Adds mariadbEngine alongside the PostgreSQL one: a three-stage probe that
separates a live-but-unreachable server from an absent one, ALTER USER password
rotation on stdin behind a session logging guard, a dynamic SET GLOBAL apply
with rollback of the installed file, file-comparison pending-restart against a
serving copy, last-known-good restore, service stop and restart, and BACKUP
STAGE quiesce on a held session with a bounded lock_wait_timeout.
The parameter-file machinery, service actions, quiesce hold bookkeeping and
client session are extracted so both engines share them, and a contract suite
runs the same cases against both through the interfaces. MariaDB is not
registered in the agent's engine table yet, so construction still refuses.
Define the composite engine interface and a newEngine() factory keyed by a per-engine layout table, so the agent's defaults, health probe and command implementation all come from the engine the image bakes rather than from PostgreSQL-shaped constants. Rename PGBin/PGData/PGUser to EngineBinDir/EngineDataDir/EngineUser, delete PGIsReady, and give engineProbe an injected state function with the pg_isready exit-code mapping moved into the PostgreSQL implementation.
setup.sh now stamps /etc/spinifex-rds/engine and buildAgentUserData emits RDS_ENGINE. The agent compares the two after registering and refuses to run when they disagree, reporting unhealthy before the bootstrap handoff is written so rds-init never unblocks and no datadir is touched.
Completes the control-plane half of the engine seam ahead of a second
engine. Engine gains maxUsernameLen, its own parameter table and its own
cross-parameter checks, and the catalog accessors become methods on it,
so no path resolves parameters without naming an engine.
PostgreSQL's table, size-derived formulas and combination checks move to
engine_postgres.go. engines is also indexed by parameter-group family,
for the callers that hold only a family string. EnginePostgres() is
deleted: the guest's reserved-username re-check now resolves the engine
through LookupEngine once at agent startup.
No behavior change.
Julian Sommer