Clean dnf cache before using it

When building a mix with a version of a package repo that contains
existing cache contents, dnf may resolve incorrect results.

For instance, Mixer does not version the local package repo, so when
incrementally building a mix with local content, dnf will continue to
re-use the same cache for the local repo. This was problematic when the
version of an rpm in the local repo was updated. In this case, the stale
cache metadata was used to incorrectly resolve files for the package.

Cleaning the dnf cache before using it prevents old cache metadata from
impacting dnf results.

Fixes #719

Signed-off-by: John Akre <john.w.akre@intel.com>

builder/bundles.go

@@ -801,11 +801,6 @@ var rpmMap map[string]bool
 var dnfDownloadDir string
 
 func buildFullChroot(b *Builder, set *bundleSet, packagerCmd []string, buildVersionDir, version string, downloadRetries int, numWorkers int) error {
-	fmt.Println("Cleaning DNF cache before full install")
-	if err := clearDNFCache(packagerCmd); err != nil {
-		return err
-	}
-
 	dnfDownloadDir = filepath.Join(buildVersionDir, "downloadedRpms")
 	err := os.MkdirAll(dnfDownloadDir, 0755)
 	if err != nil {
@@ -1052,6 +1047,12 @@ src=%s
 
 	fmt.Printf("Packager command-line: %s\n", strings.Join(packagerCmd, " "))
 
+	// Existing DNF cache content can cause incorrect queries with stale results
+	fmt.Println("Cleaning DNF cache")
+	if err := clearDNFCache(packagerCmd); err != nil {
+		return err
+	}
+
 	numWorkers := b.NumBundleWorkers
 	emptyDir, err := ioutil.TempDir("", "MixerEmptyDirForNoopInstall")
 	if err != nil {