[PATCH] sequencer: don't re-read todo for revert and cherry-pick

When 'git revert' or 'git cherry-pick --edit' is invoked with multiple commits, then after editing the first commit message is finished both these commands should continue with processing the second commit and launch another editor for its commit message, assuming there are no conflicts, of course.

Alas, this inadvertently changed with commit a47ba3c777 (rebase -i: check for updated todo after squash and reword, 2019-08-19): after editing the first commit message is finished, both 'git revert' and 'git cherry-pick --edit' exit with error, claiming that "nothing to commit, working tree clean".

The reason for the changed behaviour is twofold:

  - Prior to a47ba3c777 the up-to-dateness of the todo list file was
    only checked after 'exec' instructions, and that commit moved
    those checks to the common code path.  The intention was that this
    check should be performed after instructions spawning an editor
    ('squash' and 'reword') as well, so the ongoing 'rebase -i'
    notices when the user runs a 'git rebase --edit-todo' while
    squashing/rewording a commit message.

    However, as it happened that check is now performed even after
    'revert' and 'pick' instructions when they involved editing the
    commit message.  And 'revert' by default while 'pick' optionally
    (with 'git cherry-pick --edit') involves editing the commit
    message.

  - When invoking 'git revert' or 'git cherry-pick --edit' with
    multiple commits they don't read a todo list file but assemble the
    todo list in memory, thus the associated stat data used to check
    whether the file has been updated is all zeroed out initially.

    Then the sequencer writes all instructions (including the very
    first) to the todo file, executes the first 'revert/pick'
    instruction, and after the user finished editing the commit
    message the changes of a47ba3c777 kick in, and it checks whether
    the todo file has been modified.  The initial all-zero stat data
    obviously differs from the todo file's current stat data, so the
    sequencer concludes that the file has been modified.  Technically
    it is not wrong, of course, because the file just has been written
    indeed by the sequencer itself, though the file's contents still
    match what the sequencer was invoked with in the beginning.
    Consequently, after re-reading the todo file the sequencer
    executes the same first instruction _again_, thus ending up in
    that "nothing to commit" situation.

The todo list was never meant to be edited during multi-commit 'git revert' or 'cherry-pick' operations, so perform that "has the todo file been modified" check only when the sequencer was invoked as part of an interactive rebase.

Reported-by: Brian Norris <briannorris@chromium.org>
Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
---

On Sat, Nov 23, 2019 at 09:53:51AM +0000, Phillip Wood wrote:

That missing condition was my hunch yesterday evening as well, and while it did fix my tests and didn't break anything else, it took some time to wrap my head around some of the subtleties that are going on in the sequencer. That's why the commit message ended up this long as it did.

In the end I decided to add the new tests to 't3429-rebase-edit-todo.sh', because, though these new tests don't actually check 'rebase', that is the one test script focusing on (re-)reading the todo file in particular.

 sequencer.c                 |  2 +-
 t/t3429-rebase-edit-todo.sh | 30 ++++++++++++++++++++++++++++++
 2 files changed, 31 insertions(+), 1 deletion(-)