Re: unifying sequencer's options persisting, was Re: [PATCH v2] sequencer: fix edit handling for cherry-pick and revert messages

> Hi Dscho and Elijah
> 
> On 31/03/2021 14:48, Johannes Schindelin wrote:
>> Hi,
>>
>> On Tue, 30 Mar 2021, Elijah Newren wrote:
>>
>>> On Tue, Mar 30, 2021 at 3:13 AM Johannes Schindelin
>>> <Johannes.Schindelin@gmx.de> wrote:
>>>
>>>> I'll allow myself one tangent: the subject of the sequencer's Unix
>>>> shell script heritage seems to come up with an increasing frequency,
>>>> in particular the awful "let's write out one file per setting"
>>>> strategy.
>>>>
>>>> I would _love_ for `save_opts()` to write a JSON instead (or an INI
>>>> via the `git_config_*()` family of functions, as is done already by
>>>> the cherry-pick/revert stuff), now that we no longer have any shell
>>>> script backend (apart from `--preserve-merges`, but that one is on its
>>>> way out anyway).
>>>>
>>>> The one thing that concerns me with this idea is that I know for a
>>>> fact that some enterprisey users play games with those files inside
>>>> `<GIT_DIR>/rebase-merge` that should be considered internal
>>>> implementation details. Not sure how to deprecate that properly, I
>>>> don't think we have a sane way to detect whether users rely on these
>>>> implementation details other than breaking their expectations, which
>>>> is not really a gentle way to ask them to update their scripts.
> 
> I think it depends if users are just reading the files or writing to 
> them. If they are reading them (which my scripts do) then we could 
> continue to write the important files along side the new single-file 
> that git actually reads. I think there is a distinction between the 
> files such as git-rebase-todo which hold state and the one-line files 
> which hold the options passed by the user. For example I have scripts 
> that read git-rebase-todo, rewritten-pending, rewritten-list, amend-head 
> and check that author-script exists. If a script starts a rebase then it 
> should know what options are in effect without reading them from 
> .git/rebase-merge.
> 
>>> Ooh, I'm glad you took this tangent.  May I follow it for a second?
>>> I'd really, really like this too, for three reasons:
>>>
>>> 1) I constantly get confused about the massive duplication and
>>> difference in control structures and which ones affect which type of
>>> operations in sequencer.c.  Having them both use an ini-file would
>>> allow us to remove lots of that duplication.  I'm sure there'll still
>>> be some rebase-specific or cherry-pick-specific options, but we don't
>>> need a separate parallel structure for every piece of config.

>>> 2) In my merge-ort optimization work, rebasing 35 commits in linux.git
>>> across a massive set of 26K upstream renames has dropped rename
>>> detection time from the top spot.  And it also dropped several other
>>> things in the merge machinery from their spots as well.  Do you know
>>> what's the slowest now?  Wasted time from sequencer.c: the unnecessary
>>> process forking and all the useless disk writing (which I suspect is
>>> mostly updating the index and working directory but also writing all
>>> the individual control files under .git/rebase-merge/).  And this
>>> stuff from sequencer.c is not just barely the slowest part, it's the
>>> slowest by a wide margin.
> 
> I think we do a lot of needless writing which is unrelated to whether we 
> write to one file or may files. For example from memory picking a commit 
> involves writing the 'message', 'author-script', 'rewritten-pending' 
> (which is often immediately deleted), 'rewritten-list' (we append to 
> that one) 'CHERRY_PICK_HEAD' (which is immediately deleted unless the 
> pick has become empty), 'git-rebase-todo' is completely rewritten, and 
> 'done' is appended to. None of this is necessary. For rewording and 
> merges the only thing that needs to be written is the commit message for 
> the external process to use. Fixup and squash add a couple more files 
> into the mix.
> 
> I think we would save a lot by only syncing the state to disk when we 
> stop or run an exec command (the state needs to be synced so exec 
> commands can alter the todo list). In those cases we need to write the 
> index and possibly run an external process so writing a couple of files 
> is probably insignificant.
> 
> Where I think we can usefully consolidate is the one-line files which 
> store the options rather than state - these are read an written much 
> less frequently so I don't think they have much of a performance hit but 
> it would be much nicer to just serialize the options to a single file.
> 
>>>
>>> 3) I also want to allow cherry-picking or rebasing branches that
>>> aren't even checked out (assuming no conflicts are triggered;
>>> otherwise an error can be shown with the user asked to repeat with the
>>> operation connected to an active working directory).
> 
> Exciting!
> 
>>>  For such an
>>> operation, the difference between "cherry-pick" and "rebase" is nearly
>>> irrelevant so you'd expect the code to be the same; every time I look
>>> at the code, though, it seems that the control structures are
>>> separating these two types of operations in more areas than just the
>>> reading and writing of the config.
> 
> Yes this can be confusing, for example rebase and cherry-pick handle the 
> todo list differently. Rebase removes the command before trying to pick 
> the commit and adds it back if the pick fails for a non-conflict reason, 
> cherry-pick only removes the command if the pick is successful.
> 
> Best Wishes
> 
> Phillip
> 
>> Excellent, we're in agreement, then.
>>
>> The remaining question is: how do we want to go about it? Do we just want
>> to codify the notion that these are internal details that are nobody's
>> business, and if they are using the exact file system layout of the
>> current sequencer, then it's their responsibility to adapt?
>>
>> Ciao,
>> Dscho
>>
>