fix(packages): serve noarch Alpine index for any requested architecture (#38479)

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>

routers/api/packages/alpine/alpine.go

@@ -67,15 +67,34 @@ func GetRepositoryFile(ctx *context.Context) {
 		return
 	}
 
+	branch := ctx.PathParam("branch")
+	repository := ctx.PathParam("repository")
+	architecture := ctx.PathParam("architecture")
+
 	s, u, pf, err := packages_service.OpenFileForDownloadByPackageVersion(
 		ctx,
 		pv,
 		&packages_service.PackageFileInfo{
 			Filename:     alpine_service.IndexArchiveFilename,
-			CompositeKey: fmt.Sprintf("%s|%s|%s", ctx.PathParam("branch"), ctx.PathParam("repository"), ctx.PathParam("architecture")),
+			CompositeKey: fmt.Sprintf("%s|%s|%s", branch, repository, architecture),
 		},
 		ctx.Req.Method,
 	)
+	// A repository that only contains "noarch" packages has no per-architecture
+	// index. Since noarch packages are installable on every architecture, fall
+	// back to the noarch index so clients requesting their own architecture
+	// (e.g. x86_64) can still discover them.
+	if errors.Is(err, util.ErrNotExist) && architecture != alpine_module.NoArch {
+		s, u, pf, err = packages_service.OpenFileForDownloadByPackageVersion(
+			ctx,
+			pv,
+			&packages_service.PackageFileInfo{
+				Filename:     alpine_service.IndexArchiveFilename,
+				CompositeKey: fmt.Sprintf("%s|%s|%s", branch, repository, alpine_module.NoArch),
+			},
+			ctx.Req.Method,
+		)
+	}
 	if err != nil {
 		if errors.Is(err, util.ErrNotExist) {
 			apiError(ctx, http.StatusNotFound, err)

tests/integration/api_packages_alpine_test.go

@@ -268,6 +268,36 @@ AACAX/AKARNTyAAoAAA=`
 						AddBasicAuth(user.Name)
 					MakeRequest(t, req, http.StatusNoContent)
 				})
+
+				t.Run("NoArchOnly", func(t *testing.T) {
+					defer tests.PrintCurrentTest(t)()
+
+					// A repository that only contains noarch packages has no per-architecture index,
+					// but apk always requests the index for its own architecture (e.g. x86_64).
+					// That request must fall back to the noarch index instead of 404ing.
+					noarchRepository := repository + "-noarchonly"
+
+					req := NewRequestWithBody(t, "PUT", fmt.Sprintf("%s/%s/%s", rootURL, branch, noarchRepository), bytes.NewReader(noarchContent)).
+						AddBasicAuth(user.Name)
+					MakeRequest(t, req, http.StatusCreated)
+
+					req = NewRequest(t, "GET", fmt.Sprintf("%s/%s/%s/x86_64/APKINDEX.tar.gz", rootURL, branch, noarchRepository))
+					resp := MakeRequest(t, req, http.StatusOK)
+
+					content, err := readIndexContent(resp.Body)
+					assert.NoError(t, err)
+
+					assert.Contains(t, content, "C:Q1kbH5WoIPFccQYyATanaKXd2cJcc=\n")
+					assert.Contains(t, content, "A:noarch\n")
+
+					// The noarch index is still directly retrievable too.
+					req = NewRequest(t, "GET", fmt.Sprintf("%s/%s/%s/noarch/APKINDEX.tar.gz", rootURL, branch, noarchRepository))
+					MakeRequest(t, req, http.StatusOK)
+
+					req = NewRequest(t, "DELETE", fmt.Sprintf("%s/%s/%s/noarch/gitea-noarch-1.4-r0.apk", rootURL, branch, noarchRepository)).
+						AddBasicAuth(user.Name)
+					MakeRequest(t, req, http.StatusNoContent)
+				})
 			})
 		}
 	}