pkg/cache: configurable backend preference

pkg/cache/cache.go

@@ -51,7 +51,8 @@ type backend interface {
 }
 
 type CacheOptions struct {
-	Log *logrus.Entry
+	Log    *logrus.Entry
+	Format string
 }
 
 func WithLog(log *logrus.Entry) CacheOption {
@@ -60,6 +61,12 @@ func WithLog(log *logrus.Entry) CacheOption {
 	}
 }
 
+func WithFormat(format string) CacheOption {
+	return func(o *CacheOptions) {
+		o.Format = format
+	}
+}
+
 type CacheOption func(*CacheOptions)
 
 // New creates a new Cache. It chooses a cache implementation based
@@ -73,7 +80,7 @@ func New(cacheDir string, cacheOpts ...CacheOption) (Cache, error) {
 	for _, opt := range cacheOpts {
 		opt(opts)
 	}
-	cacheBackend, err := getDefaultBackend(cacheDir, opts.Log)
+	cacheBackend, err := getBackend(cacheDir, opts.Format, opts.Log)
 	if err != nil {
 		return nil, err
 	}
@@ -84,7 +91,7 @@ func New(cacheDir string, cacheOpts ...CacheOption) (Cache, error) {
 	return &cache{backend: cacheBackend, log: opts.Log}, nil
 }
 
-func getDefaultBackend(cacheDir string, log *logrus.Entry) (backend, error) {
+func getBackend(cacheDir string, backendName string, log *logrus.Entry) (backend, error) {
 	entries, err := os.ReadDir(cacheDir)
 	if err != nil && !errors.Is(err, os.ErrNotExist) {
 		return nil, fmt.Errorf("detect cache format: read cache directory: %v", err)
@@ -96,12 +103,21 @@ func getDefaultBackend(cacheDir string, log *logrus.Entry) (backend, error) {
 	}
 
 	if len(entries) == 0 {
-		log.WithField("backend", backends[0].Name()).Info("cache directory is empty, using preferred backend")
-		return backends[0], nil
+		if backendName == "" {
+			log.WithField("backend", backends[0].Name()).Info("cache directory is empty, using preferred backend")
+			return backends[0], nil
+		}
+		for _, b := range backends {
+			if b.Name() == backendName {
+				log.WithField("backend", backendName).Info("using preferred backend")
+				return b, nil
+			}
+		}
+		return nil, fmt.Errorf("preferred backend %q not found", backendName)
 	}
 
 	for _, backend := range backends {
-		if backend.IsCachePresent() {
+		if (backendName == "" || backend.Name() == backendName) && backend.IsCachePresent() {
 			log.WithField("backend", backend.Name()).Info("found existing cache contents")
 			return backend, nil
 		}

pkg/cache/cache_test.go

@@ -222,9 +222,11 @@ func TestCache_ListPackages(t *testing.T) {
 func genTestCaches(t *testing.T, fbcFS fs.FS) map[string]Cache {
 	t.Helper()
 
-	caches := map[string]Cache{
-		"json":      &cache{backend: newJSONBackend(t.TempDir()), log: log.Null()},
-		"pogreb.v1": &cache{backend: newPogrebV1Backend(t.TempDir()), log: log.Null()},
+	caches := make(map[string]Cache)
+	for _, format := range []string{FormatJSON, FormatPogrebV1} {
+		c, err := New(t.TempDir(), WithFormat(format), WithLog(log.Null()))
+		require.NoError(t, err)
+		caches[format] = c
 	}
 
 	for _, c := range caches {

pkg/cache/json.go

@@ -40,8 +40,10 @@ type jsonBackend struct {
 	bundles bundleKeys
 }
 
+const FormatJSON = "json"
+
 func (q *jsonBackend) Name() string {
-	return "json"
+	return FormatJSON
 }
 
 func (q *jsonBackend) IsCachePresent() bool {

pkg/cache/pogrebv1.go

@@ -31,10 +31,12 @@ func newPogrebV1Backend(baseDir string) *pogrebV1Backend {
 }
 
 const (
+	FormatPogrebV1 = "pogreb.v1"
+
 	pogrebV1CacheModeDir  = 0770
 	pogrebV1CacheModeFile = 0660
 
-	pograbV1CacheDir = "pogreb.v1"
+	pograbV1CacheDir = FormatPogrebV1
 	pogrebDigestFile = pograbV1CacheDir + "/digest"
 	pogrebDbDir      = pograbV1CacheDir + "/db"
 )
@@ -46,7 +48,7 @@ type pogrebV1Backend struct {
 }
 
 func (q *pogrebV1Backend) Name() string {
-	return pograbV1CacheDir
+	return FormatPogrebV1
 }
 
 func (q *pogrebV1Backend) IsCachePresent() bool {