Subject: Re: [PATCH 1/2] refs/reftable: don't fail empty transactions in repo without HEAD

From: Patrick Steinhardt <[email-masked]>
Date: Mon, 4 Mar 2024 07:44:10 +0100

On Wed, Feb 28, 2024 at 03:32:35AM -0800, Karthik Nayak wrote:
> Patrick Steinhardt <[email-masked]> writes:
> 
> > Under normal circumstances, it shouldn't ever happen that a repository
> > has no HEAD reference. In fact, git-update-ref(1) would fail any request
> > to delete the HEAD reference, and a newly initialized repository always
> > pre-creates it, too.
> >
> > But in the next commit, we are going to change git-clone(1) to partially
> > initialize the refdb just up to the point where remote helpers can find
> > the repository. With that change, we are going to run into a situation
> > where repositories have no refs at all.
> >
> > Now there is a very particular edge case in this situation: when
> > preparing an empty ref transacton, we end up returning whatever value
> > `read_ref_without_reload()` returned to the caller. Under normal
> > conditions this would be fine: "HEAD" should usually exist, and thus the
> > function would return `0`. But if "HEAD" doesn't exist, the function
> > returns a positive value which we end up returning to the caller.
> >
> > Fix this bug by resetting the return code to `0` and add a test.
> >
> > Signed-off-by: Patrick Steinhardt <[email-masked]>
> > ---
> >  refs/reftable-backend.c    |  1 +
> >  t/t0610-reftable-basics.sh | 13 +++++++++++++
> >  2 files changed, 14 insertions(+)
> >
> > diff --git a/refs/reftable-backend.c b/refs/reftable-backend.c
> > index a14f2ad7f4..45568818f0 100644
> > --- a/refs/reftable-backend.c
> > +++ b/refs/reftable-backend.c
> > @@ -821,6 +821,7 @@ static int reftable_be_transaction_prepare(struct ref_store *ref_store,
> >  				      &head_referent, &head_type);
> >  	if (ret < 0)
> >  		goto done;
> > +	ret = 0;
> >
> 
> So this is not really a problem in this function, it's more of that
> `refs.c:ref_transaction_prepare` checks if `ret` is non-zero.

Well, yes. I'd claim that it is a problem in this function because it
returns positive even though the transaction was prepared successfully.

> Nit: would be nice to have a comment about why overriding this value is
> ok.

True.

> >  	for (i = 0; i < transaction->nr; i++) {
> >  		struct ref_update *u = transaction->updates[i];
> > diff --git a/t/t0610-reftable-basics.sh b/t/t0610-reftable-basics.sh
> > index 6a131e40b8..c5f4d23433 100755
> > --- a/t/t0610-reftable-basics.sh
> > +++ b/t/t0610-reftable-basics.sh
> > @@ -328,6 +328,19 @@ test_expect_success 'ref transaction: writes are synced' '
> >  	EOF
> >  '
> >
> > +test_expect_success 'ref transaction: empty transaction in empty repo' '
> > +	test_when_finished "rm -rf repo" &&
> > +	git init repo &&
> > +	test_commit -C repo --no-tag A &&
> > +	COMMIT=$(git -C repo rev-parse HEAD) &&
> 
> why do we do this?

Oh, true, this isn't actually needed.

Patrick