A restore with --empty destroys before it replays: the pre-clean drops the schema in one session and the dump goes in the next, with no rollback across the two. A dump the engine cannot parse therefore does not fail harmlessly, it leaves an emptied database behind. The version each side is on decides that up front, so the refusal lands before the first session opens.
Both engines state their origin in the dump's own header and spell it differently. Postgres names the source server; MariaDB opens with mariadb-dump's own version and names the server further down, so matching the first number would read the tool on one engine and the server on the other. A pg_dumpall stream carries no version line of its own at all - the first belongs to the first database's embedded pg_dump output, arbitrarily far down - hence the deep scan.
Restoring forward across a major version stays allowed; that is the upgrade path. Only backward is refused, with --no-version-check as the way out.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
The dump replay ran statement-by-statement with autocommit. When restore --empty runs against a LIVE database, the pre-clean drops a table, the replay recreates it and autocommits, and a concurrent writer (discourse's mini_scheduler upserting scheduler_stats(id=1)) inserts the same primary key into the empty table before the dump's COPY loads it. The COPY then aborts with a duplicate-key violation under ON_ERROR_STOP and the whole restore fails. Running the replay with --single-transaction keeps the recreated table invisible to other sessions until commit, so the writer can never insert the racing row.
The --empty pre-clean stays multi-statement (\gexec, one DROP per statement): running every DROP in one transaction exhausts max_locks_per_transaction on large schemas (e.g. gitlab).
Extract the pre-clean SQL from the inline string into restore/db/empty_preclean.sql (loaded via dirname(__file__)) and declare it as package-data so it ships in the wheel. Add a unit test guarding the single-transaction/multi-statement split and an e2e that reproduces the live-writer race.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
