[hmirin]

Fixes A1, C1 and C3 from the implementation review. All three were reproduced on main before the fix.

A1 (high): a ref Git was told failed got published with the next ref

publish_checkout_head wrote HEAD and made a local commit before git push, and undid neither when the push failed. local_head() reads the working-tree HEAD first, so the unpublished manifest became the "current" head. The helper handles a whole push batch on one storage instance.

Reproduced: git push private main:refs/heads/a main:refs/heads/b with the first publish rejected by a hook. Git reported a as failed, yet the final manifest held a, b and main.

Fix: publish_checkout_head wraps a new commit_and_push_head. On any error, restore_checkout(base_commit) puts the checkout back to the last published commit:

• it resets the local branch and index to that commit (for genesis, it deletes the ref and empties the index);
• it removes the working-tree HEAD file;
• it deletes untracked files under README.md, FORMAT, manifests/ and packs/, so the failed publication's objects are not uploaded later.

If the restore itself fails, its error is attached to the original one.

C1 (medium): every push failure looked like a CAS conflict

Every failed push became CasConflict and stderr was dropped. Since #14 the helper maps that to fetch first, so a hook rejection or an authentication failure showed [rejected] (fetch first) and the git pull hint.

Fix: after a failed push, the branch is fetched again and the code acts on the observed tip, not on stderr text:

• tip equals the new commit: success, because the push landed;
• tip equals the base: an ordinary error, git push carrier branch failed: <stderr>;
• any other tip: CasConflict;
• the re-fetch itself fails: the push error is returned.

C3 (low): a garbage remote HEAD hid the conflict

head_at_commit validated the remote HEAD while building the conflict, so a stale CAS against a malformed HEAD returned invalid opaque object id. It is replaced by conflicting_head(commit) -> Option<String>, used only to fill CasConflict.actual. Reads that need a valid HEAD still validate it.

Tests

• rejected_carrier_publish_restores_the_published_head
• rejected_carrier_genesis_leaves_an_uninitialized_checkout
• failed_ref_in_a_helper_push_batch_is_not_published_by_the_next_ref. It locates the failed ref from --porcelain output and checks that the hook's message appears and fetch first does not.
• stale_compare_and_swap_conflicts_even_when_the_remote_head_is_garbage
• The existing race tests still yield CasConflict.
• cargo fmt --check, cargo clippy --all-targets -- -D warnings, cargo test --all-targets (144 passed).

Limits

• The "push reported failure but landed" path has no test; I found no deterministic way to produce it.
• If the tip moves for an unrelated reason between the failed push and the re-fetch, the result is CasConflict. The tip did move, so this is accepted.
• The push error carries git's full stderr on one line, so it can be long.