Declarative jobsets were broken by the Nix update, causing
nix cat-file to break silently.
This commit restores declarative jobsets, based on top of a commit
making it easier to see what broke.
In the past, jobsets which are automatically evaluated are evaluated
regularly, on a schedule. This schedule means a new evaluation is
created every checkInterval seconds (assuming something changed.)
This model works well for architectures where our build farm can
easily keep up with demand.
This commit adds a new type of evaluation, called ONE_AT_A_TIME, which
only schedules a new evaluation if the previous evaluation of the
jobset has no unfinished builds.
This model of evaluation lets us have 'low-tier' architectures.
For example, we could now have a jobset for ARMv7l builds, where
the buildfarm only has a single, underpowered ARMv7l builder.
Configuring that jobset as ONE_AT_A_TIME will create an evaluation
and then won't schedule another evaluation until every job of
the existing evaluation is complete.
This way, the cache will have a complete collection of pre-built
software for some commits, but the underpowered architecture will
never become backlogged in ancient revisions.
A postgresql column which is non-null and unique is treated with
the same optimisations as a primary key, so we have no need to
try and recreate the `id` as the primary key.
No read paths are impacted by this change, and the database will
automatically create an ID for each insert. Thus, no code needs to
change.
hydra.nixos.org is already running this rev, and it should be safe to
apply to everyone else. If we make changes to this migration, we'll
need to write another migration anyway.
Lowercasing is due to postgresql not having case-sensitive table names.
It always technically workde before, but those table names never
existed literally.
The switch to generating from postgresql is to handle an upcoming
addition of an auto-incrementign ID to the Jobset table. Sqlite doesn't
seem to be able to handle the table having an auto incrementing ID
field which isn't the primary key, but we can't change the primary
key trivially.
Since hydra doesn't support sqlite and hasn't for many year anyway,
it is easier to just generate from pgsql directly.
