Self-review of this branch caught a data-loss bug I introduced with the
per-app / group files ceilings: the quota check ran AFTER the blob write.
`write_atomic_at` renames the new bytes over the FINAL path, so by the time
the ceiling refused an update the previous bytes were already gone. The
per-app path then unlinked the blob (row survives, file destroyed — every
read 404s); the group path left the new bytes in place under the old row's
checksum (every read fails `Corrupted`). Either way a user permanently lost
a file merely by exceeding a quota — a strictly worse outcome than the
unchecked-update bypass the ceiling was added to close.
The check now precedes the write on create AND update, per-app and group. As
a bonus this stops an over-quota caller driving unbounded write+unlink disk
churn: the ceiling now bounds I/O, not just stored bytes. The blob still goes
down inside the transaction, under the advisory lock, so a rollback unlinks
it and nothing can reference it in between.
The original test passed against the bug: it asserted the refused update did
not change the stored byte TOTAL — true, while the blob was already
destroyed. The regression test now reads the file back through the
checksum-verifying `FsFilesRepo::get`, and was confirmed to fail with
`Corrupted` against the old ordering.
Also in the KV writer (same file): drop the redundant pre-read on the hottest
write path. `set`/`set_if` did a SELECT purely to learn whether the write
would add a row, when the upsert already returns the previous value. Check
after the insert instead (`>` not `>=`) and let the transaction roll back on
refusal — identical outcome, one round-trip fewer, and an update pays nothing
at all. `kv_repo::get_on` and `FsFilesRepo::final_path` are now unused and
deleted; `FilesRepo::delete` delegates to the `delete_meta_on` + `unlink_blob`
helpers rather than re-implementing them.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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