Function verify_last_link 

pub async fn verify_last_link(
    conn: &Connection,
    archive_path: Option<&Path>,
    snapshots_dir: &Path,
) -> Result<Option<ChainCheck>>

Check the newest link of the snapshot chain (0.15.19, review C-18).

What it is for

verify_snapshot_chain is right and unaffordable: two folds, one of them from genesis over the whole log. Its own rustdoc calls scheduling it the open problem, and nothing schedules it, so in practice a composition defect is copied forward with nothing looking. This is the cheap half of the same idea — re-derive snapshot n from snapshot n−1 and compare — which costs one anchored delta and can therefore run whenever a snapshot is written. The snapshot cadence does exactly that and logs a divergence.

Ok(None) when there are not two snapshots to compare, which is a young database and not a fault.

What it catches, and what it does not

It catches a defect as it is introduced: a snapshot that does not survive its own serialize/load round trip, an apply_to that composes differently from how it was composed, or a delta that has stopped covering the window between the two anchors. That last one is the practical case — rows archived out of the hot log between the two writes, with no archive path given here to fold them back in.

It does not catch a defect inherited from further back. If the chain went wrong at link three and every link since has composed faithfully onto it, this agrees at every one of them, because both sides descend from the same wrong state. Only a genesis fold answers that, which is what verify_snapshot_chain is and why it stays.

Pass archive_path whenever there is an archive. Without it the delta is folded from the hot log alone, and a link spanning an archive session will disagree for a reason that is not a defect.

It reports; it does not repair

verify_snapshot_chain’s reasoning, unchanged: under Doctrine VI a snapshot is derivative, so the repair is delete the snapshots, which is one line and the caller’s to run. Rewriting the file here would destroy the only evidence that composition has a bug.