Re: [PATCH 1/4] wt-status: read HEAD and ORIG_HEAD via the refdb

[Patrick Steinhardt <ps@pks.im>]

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On Wed, Nov 29, 2023 at 04:45:00PM -0500, Taylor Blau wrote:
> On Wed, Nov 29, 2023 at 09:14:12AM +0100, Patrick Steinhardt wrote:
> > We read both the HEAD and ORIG_HEAD references directly from the
> > filesystem in order to figure out whether we're currently splitting a
> > commit. If both of the following are true:
> >
> >   - HEAD points to the same object as "rebase-merge/amend".
> >
> >   - ORIG_HEAD points to the same object as "rebase-merge/orig-head".
> >
> > Then we are currently splitting commits.
> >
> > The current code only works by chance because we only have a single
> > reference backend implementation. Refactor it to instead read both refs
> > via the refdb layer so that we'll also be compatible with alternate
> > reference backends.
> >
> > Note that we pass `RESOLVE_REF_NO_RECURSE` to `read_ref_full()`. This is
> > because we didn't resolve symrefs before either, and in practice none of
> > the refs in "rebase-merge/" would be symbolic. We thus don't want to
> > resolve symrefs with the new code either to retain the old behaviour.
> >
> > Signed-off-by: Patrick Steinhardt <ps@pks.im>
> > ---
> >  wt-status.c | 17 +++++++++--------
> >  1 file changed, 9 insertions(+), 8 deletions(-)
> >
> > diff --git a/wt-status.c b/wt-status.c
> > index 9f45bf6949..fe9e590b80 100644
> > --- a/wt-status.c
> > +++ b/wt-status.c
> > @@ -1295,26 +1295,27 @@ static char *read_line_from_git_path(const char *filename)
> >  static int split_commit_in_progress(struct wt_status *s)
> >  {
> >  	int split_in_progress = 0;
> > -	char *head, *orig_head, *rebase_amend, *rebase_orig_head;
> > +	struct object_id head_oid, orig_head_oid;
> > +	char *rebase_amend, *rebase_orig_head;
> >
> >  	if ((!s->amend && !s->nowarn && !s->workdir_dirty) ||
> >  	    !s->branch || strcmp(s->branch, "HEAD"))
> >  		return 0;
> >
> > -	head = read_line_from_git_path("HEAD");
> > -	orig_head = read_line_from_git_path("ORIG_HEAD");
> > +	if (read_ref_full("HEAD", RESOLVE_REF_NO_RECURSE, &head_oid, NULL) ||
> > +	    read_ref_full("ORIG_HEAD", RESOLVE_REF_NO_RECURSE, &orig_head_oid, NULL))
> 
> Switching to read_ref_full() here is going to have some slightly
> different behavior than just reading out the contents of
> "$GIT_DIR/HEAD", but I think that it should be OK.
> 
> Before we would not have complained, if, for example, the contents of
> "$GIT_DIR/HEAD" were malformed, but now we will. I think that's OK,
> especially given that if that file is bogus, we'll have other problems
> before we get here ;-).
> 
> Are there any other gotchas that we should be thinking about?

Not that I can think of. As you say, a repository with malformed HEAD
will run into other problems anyway. And `read_ref_full()` would return
errors if these refs were malformed, which would cause us to exit early
from anyway. So unless "rebase-merge/amend" and "rebase-merge/orig-head"
contained the same kind of garbage we'd retain the same behaviour as
before, and that shouldn't really be happening.

One interesting bit is that we don't set `RESOLVE_REF_READING`, so
`read_ref_full()` may return successfully even if the ref doesn't exist.
But in practice this is fine given that the resulting oid would be
cleared in that case.

Patrick

> > +		return 0;
> > +
> >  	rebase_amend = read_line_from_git_path("rebase-merge/amend");
> >  	rebase_orig_head = read_line_from_git_path("rebase-merge/orig-head");
> >
> > -	if (!head || !orig_head || !rebase_amend || !rebase_orig_head)
> > +	if (!rebase_amend || !rebase_orig_head)
> >  		; /* fall through, no split in progress */
> >  	else if (!strcmp(rebase_amend, rebase_orig_head))
> > -		split_in_progress = !!strcmp(head, rebase_amend);
> > -	else if (strcmp(orig_head, rebase_orig_head))
> > +		split_in_progress = !!strcmp(oid_to_hex(&head_oid), rebase_amend);
> > +	else if (strcmp(oid_to_hex(&orig_head_oid), rebase_orig_head))
> 
> I did a double take at these strcmp(oid_to_hex(...)) calls, but I think
> that they are the best that we can do given that we're still reading the
> contents of "rebase-merge/amend" and "rebase-merge/orig-head" directly.
> 
> I suppose we could go the other way and turn their contents into
> object_ids and then use oidcmp(), but it doesn't seem worth it IMHO.
> 
> Thanks,
> Taylor

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