A config deploy could silently discard committed agent config. sock's
icon commit carried a full proposal/approved/building/deployed tag set
yet was not an ancestor of `main` — genuinely deployed, then dropped.
Two gaps compounded.
`prepare_applied_target` is documented as "fast-forward applied/main to
target", but `git_update_ref` is `git update-ref <ref> <target>` with no
old-value guard: an unconditional force move. Anything reachable from the
old `main` but not from `target` leaves the branch without a word.
And nothing checked that it *was* a fast-forward. `run_deploy_merge_verify`
asserts exactly one thing about history — that the live PR head still
equals the reviewed sha. That is a drift gate on the *head*; it says
nothing about the *base*. A PR opened from a stale base passes it
unchanged and then rewinds `main` when it lands.
Adds, in the order they run:
- an ancestry gate as step 3 of MergeVerify — the reviewed head must
descend from `applied/main`, else bail before the irreversible merge.
It sits after the fetch (the commit has to be local to check
reachability) and before the eval, so it stays inside the region where
nothing is mutated and the node is still safely retryable.
- `git_update_ref_cas`, used for the `applied/main` move.
`git update-ref <ref> <new> <old>` refuses, and leaves the ref alone,
when the current value is not `old`. The ancestry gate only proves the
target is safe against the `main` observed *then*; the CAS is what
keeps that proof true *now*. `run_deploy_apply` already reads
`prev_main` to park the rollback ref, so that value is threaded in —
re-reading it inside the callee would reintroduce the race.
`git_is_ancestor` returns `Ok(false)` for exit 1 rather than treating
"not an ancestor" as a failure. Its doc comment notes this is not the
"did this branch land upstream" question: a squash-merge rewrites the
commit, so `--is-ancestor` correctly answers false for a branch whose
contents were merged. Different question, same command.
Tests cover both directions of the ancestry check, and that a stale CAS
both errors *and* leaves the ref where it was — a guard that fails while
still moving the ref would be worse than none.
Not covered here, deliberately: the non-PR apply path also writes `main`
and wants the same treatment. Kept separate to stay reviewable.
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