Re: [PATCH v3 4/7] sequencer: treat error reading HEAD as unborn branch

[Junio C Hamano <gitster@pobox.com>]

Junio C Hamano <gitster@pobox.com> writes:

> It is not a good code hygiene to assume that a failure to read HEAD
> always means we are on an unborn branch, even if that is the most
> likely cause of the failure.  We may instead want to positively
> determine that we are on an unborn state, by seeing if the HEAD is a
> symbolic ref that points at a ref in refs/heads/* hierarchy, and
> that ref does not exist.

I suspect that you are almost there.

+	if (!resolve_ref_unsafe("HEAD", RESOLVE_REF_READING, &head_oid, NULL)) {
+		/*
+		 * Treat an error reading HEAD as an unborn branch.
+		 */

After we see this error, if we make a call to resolve_ref_unsafe()
with RESOLVE_REF_NO_RECURSE, the call should return the branch that
we are on but is not yet born, and &oid will get the null_oid.  I am
not sure if there is a way to combine the two calls into one, but
because the failure case (i.e. doing anything on an unborn branch)
is a rare case that happens only once before actually giving birth
to a new branch, it probably is not worth spending extra brain
cycles on it and just use a simple and stupid "when resolving fails,
see if we are in a rare salvageable case with extra code" approach.