shadow-repos: check if path to shadow repo existing before trying to call SCM object.

- sometimes cleanups of shadow repos can occur. We should fail only if shadow repo path
is there, and we can create a repo object from it.

rhodecode/apps/repository/views/repo_pull_requests.py

@@ -433,12 +433,14 b' class RepoPullRequestsView(RepoAppView, '
         source_scm = source_repo.scm_instance()
         target_scm = target_repo.scm_instance()
 
-        # try first shadow repo, fallback to regular repo
+        shadow_scm = None
         try:
-            commits_source_repo = pull_request_latest.get_shadow_repo()
+            shadow_scm = pull_request_latest.get_shadow_repo()
         except Exception:
             log.debug('Failed to get shadow repo', exc_info=True)
-            commits_source_repo = source_scm
+        # try first the existing source_repo, and then shadow
+        # repo if we can obtain one
+        commits_source_repo = source_scm or shadow_scm
 
         c.commits_source_repo = commits_source_repo
         c.ancestor = None  # set it to None, to hide it from PR view

rhodecode/model/db.py

@@ -3763,6 +3763,7 b' class PullRequest(Base, _PullRequestBase'
         vcs_obj = self.target_repo.scm_instance()
         shadow_repository_path = vcs_obj._get_shadow_repository_path(
             workspace_id)
+        if os.path.isdir(shadow_repository_path):
         return vcs_obj._get_shadow_instance(shadow_repository_path)