Re: [PATCH v2] sequencer: do not squash 'reword' commits when we hit conflicts

From: Johannes Schindelin <johannes.schindelin@gmx.de>
Date: Jun 17, 2018, 19:28 UTC
Message-ID: <nycvar.QRO.7.76.6.1806172048270.77@tvgsbejvaqbjf.bet>
In-Reply-To: <d62d834a-58dd-590a-cf71-028220323bf9@talktalk.net>

Hi Phillip,

On Sun, 17 Jun 2018, Phillip Wood wrote:

Show 40 quoted lines

> On 17/06/18 06:37, Elijah Newren wrote:
> > Ever since commit 18633e1a22 ("rebase -i: use the rebase--helper builtin",
> > 2017-02-09), when a commit marked as 'reword' in an interactive rebase
> > has conflicts and fails to apply, when the rebase is resumed that commit
> > will be squashed into its parent with its commit message taken.
> > 
> > The issue can be understood better by looking at commit 56dc3ab04b
> > ("sequencer (rebase -i): implement the 'edit' command", 2017-01-02), which
> > introduced error_with_patch() for the edit command.  For the edit command,
> > it needs to stop the rebase whether or not the patch applies cleanly.  If
> > the patch does apply cleanly, then when it resumes it knows it needs to
> > amend all changes into the previous commit.  If it does not apply cleanly,
> > then the changes should not be amended.  Thus, it passes !res (success of
> > applying the 'edit' commit) to error_with_patch() for the to_amend flag.
> > 
> > The problematic line of code actually came from commit 04efc8b57c
> > ("sequencer (rebase -i): implement the 'reword' command", 2017-01-02).
> > Note that to get to this point in the code:
> >    * !!res (i.e. patch application failed)
> >    * item->command < TODO_SQUASH
> >    * item->command != TODO_EDIT
> >    * !is_fixup(item->command) [i.e. not squash or fixup]
> > So that means this can only be a failed patch application that is either a
> > pick, revert, or reword.  For any of those cases we want a new commit, so
> > we should not set the to_amend flag.
> 
> Unfortunately I'm not sure it's that simple. Looking and do_pick() sometimes
> reword amends HEAD and sometimes it does not. In the "normal" case then the
> commit is picked and committed with '--edit'. However when fast-forwarding the
> code fast forwards to the commit to be reworded and then amends it. If the
> root commit is being reworded it takes the same code path. While these cases
> cannot fail with conflicts, it is possible for the user to cancel the commit
> or for them to fail due to collisions with untracked files.
> 
> If I remember correctly the shell version always picks the commit and then
> calls 'git commit --amend' afterwards which is less efficient but consistent.
> 
> I'm afraid I don't have a simple solution for fixing this, as currently
> pick_commits() does not know if the commit was called with AMEND_MSG, I guess
> that means adding some kind of flag for do_pick() to set.

Oh, you're right, the fast-forwarding path would pose a problem. I think there is an easy way to resolve this, though: in the case that we do want to amend the to-be-reworded commit, we simply have to see whether HEAD points to the very same commit mentioned in the `reword` command:

-- snip --

diff --git a/sequencer.c b/sequencer.c
index 2dad7041960..99d33d4e063 100644
--- a/sequencer.c
+++ b/sequencer.c
@@ -3691,10 +3691,22 @@ static int pick_commits(struct todo_list
*todo_list, struct replay_opts *opts)
                                        intend_to_amend();
                                return error_failed_squash(item->commit, opts,
                                        item->arg_len, item->arg);
-                       } else if (res && is_rebase_i(opts) && item->commit)
+                       } else if (res && is_rebase_i(opts) && item->commit) {
+                               int to_amend = 0;
+
+                               if (item->command == TODO_REWORD) {
+                                       struct object_id head;
+
+                                       if (!get_oid("HEAD", &head) &&
+					    !oidcmp(&item->commit->object.oid,
+                                                   &head))
+                                               to_amend = 1;
+                               }
+
                                return res | error_with_patch(item->commit,
                                        item->arg, item->arg_len, opts, res,
-                                       item->command == TODO_REWORD);
+                                       to_amend);
+                       }
                } else if (item->command == TODO_EXEC) {
                        char *end_of_arg = (char *)(item->arg + item->arg_len);
                        int saved = *end_of_arg;
-- snap --

Note that

- this patch is only compile-tested, and

- it is on top of my sequencer-shears branch thicket, so it might not
  apply cleanly to master, and

- it could probably use a comment what we are doing here (see whether we
  wanted to amend a fast-forwarded commit).

What do you think about this approach?
Dscho