Stop adding repo entries for empty tags for local builds

DNF fails on Fedora 31 when there is a repo entry to an empty directory.

module_build_service/builder/MockModuleBuilder.py

@@ -373,18 +373,22 @@ class MockModuleBuilder(GenericBuilder):
                 repo_name = tag = source
                 koji_config = get_koji_config(self.config)
                 koji_session = koji.ClientSession(koji_config.server, opts=koji_config)
+                # Check to see if there are any external repos tied to the tag
+                for ext_repo in koji_session.getTagExternalRepos(tag):
+                    self._add_repo(ext_repo["external_repo_name"], ext_repo["url"])
+
                 repo = koji_session.getRepo(repo_name)
                 if repo:
                     baseurl = koji.PathInfo(topdir=koji_config.topurl).repo(repo["id"], repo_name)
                     baseurl = "{0}/{1}/".format(baseurl, self.arch)
                 else:
                     repo_dir = os.path.join(self.config.cache_dir, "koji_tags", tag)
-                    create_local_repo_from_koji_tag(
+                    should_add_repo = create_local_repo_from_koji_tag(
                         self.config, tag, repo_dir, [self.arch, "noarch"])
+                    if not should_add_repo:
+                        continue
                     baseurl = "file://" + repo_dir
-                # Check to see if there are any external repos tied to the tag
-                for ext_repo in koji_session.getTagExternalRepos(repo_name):
-                    self._add_repo(ext_repo["external_repo_name"], ext_repo["url"])
+
             self._add_repo(repo_name, baseurl)
         self._write_mock_config()
 

module_build_service/builder/utils.py

@@ -72,6 +72,8 @@ def create_local_repo_from_koji_tag(config, tag, repo_dir, archs=None):
     Downloads the packages build for one of `archs` (defaults to ['x86_64',
     'noarch']) in Koji tag `tag` to `repo_dir` and creates repository in that
     directory. Needs config.koji_profile and config.koji_config to be set.
+
+    If the there are no builds associated with the tag, False is returned.
     """
 
     # Placed here to avoid py2/py3 conflicts...
@@ -92,6 +94,10 @@ def create_local_repo_from_koji_tag(config, tag, repo_dir, archs=None):
     except koji.GenericError:
         log.exception("Failed to list rpms in tag %r" % tag)
 
+    if not builds:
+        log.debug("No builds are associated with the tag %r", tag)
+        return False
+
     # Reformat builds so they are dict with build_id as a key.
     builds = {build["build_id"]: build for build in builds}
 
@@ -162,3 +168,5 @@ def create_local_repo_from_koji_tag(config, tag, repo_dir, archs=None):
 
         log.info("Creating local repository in %s" % repo_dir)
         execute_cmd(["/usr/bin/createrepo_c", repo_dir])
+
+    return True