# RB-02 CRITICAL: multi-statement migrations can half-apply

**Severity:** CRITICAL
**Path:** `crates/storage/src/migrations.rs:263-299`
**Source:** [2026-04-22 code review](../code-review-2026-04-22.md#c3--multi-statement-migrations-can-half-apply)
**Labels:** release-blocker, critical, data-integrity, storage
**Status:** RESOLVED (2026-04-22)

## Resolution

Extracted `run_migrations_slice(pool, migrations)` as the single code
path that applies pending migrations. For each pending version it
opens a `Transaction` via `pool.begin()`, applies every split statement
on that transaction, records the `schema_version` row inside the same
transaction, and finally `tx.commit()`s. A failure anywhere in the
sequence — statement, version insert, commit — rolls the whole
migration back.

`run_migrations` delegates to `run_migrations_slice(pool, MIGRATIONS)`
and the test helper `run_migrations_up_to` to a filtered subset, so
only one version of the apply logic exists.

Regression test `multi_statement_migration_rolls_back_on_failure`
feeds a poisoned v9 migration (`CREATE TABLE poison_marker; SELECT
this_function_does_not_exist()`) through `run_migrations_slice`. The
call returns `Err`, and post-call `SELECT COUNT(*) FROM poison_marker`
fails with "no such table" while `MAX(schema_version)` remains at 8.

SQLite DDL participates in transactions, so this is sufficient for the
Kon schema. If any future migration needs a statement that implicitly
commits (`VACUUM`, `REINDEX`, `ATTACH`) — none do today — it must be
split into its own non-transactional migration. Reviewer's job to flag.

## Problem

`run_migrations` executes each statement individually and only records the schema version after the full migration succeeds. If a multi-statement migration (v5, v6, v8 — any containing more than one `CREATE` / `ALTER` / `UPDATE`) fails mid-run, or the process is killed between statements, the schema can end up partially changed while still appearing unapplied. The next startup replays the same migration against the mutated database, which can fail in confusing ways or corrupt data further.

## Acceptance

- Every migration runs inside a single `BEGIN` / `COMMIT` transaction.
- The version row update happens inside the same transaction — atomic success or no change.
- Regression test: a migration that panics partway through leaves the database at the previous schema version with no partial changes visible on restart.

## Fix scope

Medium. Wrap each migration in `pool.begin()` / `tx.commit()`. The version update and the migration statements all execute on the same `Transaction` handle. Needs careful review of any migration that uses implicit commits (SQLite `VACUUM`, `REINDEX`, `ATTACH` — none of which Kon currently uses, but the review pattern should guard against future additions).

## Dependencies

- Coupled with RB-03 (any v9 migration adding the transcript-profile FK must itself be transactional — this fix is a prerequisite).
