Re: [PATCH v3 2/6] notes: replace pseudorefs with real refs

[Junio C Hamano <gitster@pobox.com>]

Johan Herland <johan@herland.net> writes:

> On Wed, Jul 29, 2015 at 7:01 AM, Junio C Hamano <gitster@pobox.com> wrote:
>> Johan Herland <johan@herland.net> writes:
>>
>>> I believe it is a bad compromise. It complicates the code, and it
>>> provides a concurrent notes merges that is unnecessarily tied to (and
>>> dependent on) worktrees. For example, if I wanted to perform N
>>> concurrent notes merges in a project that happens to have a huge
>>> worktree, I would now have to create N copies of the huge worktree...
>>
>> Who said worktree has to be populated?  You should be able to have
>> an absolutely empty checkout just like "clone --no-checkout".
>
> IMHO that's an insane workaround that only serves to highlight the
> conceptual problems of binding notes merges (as they are implemented
> today) to worktrees.

Actually, the name "linked worktree" is probably a misnomer.

There is nothing fundamental in the mechanism or in the concept that
says that these multiple $GIT_DIR's must not be a bare one.  The
main thing the separation between $GIT_DIR and $GIT_COMMON_DIR
affords you is that you can have some things shared across them
(e.g. refs/*, objects) while making others per $GIT_DIR (e.g. HEAD,
index, etc.).

With that in mind, it is not an insane workaround but a very natural
mechanism suited exactly for what you want to do: using a feature
(e.g. "notes merge") that currently can have at most one instance
running at a time because it stores its state inside $GIT_DIR, and
you want to have N concurrent one going.  You keep that "state per
running instance" inside $GIT_DIR (i.e. not shared) and use the
"linked worktree" mechanism to have multiple $GIT_DIR, connected
to the same $GIT_COMMON_DIR.

> But, whatever. This is unrelated to David's current effort, and I
> don't want to hold that up, so please move along, nothing to see here.

I need this part from an earlier message answered to unblock David's
topic:

    Now we are getting somewhere.  So is there more that is needed
    than separating NOTES_MERGE_REF per worktree to make this work
    (remember, multiple notes-merge in a single worktree is a
    non-goal, just like multiple merge in a single worktree is not
    supported today and will not be)?  Is there some other state
    that is not captured by NOTES_MERGE_REF and friends that you
    would end up recording a wrong merge result, if two worktrees
    that have NOTES_MERGE_REF pointing at a different ref in
    refs/notes/* try to do the notes-merge at the same time?

If we do not change anything (not even applying the [v3 2/6] patch
we are discussing), all these things prefixed with NOTES_ will
become per $GIT_DIR with linked worktrees.

    NOTES_EDITMSG, NOTES_MERGE_REF, NOTES_MERGE_PARTIAL,
    NOTES_MERGE_WORKTREE

The user could attempt to start different notes merges in her
multiple $GIT_DIRs.  The question is to what degree we want to
support her.

Is it sufficient to have these per $GIT_DIR, when the user has two
$GIT_DIRs connected to the same repository and wants to do two
"notes merge" acting on different ref in refs/notes/*?  Or are there
some other states in the shared part kept, which would be stomped on
by simultaneously running "notes merge" instances in different
$GIT_DIRs, that make this not to work?  Any other problems in the
remainder of the current implementation of "notes merge"?

If there are reasons/limitations that make simultaneous "notes
merge" of different notes in different $GIT_DIRs impossible, then I
agree we shouldn't bother with [v3 2/6] patch.  We should just
declare "do not do it, it does not (yet) work".

But if there isn't, [v3 2/6] is the absolute minimum thing we could
do to make "notes merge" usable by making sure that the user does
not attempt merging the same refs/notes/commits in two different
places.