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

[Johan Herland <johan@herland.net>]

On Wed, Jul 29, 2015 at 6:37 PM, Junio C Hamano <gitster@pobox.com> wrote:
> 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.

Makes sense, although currently, IINM, those multiple $GIT_DIRs must
be associated with strictly different branches, which is completely
unrelated to the desired notes-merge restriction (which applies to
notes refs - not branches). But this has been discussed to death,
already.

>> 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?

I believe the answer to both questions is "No".

> If we do not change anything (not even applying the "[PATCH] notes: handle multiple worktrees" 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"?

Still, I believe the answer is "No".

> 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 "[PATCH] notes: handle multiple worktrees" patch.  We should just
> declare "do not do it, it does not (yet) work".
>
> But if there isn't, "[PATCH] notes: handle multiple worktrees" 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.

Sure. There's no point in delaying a patch that works well in practice
just because I have a delusion of a theoretically cleaner solution
that won't make any difference in practice.


...Johan

-- 
Johan Herland, <johan@herland.net>
www.herland.net