[cmd/fetch_repo] make cache corruption failures more clear

cmd/fetch_repo/module.go

@@ -101,9 +101,8 @@ func fetchModule(dest, importpath, version, sum string) error {
 	if err := json.Unmarshal(buf.Bytes(), &dl); err != nil {
 		if bufErr.Len() > 0 {
 			return fmt.Errorf("%s %s: %s", cmd.Path, strings.Join(cmd.Args, " "), bufErr.Bytes())
-		} else {
-			return fmt.Errorf("%s %s: %v", cmd.Path, strings.Join(cmd.Args, " "), err)
 		}
+		return fmt.Errorf("%s %s: %v", cmd.Path, strings.Join(cmd.Args, " "), err)
 	}
 	if dl.Error != "" {
 		return errors.New(dl.Error)
@@ -116,18 +115,20 @@ func fetchModule(dest, importpath, version, sum string) error {
 	}
 
 	// Copy the module to the destination.
-	err = copyTree(dest, dl.Dir)
-	if err != nil {
+	if err := copyTree(dest, dl.Dir); err != nil {
 		return fmt.Errorf("failed copying repo: %w", err)
 	}
 
-	// Verify sum
+	// Verify sum of the directory itself against the go.sum.
 	repoSum, err := dirhash.HashDir(dest, importpath+"@"+version, dirhash.Hash1)
 	if err != nil {
 		return fmt.Errorf("failed computing sum: %w", err)
 	}
 
 	if repoSum != sum {
+		if goModCache := os.Getenv("GOMODCACHE"); modcacherw && goModCache != "" {
+			return fmt.Errorf("resulting module with sum %s; expected sum %s, Please try clearing your module cache directory %q", repoSum, sum, goModCache)
+		}
 		return fmt.Errorf("resulting module with sum %s; expected sum %s", repoSum, sum)
 	}