Fix github_alpine .apk redirect to resolve stored FilePath

apk reconstructs the download URL itself as <arch>/<name>-<version>.apk
because APKINDEX carries no filename field (unlike rpm's <location> or
deb's Filename:). ServeRemote forwarded that synthesized path verbatim
into the releases_remote redirect, pointing at a nonexistent,
allowlist-denied github.com path (404/403).

Look up the cached metadata row by arch plus the full reconstructed
filename (no hyphen-split, so -rN suffixes are preserved) and redirect
to the stored github-relative FilePath. Unknown packages now 404 instead
of redirecting to a bad path.

internal/provider/alpine/github.go

@@ -146,14 +146,49 @@ func (p *GitHubProvider) ServeRemote(w http.ResponseWriter, r *http.Request, rem
 			http.Error(w, "github_alpine remote has no releases_remote configured for downloads", http.StatusInternalServerError)
 			return true
 		}
-		loc := strings.TrimRight(proxyBaseURL, "/") + "/api/v1/remote/" + remote.ReleasesRemote + "/" + strings.TrimLeft(path, "/")
-		http.Redirect(w, r, loc, http.StatusFound)
+		p.serveApkRedirect(w, r, remote, path, proxyBaseURL, store)
 		return true
 	}
 
 	return false
 }
 
+// serveApkRedirect resolves an apk-reconstructed download path — apk builds
+// "<arch>/<name>-<version>.apk" itself because APKINDEX carries no filename — to
+// the real github-relative asset path stored on the metadata row, then redirects
+// to the backend releases_remote. Passing the inbound path through verbatim would
+// point at a nonexistent, allowlist-denied github.com path.
+func (p *GitHubProvider) serveApkRedirect(w http.ResponseWriter, r *http.Request, remote models.Remote, path, proxyBaseURL string, store provider.RemoteMetadataStore) {
+	arch := strings.TrimSuffix(path[:strings.LastIndex(path, "/")+1], "/")
+	basename := path[strings.LastIndex(path, "/")+1:]
+	if arch == "" || strings.Contains(arch, "/") {
+		http.Error(w, "apk download must be requested per-arch: <arch>/<name>-<version>.apk", http.StatusNotFound)
+		return
+	}
+
+	reader, ok := store.(provider.AlpineMetadataReader)
+	if !ok {
+		http.Error(w, "alpine metadata not available", http.StatusInternalServerError)
+		return
+	}
+	sctx, cancel := context.WithTimeout(context.WithoutCancel(r.Context()), p.serveTimeout)
+	defer cancel()
+	rows, err := reader.ListAlpineMetadataEntries(sctx, remote.Name)
+	if err != nil {
+		http.Error(w, err.Error(), http.StatusInternalServerError)
+		return
+	}
+
+	for _, row := range rows {
+		if row.Arch == arch && row.Name+"-"+row.Version+".apk" == basename {
+			loc := strings.TrimRight(proxyBaseURL, "/") + "/api/v1/remote/" + remote.ReleasesRemote + "/" + strings.TrimLeft(row.FilePath, "/")
+			http.Redirect(w, r, loc, http.StatusFound)
+			return
+		}
+	}
+	http.Error(w, "package not found", http.StatusNotFound)
+}
+
 func (p *GitHubProvider) serveIndex(w http.ResponseWriter, r *http.Request, remote models.Remote, path string, store provider.RemoteMetadataStore) {
 	arch := strings.TrimSuffix(path, "APKINDEX.tar.gz")
 	arch = strings.Trim(arch, "/")

internal/provider/alpine/github_test.go

@@ -273,10 +273,13 @@ func TestGitHubServeRemoteIndexAndRedirect(t *testing.T) {
 		t.Fatalf("aarch64 index should not carry the x86_64 package: %s", got)
 	}
 
-	// An .apk request redirects to the backend releases_remote.
+	// An .apk request arrives in apk's reconstructed shape
+	// "<arch>/<name>-<version>.apk" (APKINDEX carries no filename), NOT as the
+	// github-relative FilePath. ServeRemote must resolve it back to the stored
+	// FilePath before redirecting to the backend releases_remote.
 	rec = httptest.NewRecorder()
-	req = httptest.NewRequest(http.MethodGet, "/api/v1/remote/acme-apk/"+demoPath, nil)
-	if !p.ServeRemote(rec, req, remote, demoPath, proxyBase, store) {
+	req = httptest.NewRequest(http.MethodGet, "/api/v1/remote/acme-apk/x86_64/demo-1.2.3-r0.apk", nil)
+	if !p.ServeRemote(rec, req, remote, "x86_64/demo-1.2.3-r0.apk", proxyBase, store) {
 		t.Fatal("ServeRemote did not handle .apk")
 	}
 	if rec.Code != http.StatusFound {
@@ -284,7 +287,30 @@ func TestGitHubServeRemoteIndexAndRedirect(t *testing.T) {
 	}
 	wantLoc := proxyBase + "/api/v1/remote/github/" + demoPath
 	if got := rec.Header().Get("Location"); got != wantLoc {
-		t.Fatalf("Location = %q, want %q", got, wantLoc)
+		t.Fatalf("Location = %q, want %q (must be the stored FilePath, not the inbound path)", got, wantLoc)
+	}
+}
+
+// An apk download whose reconstructed "<arch>/<name>-<version>.apk" matches no
+// cached row must 404, never redirect to a bad path.
+func TestGitHubServeRemoteApkRedirectNotFound(t *testing.T) {
+	fx := newGitHubFixture(t, true)
+	p := newTestProvider()
+	store := newFakeStore()
+	remote := fx.remote()
+
+	// Warm the cache so the store is populated but lacks the requested package.
+	if err := p.scan(context.Background(), remote, store); err != nil {
+		t.Fatalf("warm scan: %v", err)
+	}
+
+	rec := httptest.NewRecorder()
+	req := httptest.NewRequest(http.MethodGet, "/api/v1/remote/acme-apk/x86_64/nope-9.9.9.apk", nil)
+	if !p.ServeRemote(rec, req, remote, "x86_64/nope-9.9.9.apk", "https://x", store) {
+		t.Fatal("ServeRemote did not handle .apk")
+	}
+	if rec.Code != http.StatusNotFound {
+		t.Fatalf("want 404 for unknown package, got %d (Location=%q)", rec.Code, rec.Header().Get("Location"))
 	}
 }