Re: [PATCH] glossary: describe "worktree"

[Elijah Newren <newren@gmail.com>]

On Thu, Feb 10, 2022 at 8:35 AM Junio C Hamano <gitster@pobox.com> wrote:
>
> Elijah Newren <newren@gmail.com> writes:
>
> >>    One thing that makes me worried somewhat is what I did not touch,
> >>    namely, how pseudo refs are defined.  I know MERGE_HEAD is very
> >>    special and it may be impossible to coax it into refs API for
> >>    writing, so the text there makes sense for it, but there are
> >>    other all-caps-and-directly-under-dot-git-directory files like
> >>    ORIG_HEAD and CHERRY_PICK_HEAD that are written using the refs
> >>    API, so the description would have to be updated there.
> >
> > I'm not quite following; why would the description need to be updated?
> >  Sure MERGE_HEAD is written without using the refs API, but we didn't
> > mention how the pseduorefs were written in the description, and all of
> > MERGE_HEAD, CHERRY_PICK_HEAD, ORIG_HEAD, REVERT_HEAD get written
> > per-worktree so doesn't "pseudorefs like MERGE_HEAD" cover it as far
> > as the reader is concerned?
>
> Here is how pseudo refs are defined.
>
> [[def_pseudoref]]pseudoref::
>         Pseudorefs are a class of files under `$GIT_DIR` which behave
>         like refs for the purposes of rev-parse, but which are treated
>         specially by git.  Pseudorefs both have names that are all-caps,
>         and always start with a line consisting of a
>         <<def_SHA1,SHA-1>> followed by whitespace.  So, HEAD is not a
>         pseudoref, because it is sometimes a symbolic ref.  They might
>         optionally contain some additional data.  `MERGE_HEAD` and
>         `CHERRY_PICK_HEAD` are examples.  Unlike
>         <<def_per_worktree_ref,per-worktree refs>>, these files cannot
>         be symbolic refs, and never have reflogs.  They also cannot be
>         updated through the normal ref update machinery.  Instead,
>         they are updated by directly writing to the files.  However,
>         they can be read as if they were refs, so `git rev-parse
>         MERGE_HEAD` will work.
>
> Points that may need to be looked at in the world where files
> backend is not the only ref backend are:

Ah, sorry, I assumed in "the description would have to be updated
there" you used "there" to refer to some part of your new patch text.
Re-reading, I can see you did specify the pseudoref section, but I
just somehow missed it.  Sorry about that.

>  - "are ... files under `$GIT_DIR`" may no longer be true, once some
>    of them are stored in reftable, for example.
>
>  - "followed by whitespace" may be an irrelevant detail for the
>    purpose of this paragraph.
>
>  - CHERRY_PICK_HEAD, as written in sequencer.c::do_pick_commit(),
>    use update_ref() to write a named file out, so "followed by
>    whitesspace" (and other cruft, like MERGE_HEAD does) certainly
>    does not apply.
>
>  - Also "cannot be updated through the normal ref update machinery"
>    is no longer true.  sequencer.c::do_pick_commit() even calls
>    update_ref() with REF_NO_DEREF to ensure "cannot be symbolic
>    refs".
>
>  - "never have reflogs" would make sense for the current set of
>    pseudorefs (does reflog on CHERRY_PICK_HEAD, for example, have
>    real use case?), but I do not know if it stays that way.  I do
>    not care too deeply either way, but I want to avoid over
>    specifying things.
>
> What worries me the most is that we cannot simply say "all-caps
> names that end with '_HEAD' all behave like refs except that they
> will not be symrefs without reflog." MERGE_HEAD is the only known
> exception if I am not mistaken, and I am OK to single it out as an
> oddball.  The current description however gives that there are a lot
> more differences _among_ pseudorefs.

Makes sense; thanks for clarifying for me.