Re-use cache when downloading charts

Previously, when a chart has been downloaded, it had been put in the cache. However, this has happened unconditionally,
so that the cache hasn't actually been used for anything. This patch attempts to fix this and checks if the target file
exists in the cache before downloading.

Fixes: #12607, #12602

Signed-off-by: Nikolai Prokoschenko <nikolai.prokoschenko@kurzdigital.com>

pkg/downloader/chart_downloader.go

@@ -92,25 +92,28 @@ func (c *ChartDownloader) DownloadTo(ref, version, dest string) (string, *proven
 		return "", nil, err
 	}
 
-	g, err := c.Getters.ByScheme(u.Scheme)
-	if err != nil {
-		return "", nil, err
-	}
-
-	data, err := g.Get(u.String(), c.Options...)
-	if err != nil {
-		return "", nil, err
-	}
-
 	name := filepath.Base(u.Path)
 	if u.Scheme == registry.OCIScheme {
 		idx := strings.LastIndexByte(name, ':')
 		name = fmt.Sprintf("%s-%s.tgz", name[:idx], name[idx+1:])
 	}
+	g, err := c.Getters.ByScheme(u.Scheme)
+	if err != nil {
+		return "", nil, err
+	}
 
 	destfile := filepath.Join(dest, name)
-	if err := fileutil.AtomicWriteFile(destfile, data, 0644); err != nil {
-		return destfile, nil, err
+
+	_, err = os.Stat(destfile)
+	if err != nil {
+		data, err := g.Get(u.String(), c.Options...)
+		if err != nil {
+			return "", nil, err
+		}
+
+		if err := fileutil.AtomicWriteFile(destfile, data, 0644); err != nil {
+			return destfile, nil, err
+		}
 	}
 
 	// If provenance is requested, verify it.