fix: fall back to GET when APKINDEX HEAD is unsupported

pkg/apk/apk/index.go

@@ -131,16 +131,6 @@ func (i *indexCache) get(ctx context.Context, repoName, repoURL string, keys map
 			return nil, fmt.Errorf("unable to add auth to request: %w", err)
 		}
 
-		resp, err := client.Do(head)
-		if err != nil {
-			return nil, err
-		}
-		defer resp.Body.Close()
-
-		if resp.StatusCode != http.StatusOK {
-			return nil, fmt.Errorf("unexpected status code %d", resp.StatusCode)
-		}
-
 		fetchAndParse := func(etag string) (NamedIndex, error) {
 			b, err := fetchRepositoryIndex(ctx, u, etag, opts)
 			if err != nil {
@@ -153,6 +143,19 @@ func (i *indexCache) get(ctx context.Context, repoName, repoURL string, keys map
 			return NewNamedRepositoryWithIndex(repoName, repoRef.WithIndex(idx)), nil
 		}
 
+		resp, err := client.Do(head)
+		if err != nil {
+			return nil, err
+		}
+		defer resp.Body.Close()
+
+		if resp.StatusCode != http.StatusOK {
+			if resp.StatusCode == http.StatusMethodNotAllowed || resp.StatusCode == http.StatusNotImplemented {
+				return fetchAndParse("")
+			}
+			return nil, fmt.Errorf("unexpected status code %d", resp.StatusCode)
+		}
+
 		etag, ok := etagFromResponse(resp)
 		if !ok {
 			// If there's no etag, we can't cache it, so just return the result.

pkg/apk/apk/repo_test.go

@@ -18,6 +18,7 @@ import (
 	"context"
 	"encoding/base32"
 	"fmt"
+	"io"
 	"io/fs"
 	"maps"
 	"net/http"
@@ -91,6 +92,21 @@ guyM+Ks3c29KlRf3iX35Gt0CAwEAAQ==
 	testArch = "aarch64"
 )
 
+type headMethodNotAllowedTransport struct {
+	wrapped http.RoundTripper
+}
+
+func (t *headMethodNotAllowedTransport) RoundTrip(request *http.Request) (*http.Response, error) {
+	if request.Method == http.MethodHead {
+		return &http.Response{
+			StatusCode: http.StatusMethodNotAllowed,
+			Body:       io.NopCloser(strings.NewReader("method not allowed")),
+		}, nil
+	}
+
+	return t.wrapped.RoundTrip(request)
+}
+
 func TestGetRepositoryIndexes(t *testing.T) {
 	prepLayout := func(t *testing.T, tr http.RoundTripper, cache string, repos []string) *APK {
 		src := apkfs.NewMemFS()
@@ -184,6 +200,16 @@ func TestGetRepositoryIndexes(t *testing.T) {
 		require.NoError(t, err, "unable to read previous index file")
 		require.Equal(t, index1, index2, "index files do not match")
 	})
+	t.Run("head method not allowed falls back to get", func(t *testing.T) {
+		tmpDir := t.TempDir()
+		a := prepLayout(t, &headMethodNotAllowedTransport{
+			wrapped: &testLocalTransport{root: testPrimaryPkgDir, basenameOnly: true},
+		}, tmpDir, nil)
+
+		indexes, err := a.GetRepositoryIndexes(context.Background(), false)
+		require.NoErrorf(t, err, "unable to get indexes")
+		require.Greater(t, len(indexes), 0, "no indexes found")
+	})
 	t.Run("repo url with http basic auth", func(t *testing.T) {
 		tmpDir := t.TempDir()
 		a := prepLayout(t, &testLocalTransport{