db/db_checkout_error.ts

DbCheckoutError — a failed pool checkout, kept apart from a failed query.

A request that cannot check a connection out of the pool (the wait timed out, a new connection's connect timed out or was refused, the pool is closed) finds the database unavailable; every request surface answers it with a 503 (service_unavailable), where a query that fails on a connection already checked out keeps its 500. The pool built by create_pg_pool (db/db_pg.ts) throws one from its connect, which per-statement pool.query and each transaction's pool.connect() both run through. to_thrown_jsonrpc_error (http/jsonrpc_errors.ts) maps it, so the dispatcher, the route catch, and the server's onError all answer it the same way. Twin of the Rust spine's pool.get() failures, which answer fuz_http::db_unavailable / db_unavailable_response.

PGlite and pglet have no pool, so neither ever throws one.