This should NOT be backported to 20.09!
When 21.03 is released, the DB changes are about a year old and
operators had two release cycles for the upgrade. At this point it
should be fair to remove the compat layer to reduce the complexity of
the module itself.
We already set the relevant env vars in the systemd services. That does
not help one when executing any of the executables outside a service,
e.g. when creating a new user.
Also removed `pkgs.hydra-flakes` since flake-support has been merged
into master[1]. Because of that, `pkgs.hydra-unstable` is now compiled
against `pkgs.nixFlakes` and currently requires a patch since Hydra's
master doesn't compile[2] atm.
[1] https://github.com/NixOS/hydra/pull/730
[2] https://github.com/NixOS/hydra/pull/732
Upgrades Hydra to the latest master/flake branch. To perform this
upgrade, it's needed to do a non-trivial db-migration which provides a
massive performance-improvement[1].
The basic ideas behind multi-step upgrades of services between NixOS versions
have been gathered already[2]. For further context it's recommended to
read this first.
Basically, the following steps are needed:
* Upgrade to a non-breaking version of Hydra with the db-changes
(columns are still nullable here). If `system.stateVersion` is set to
something older than 20.03, the package will be selected
automatically, otherwise `pkgs.hydra-migration` needs to be used.
* Run `hydra-backfill-ids` on the server.
* Deploy either `pkgs.hydra-unstable` (for Hydra master) or
`pkgs.hydra-flakes` (for flakes-support) to activate the optimization.
The steps are also documented in the release-notes and in the module
using `warnings`.
`pkgs.hydra` has been removed as latest Hydra doesn't compile with
`pkgs.nixStable` and to ensure a graceful migration using the newly
introduced packages.
To verify the approach, a simple vm-test has been added which verifies
the migration steps.
[1] https://github.com/NixOS/hydra/pull/711
[2] https://github.com/NixOS/nixpkgs/pull/82353#issuecomment-598269471
This applies [hydra PR #432](https://github.com/NixOS/hydra/pull/432)
to the NixOS module in nixpkgs:
```
commit 4efd078977e5ea20e1104783efc324cba11690bc
Author: Bas van Dijk <v.dijk.bas@gmail.com>
Date: Sun Dec 11 15:35:38 2016 +0100
Only set buildMachinesFiles when nix.buildMachines is defined
```
The following commit from 2016 in hydra removed the `--option
build-use-substitutes` from the hydra-queue-runner service:
```
commit ee2e9f5335c8c0288c102975b506f6b275793cfe
Author: Eelco Dolstra <edolstra@gmail.com>
Date: Fri Oct 7 20:23:05 2016 +0200
Update to reflect BinaryCacheStore changes
BinaryCacheStore no longer implements buildPaths() and ensurePath(),
so we need to use copyPath() / copyClosure().
```
It would be better if the hydra module in NixOS matches the upstream
module.
During the last update, `hydra-notify` was rewritten as a daemon which
listens to postgresql notifications for each build[1]. The module
uses the `hydra-notify.service` unit from upstream's Hydra module and
the VM test ensures that email notifications are sent properly.
Also updated `hydra-init.service` to install `pg_trgm` on a local
database if needed[2].
[1] c7861b85c4
[2] 8a0a5ec3a3
This was originally removed in d4d0e449d7.
The intent was not to maintain hydra expression at two places.
Nowadays we have enough devs to maintain this despite copy/pasta.
This should encourage more people to use Hydra, which is a really
great piece of software together with Nix.
Tested a deploy using https://github.com/peti/hydra-tutorial
