Improve error handling in git-rebase-update

git_rebase_update.py

 #!/usr/bin/env python
 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 """
 Tool to update all branches to have the latest changes from their upstreams.
 """
 
 import argparse
@@ skipping 112 matching lines @@
     back_ups += 1
     parent = git.run('rev-parse', parent+'~')
 
   if back_ups:
     logging.debug('Backed parent up by %d from %s to %s',
                   back_ups, orig_parent, parent)
 
   if git.hash_one(parent) != start_hash:
     # Try a plain rebase first
     print 'Rebasing:', branch
-    if not git.rebase(parent, start_hash, branch, abort=True).success:
+    rebase_ret = git.rebase(parent, start_hash, branch, abort=True)
+    if not rebase_ret.success:
       # TODO(iannucci): Find collapsible branches in a smarter way?
       print "Failed! Attempting to squash", branch, "...",
       squash_branch = branch+"_squash_attempt"
       git.run('checkout', '-b', squash_branch)
       git.squash_current_branch(merge_base=start_hash)
 
       # Try to rebase the branch_squash_attempt branch to see if it's empty.
       squash_ret = git.rebase(parent, start_hash, squash_branch, abort=True)
       empty_rebase = git.hash_one(squash_branch) == git.hash_one(parent)
       git.run('checkout', branch)
       git.run('branch', '-D', squash_branch)
       if squash_ret.success and empty_rebase:
         print 'Success!'
         git.squash_current_branch(merge_base=start_hash)
         git.rebase(parent, start_hash, branch)
       else:
         # rebase and leave in mid-rebase state.
-        git.rebase(parent, start_hash, branch)
+        # This second rebase attempt should always fail in the same
+        # way that the first one does.  If it magically succeeds then
+        # something very strange has happened.
+        second_rebase_ret = git.rebase(parent, start_hash, branch)
+        assert(not second_rebase_ret.success)

[iannucci, 2014/10/13 17:41:39]

I would actually throw an AssertionError with the text that you have in the
comment. Otherwise it just looks like the tool is brokened

         print "Failed!"
         print
-        print "Here's what git-rebase had to say:"
-        print squash_ret.message
+        print "Here's what git-rebase (squashed) had to say:"
         print
+        print squash_ret.stdout
+        print squash_ret.stderr
         print textwrap.dedent(
-        """
+        """\
         Squashing failed. You probably have a real merge conflict.
 
         Your working copy is in mid-rebase. Either:
          * completely resolve like a normal git-rebase; OR
          * abort the rebase and mark this branch as dormant:
                git config branch.%s.dormant true
 
         And then run `git rebase-update` again to resume.
         """ % branch)
         return False
@@ skipping 83 matching lines @@
           % (return_branch, root_branch)
         )
       git.run('checkout', root_branch)
     git.set_config(STARTING_BRANCH_KEY, '')
 
   return retcode
 
 
 if __name__ == '__main__':  # pragma: no cover
   sys.exit(main(sys.argv[1:]))