Gclient with git cache: delete conflicting mirror.

gclient_scm.py

Index: gclient_scm.py
diff --git a/gclient_scm.py b/gclient_scm.py
index 4e3ab8264a81dd9de4ea246c02b18014354331e2..12ef85401ffef807bc9532064859cee1fce5cc1f 100644
--- a/gclient_scm.py
+++ b/gclient_scm.py
@@ -461,8 +461,12 @@ class GitWrapper(SCMWrapper):
             self.checkout_path, '.git', 'objects', 'info', 'alternates'),
             'w') as fh:
           fh.write(os.path.join(url, 'objects'))
+      self._EnsureValidHeadObjectOrCheckout(revision, options, url)
       self._FetchAndReset(revision, file_list, options)
+
       return_early = True
+    else:
+      self._EnsureValidHeadObjectOrCheckout(revision, options, url)
 
     if return_early:
       return self._Capture(['rev-parse', '--verify', 'HEAD'])
@@ -1039,6 +1043,29 @@ class GitWrapper(SCMWrapper):
       raise gclient_utils.Error('git version %s < minimum required %s' %
                                 (current_version, min_version))
 
+  def _EnsureValidHeadObjectOrCheckout(self, revision, options, url):
+    # Special case handling if all 3 conditions are met:
+    #   * the mirros have recently changed, but deps destination remains same,
+    #   * the git histories of mirrors are conflicting.
+    #   * git cache is used
+    # This manifests itself in current checkout having invalid HEAD commit on
+    # most git operations. Since git cache is used, just deleted the .git
+    # folder, and re-create it by cloning.
+    try:
+      self._Capture(['rev-list', '-n', '1', 'HEAD'])
+    except subprocess2.CalledProcessError as e:
+      if ('fatal: bad object HEAD' in e.stderr

[Ryan Tseng, 2015/08/24 22:40:46]

This sort of error detection scares me :/.  Theres no way to know if it'll carry
over from version to verison.

though failing to run rev-list on HEAD should be signal enough that HEAD is
broken somehow?

[tandrii(chromium), 2015/08/25 08:00:52]

So, I thought this way:

best case -> it works
ok case -> the error changes, so our waterfall gets red. Oh well, not any worse
than now.
worst case -> the error is same, but for some other weird reason. Then we'll mv
.git to _bad_scm, even though we didn't have to. Still tolerable.

+          and self.cache_dir and self.cache_dir in url):
+        self.Print((
+          'Likely due to DEPS change with git cache_dir, '
+          'the current commit points to no longer existing object.\n'
+          '%s' % e)
+        )
+        self._DeleteOrMove(options.force)
+        self._Clone(revision, url, options)
+      else:
+        raise
+
   def _IsRebasing(self):
     # Check for any of REBASE-i/REBASE-m/REBASE/AM. Unfortunately git doesn't
     # have a plumbing command to determine whether a rebase is in progress, so