Re: [PATCH v4 20/21] refs: add LMDB refs storage backend

[David Turner <dturner@twopensource.com>]

On Mon, 2016-02-15 at 16:57 +0700, Duy Nguyen wrote:
> On Sun, Feb 14, 2016 at 7:04 PM, Duy Nguyen <pclouds@gmail.com>
> wrote:
> > On Sat, Feb 6, 2016 at 2:44 AM, David Turner <
> > dturner@twopensource.com> wrote:
> > > +static char *get_refdb_path(const char *base)
> > > +{
> > > +       static struct strbuf path_buf = STRBUF_INIT;
> > > +       strbuf_reset(&path_buf);
> > > +       strbuf_addf(&path_buf, "%s/refdb", base);
> > > +       return path_buf.buf;
> > > +}
> > ...
> > > +static int lmdb_init_db(struct strbuf *err, int shared)
> > > +{
> > > +       /*
> > > +        * To create a db, all we need to do is make a directory
> > > for
> > > +        * it to live in; lmdb will do the rest.
> > > +        */
> > > +
> > > +       if (!db_path)
> > > +               db_path =
> > > xstrdup(real_path(get_refdb_path(get_git_common_dir())));
> > 
> > This works for multiple worktrees. But scripts may have harder time
> > getting the path. The recommended way is "git rev-parse --git-path
> > refdb" but because "refdb" is not registered in
> > path.c:common_list[],
> > that command becomes git_path("refdb") instead of
> > get_refdb(get_git_... like here. And I will need to know that
> > .git/refdb is _not_ per-worktree when I migrate/convert main
> > worktree
> > (it's very likely I have to go that route to solve .git/config
> > issue
> > in multi worktree).
> > 
> > The solution is register refdb to common_list[] and you can do
> > git_path("refdb") here. But then what happens when another backend
> > is
> > added? Will the new backend use the same path "refdb", or say
> > "refdb.sqlite"? If all backends share the name "refdb", why can't
> > we
> > just reuse "refs" instead because the default filesystem-based
> > backend
> > is technically just another backend?
> 
> To answer myself: I forgot that there were per-worktree refs (e.g.
> refs/bisect). It makes me wonder if we should put per-worktree refs
> to
> lmdb as well (maybe one per worktree if we don't want to put all in
> one db). One of the advantages of moving away from fs-based backend
> is
> the ability to deal with case sensitivity, "nested" refs (e.g. a/b
> and
> a/b/c are both refs).

I don't think case-sensitivity is a huge problem here: per-worktree
refs are now HEAD or refs/bisect/*, and refs/bisect is controlled
entirely by the bisection machinery (that is, it won't have user
-defined refs with case conflicts).

>  With this split, I think some refs are still
> left behind.. Sorry if this was discussed before, I haven't followed
> this closely.

Previous discussion:
http://comments.gmane.org/gmane.comp.version-control.git/275097