[FIX] runbot_merge: error in maintenance, and tracking

Before this, we would check if a repository had a name and run
maintenance on it, leading to repeated (but unnoticed until now
because I didn't monitor it) tracebacks as the maintenance cron would
fail to find the local repo then run maintenance on nowhere anyway.

Also augment the repo-finding process to try and get better
information about what it's doing when it fails, rather than failing
completely silently.

runbot_merge/git.py

@@ -4,6 +4,7 @@ import logging
 import os
 import pathlib
 import resource
+import stat
 import subprocess
 from typing import Optional, TypeVar, Union, Sequence, Tuple, Dict
 
@@ -41,6 +42,14 @@ def get_local(repository, prefix: Optional[str]) -> 'Optional[Repo]':
         repo.config('--add', 'remote.origin.fetch', '^refs/heads/tmp.*')
         repo.config('--add', 'remote.origin.fetch', '^refs/heads/staging.*')
         return repo
+    else:
+        _logger.warning(
+            "Unable to acquire %s: %s",
+            repo_dir,
+            "doesn't exist" if not repo_dir.exists()\
+        else oct(stat.S_IFMT(repo_dir.stat().st_mode))
+        )
+        return None
 
 
 ALWAYS = ('gc.auto=0', 'maintenance.auto=0')

runbot_merge/models/crons/git_maintenance.py

@@ -25,7 +25,7 @@ class GC(models.TransientModel):
         # run on all repos with a forwardport target (~ forwardport enabled)
         for repo in self.env['runbot_merge.repository'].search([]):
             repo_git = get_local(repo, prefix=None)
-            if not repo:
+            if not repo_git:
                 continue
 
             _gc.info('Running maintenance on %s', repo.name)