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Apparently a transaction doesn't help here as it only protects from seeing incomplete changes from other transactions*. But the new jobs are likely not added in a transactional way.

I also tested this by adding the following to a controller:

$self->app->schema->txn_do(sub {
    log_debug('starting transaction');
    sleep 10; # in the meantime, insert a new worker via psql
    log_debug('actual lookup: ' . $self->app->schema->resultset('Workers')->count); # the count is incremented
});

This still leaves the question how to handle it instead. We could make scheduling an iso a transaction. But in general new jobs can always be added so the asset cleanup should likely be able to handle it.

* see https://www.postgresql.org/docs/8.3/tutorial-transactions.html, "when multiple transactions are running concurrently, each one should not be able to see the incomplete changes made by others"

The database operations for scheduling ISOs are actually already a transaction. Maybe the jobs which are added in the meantime are added elsewhere?