The §11.6 M4 byte quota mixed two incompatible byte measures: `used` came from
Postgres as `octet_length(value::text)` (jsonb CANONICAL text — whitespace after
`:` and `,`, keys normalized), while `old_len`/`new_len` were computed in Rust
with `serde_json::to_vec` (COMPACT, no spaces). The projection
`used - old_len + new_len` therefore subtracted and added a *smaller* measure
than what is actually stored, so a group could be admitted over its ceiling —
the cap drifted permissive. The two forms are not reconcilable in Rust (jsonb
also reorders/dedups keys), so the projection has to be measured by Postgres.
Add `projected_total_bytes` to `GroupKvRepo` (keyed by collection+key) and
`GroupDocsRepo` (keyed by the replaced doc id, `None` on create). Each computes
`SUM(canonical) - existing_row(canonical) + new_value(canonical)` in ONE query,
binding the new value as compact TEXT and re-parsing it through `::jsonb::text`
so PG measures it in exactly the form it stores. All three terms now share one
metric. The services call it instead of the mixed Rust math; the near-cap
fast-path (`rows_after * max_value_bytes <= ceiling` → skip the SUM) is kept, so
the common case still does no extra work. The docs `update` path no longer needs
its old-doc fetch (the subtraction happens in SQL), removing a read.
The in-memory test repos implement the same projection with their own (compact)
metric, so the quota unit tests keep their exact byte arithmetic. No schema
change.
Note: the check-then-write TOCTOU (concurrent writers can each pass and
collectively overshoot) is NOT addressed here — it needs a transaction spanning
the check and the write, the same missing infrastructure as the transactional
outbox. Tracked separately.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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