With #83290 merged the build would fail during the configuration phase:
```
configuring
ERROR at //gn/BUILDCONFIG.gn:85:14: Script returned non-zero exit code.
is_clang = exec_script("gn/is_clang.py",
^----------
Current dir: /build/source/out/Release/
Command: python /build/source/gn/gn/is_clang.py cc c++
Returned 2.
stderr:
python: can't open file '/build/source/gn/gn/is_clang.py': [Errno 2] No such file or directory
```
It's impossible to move two major-versions forward when upgrading
Nextcloud. This is an issue when comming from 19.09 (using Nextcloud 16)
and trying to upgrade to 20.03 (using Nextcloud 18 by default).
This patch implements the measurements discussed in #82056 and #82353 to
improve the update process and to circumvent similar issues in the
future:
* `pkgs.nextcloud` has been removed in favor of versioned attributes
(currently `pkgs.nextcloud17` and `pkgs.nextcloud18`). With that
approach we can safely backport major-releases in the future to
simplify those upgrade-paths and we can select one of the
major-releases as default depending on the configuration (helpful to
decide whether e.g. `pkgs.nextcloud17` or `pkgs.nextcloud18` should be
used on 20.03 and `master` atm).
* If `system.stateVersion` is older than `20.03`, `nextcloud17` will be
used (which is one major-release behind v16 from 19.09). When using a
package older than the latest major-release available (currently v18),
the evaluation will cause a warning which describes the issue and
suggests next steps.
To make those package-selections easier, a new option to define the
package to be used for the service (namely
`services.nextcloud.package`) was introduced.
* If `pkgs.nextcloud` exists (e.g. due to an overlay which was used to
provide more recent Nextcloud versions on older NixOS-releases), an
evaluation error will be thrown by default: this is to make sure that
`services.nextcloud.package` doesn't use an older version by accident
after checking the state-version. If `pkgs.nextcloud` is added
manually, it needs to be declared explicitly in
`services.nextcloud.package`.
* The `nixos/nextcloud`-documentation contains a
"Maintainer information"-chapter which describes how to roll out new
Nextcloud releases and how to deal with old (and probably unsafe)
versions.
Closes#82056
Niklas Hambüchen