fix(dbconn): keep the execution bounds where a pooler would drop them

[aparajon]

lock_timeout and statement_timeout are how pg-sprite bounds every statement it issues (LK-2). They are session settings, and they were sent only as startup parameters — which a pooler is free to drop. PgBouncer drops anything in ignore_startup_parameters, in session mode as well as transaction mode, so the connection succeeds and the bounds are simply absent. Measured against a real PgBouncer: asked for lock_timeout=3s, statement_timeout=30s, got 0 and 0. Unbounded, reported as fine.

That matters beyond a misconfiguration: hosted PostgreSQL hands out a pooled connection string by default, so this is what an ordinary adopter gets.

Three steps, each catching what the one before it cannot:

  1  apply     startup params, plus a SET on every connection
               a pooler may drop a parameter; none strips a statement,
               so session-mode pooling now works correctly

  2  verify    read the bounds back off pg_settings
               refuse when the session did not keep them

  3  prove     take an advisory lock, hold a transaction open on a
               second connection so the first is forced to rebind,
               and refuse when the lock does not follow

Step 3 is not redundant. In transaction mode PgBouncer runs no server_reset_query by default, so the SET from step 1 sticks on a reused backend and step 2 reports healthy on a connection whose next statement lands on a backend that never saw it. Only forcing the rebind settles it.

The proof has no false positives — each reading is a fact about the connection in hand, not a guess about what sits behind it. It is one-sided the other way: an idle pooler with spare backends can still pass it, so it is a guard against the connection string a platform hands you, not a substitute for pointing pg-sprite at a session-mode endpoint. The refusal says which endpoint to use rather than failing opaquely.

LK-2: extends enforcement. The invariant text is unchanged — the bound was always required; this makes its absence detectable instead of silent.