Improve migration error message "cannot run inside a transaction block"

[Turbo87]

Backend

PostgreSQL

What do you want to add to diesel?

When migrations fail because the operation is not allowed to run inside transactions, the error message just shows:

QueryError(DieselMigrationName { name: "2025-03-14-192013_migration", version: MigrationVersion("20250314192013") }, DatabaseError(Unknown, "CREATE INDEX CONCURRENTLY cannot run inside a transaction block"))

The error message should be enhanced by linking to e.g. https://docs.diesel.rs/master/diesel_migrations/struct.FileBasedMigrations.html and mentioning that a metadata.toml file can be used to disable transaction usage for the migration.

Additionally, if the migration contains multiple commands, Postgres will open an implicit transaction even if run_in_transaction = false was used, resulting in the same error message (see #4552). If we detect run_in_transaction = false and the error above, we should instead suggest to split the migration into multiple single-instruction migrations to resolve the problem.

[weiznich]

Thanks for opening this request. I think that's a good idea. Migrations are executed here

We would need to change this line to inspect the returned error and see if it contains something like the error message mentioned in the OP.

For the second branch we could likely also check if the input contains more than one ; and use that as heuristic if there are more than one instruction in there. Likely it's also a good idea to guard this on DB being diesel::pg::Pg as it might not apply to other backends. Such a check can be done by just comparing the type id of DB to that one from diesel::pg::Pg at runtime.