued_proof_branch "$branch"; then ...

A skipped goal is added to the in-pass seen-set so its sibling branches are skipped too.

4. Residual window

A millisecond TOCTOU remains between goal_taken_fresh and gh pr create (no cross-process lock). First-merge-wins (ADR-004) keeps any residual duplicate sound — it cannot merge twice — only wasteful. A fully atomic dispatch lock and the upstream prove-time race fix are deferred (ADR-071 Options 2 & 3).

5. Test

test_dispatch_skips_taken_midpass: a goal that passes the pass-start checks but for which goal_taken_fresh returns true is not dispatched (0 PRs opened). test_dispatch_goal_dedup continues to pass (fresh check is transparent when the goal is genuinely free).