Fix cache extractor redownloading up-to-date caches for Go Cloud URLs

For pre-signed URLs, the runner helper checks that the `Last-Modified`
header of the cache blob is later than the modification time of the
downloaded cache file. This avoids having to download an unchanged
cache file.

This was previously not happening for Go Cloud URLs. This commit
adds support for this checking.

Note that the Last-Modified time appears to be updated automatically
by Go Cloud, so we don't to set this header as we do with presigned
URLs in the cache archiver.

Changelog: fixed

Author: stanhu

commands/helpers/cache_extractor.go

@@ -49,9 +49,13 @@ func (c *CacheExtractorCommand) getClient() *CacheClient {
 }
 
 func checkIfUpToDate(path string, resp *http.Response) (bool, time.Time) {
-	fi, _ := os.Lstat(path)
 	date, _ := time.Parse(http.TimeFormat, resp.Header.Get("Last-Modified"))
-	return fi != nil && !date.After(fi.ModTime()), date
+	return isLocalCacheFileUpToDate(path, date), date
+}
+
+func isLocalCacheFileUpToDate(path string, date time.Time) bool {
+	fi, _ := os.Lstat(path)
+	return fi != nil && !date.After(fi.ModTime())
 }
 
 func getRemoteCacheSize(resp *http.Response) int64 {
@@ -145,6 +149,11 @@ func (c *CacheExtractorCommand) handleGoCloudURL() error {
 		return err
 	}
 
+	if isLocalCacheFileUpToDate(c.File, attrs.ModTime) {
+		logrus.Infoln(filepath.Base(c.File), "is up to date")
+		return nil
+	}
+
 	reader, err := b.NewReader(ctx, objectName, nil)
 	if err != nil {
 		return err

commands/helpers/cache_extractor_test.go

@@ -288,3 +288,25 @@ func TestCacheExtractorRemoteServerFailOnInvalidServer(t *testing.T) {
 	_, err := os.Stat(cacheExtractorTestArchivedFile)
 	assert.Error(t, err)
 }
+
+func TestIsLocalCacheFileUpToDate(t *testing.T) {
+	tmpDir := t.TempDir()
+	cacheFile := path.Join(tmpDir, "cache-file")
+
+	// Create cache file
+	err := os.WriteFile(cacheFile, []byte("test content"), 0644)
+	require.NoError(t, err)
+
+	// Set a specific modification time
+	modTime := time.Now()
+	err = os.Chtimes(cacheFile, modTime, modTime)
+	require.NoError(t, err)
+
+	// Test when remote file is older (cache is up to date)
+	result := isLocalCacheFileUpToDate(cacheFile, modTime.Add(-1*time.Hour))
+	require.True(t, result, "Cache should be up to date when remote file is older")
+
+	// Test when remote file is newer (cache is outdated)
+	result = isLocalCacheFileUpToDate(cacheFile, modTime.Add(1*time.Hour))
+	require.False(t, result, "Cache should be outdated when remote file is newer")
+}