This gets rid of the openHydraDB function and ensures that we
open the database in a consistent way.
Also drop the PostgreSQL sequence hacks. They don't seem to be
necessary anymore.
When checking whether the jobset is unchanged, we need to compare with
the previous JobsetEval regardless of whether it had new builds.
Otherwise we'll keep adding new JobsetEval rows.
* Don't use isCurrent anymore; instead look up builds in the previous
jobset evaluation. (The isCurrent field is still maintained because
it's still used in some other places.)
* To determine whether to perform an evaluation, compare the hash of
the current inputs with the inputs of the previous jobset
evaluation, rather than checking if there was ever an evaluation
with those inputs. This way, if the inputs of an evaluation change
back to a previous state, we get a new jobset evaluation in the
database (and thus the latest jobset evaluation correctly represents
the latest state of the jobset).
* Improve performance by removing some unnecessary operations and
adding an index.
The abbreviated Git revision hash (e.g. "267480b") is typically
contained in ‘gitTag’ as well, but the latter can contain other
elements as well, e.g., the delta to the closest tag. That may
be undesirable in version strings, so this is an alternative.
The ‘revCount’ attribute is the number of commits in the history
of the revision. This is useful if you need a monotonically
increasing version number.
The ‘gitTag’ attribute is the output of ‘git describe’, e.g.
‘v1.0.4-14-g2414721’ to indicate that the current revision is 14
commits after the tag ‘v1.0.4’.
In hydra-evaluator, reuse an SVN working copy between runs (similar to
what we do with Git and other input types). This reduces network
traffic in the common case.
Also, don't use nix-prefetch-svn. It doesn't do anything useful.
even when all the remote build slots are in use. The evaluator can
cause builds if Nix expressions import derivations (e.g. in
pkgs/build-support/vm to compute the RPM/Deb closures). If there
are no free build slots, the evaluator can hang for a long time.
