Fix migration diffid atomic write

Fixes #20267

Signed-off-by: Tonis Tiigi <tonistiigi@gmail.com>

migrate/v1/migratev1.go

@@ -160,7 +160,12 @@ func calculateLayerChecksum(graphDir, id string, ls checksumCalculator) error {
 		return err
 	}
 
-	if err := ioutil.WriteFile(filepath.Join(graphDir, id, migrationDiffIDFileName), []byte(diffID), 0600); err != nil {
+	tmpFile := filepath.Join(graphDir, id, migrationDiffIDFileName+".tmp")
+	if err := ioutil.WriteFile(tmpFile, []byte(diffID), 0600); err != nil {
+		return err
+	}
+
+	if err := os.Rename(tmpFile, filepath.Join(graphDir, id, migrationDiffIDFileName)); err != nil {
 		return err
 	}
 
@@ -422,7 +427,11 @@ func migrateImage(id, root string, ls graphIDRegistrar, is image.Store, ms metad
 		history = parentImg.History
 	}
 
-	diffID, err := ioutil.ReadFile(filepath.Join(root, graphDirName, id, migrationDiffIDFileName))
+	diffIDData, err := ioutil.ReadFile(filepath.Join(root, graphDirName, id, migrationDiffIDFileName))
+	if err != nil {
+		return err
+	}
+	diffID, err := digest.ParseDigest(string(diffIDData))
 	if err != nil {
 		return err
 	}