Fixes#38456
## Problem
The Alpine package registry serves one `APKINDEX.tar.gz` per
architecture. When a repository contains only `noarch` packages (no
architecture-specific packages), only the `noarch` index is built.
Because `apk` substitutes `$ARCH` with the host architecture and
requests e.g. `x86_64/APKINDEX.tar.gz`, such a repository returned HTTP
404 and was unusable, matching the report in #38456.
## Fix
`GetRepositoryFile` now falls back to the `noarch` index when the
requested architecture has no index of its own, mirroring the fallback
already present in the sibling `DownloadPackageFile` handler. `noarch`
packages are installable on every architecture, so serving them for any
requested architecture is correct. The index-build side is unchanged;
only the serving path gains the fallback, so mixed repositories (which
already merge `noarch` into each per-architecture index) are unaffected.
## AI assistance disclosure
This change was implemented with the help of an AI coding assistant,
which gitea's CONTRIBUTING.md explicitly welcomes when disclosed. I have
reviewed the change, understand it, and can explain and defend it.
## Tests
Added a `NoArchOnly` subtest to `TestPackageAlpine` that publishes only
a `noarch` package to a fresh repository and asserts that `GET
.../x86_64/APKINDEX.tar.gz` now returns `200` (previously `404`) and
that the served index lists the noarch package. Verified locally with
`go build`/`go vet` on the changed package and a compile of the
integration test package (`go test -c`); the full integration run relies
on CI.
---------
Signed-off-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Fixes#26691
Revert #24972
The alpine package manager expects `noarch` packages in the index of
other architectures too.
---------
Co-authored-by: Lauris BH <lauris@nix.lv>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
