refs: do not check ref existence in `is_root_ref()`

Before this patch series, root refs except for "HEAD" and our special
refs were classified as pseudorefs. Furthermore, our terminology
clarified that pseudorefs must not be symbolic refs. This restriction
is enforced in `is_root_ref()`, which explicitly checks that a supposed
root ref resolves to an object ID without recursing.

This has been extremely confusing right from the start because (in old
terminology) a ref name may sometimes be a pseudoref and sometimes not
depending on whether it is a symbolic or regular ref. This behaviour
does not seem reasonable at all and I very much doubt that it results in
anything sane.

Last but not least, the current behaviour can actually lead to a
segfault when calling `is_root_ref()` with a reference that either does
not exist or that is a symbolic ref because we never initialized `oid`,
but then read it via `is_null_oid()`.

We have now changed terminology to clarify that pseudorefs are really
only "MERGE_HEAD" and "FETCH_HEAD", whereas all the other refs that live
in the root of the ref hierarchy are just plain refs. Thus, we do not
need to check whether the ref is symbolic or not. In fact, we can now
avoid looking up the ref completely as the name is sufficient for us to
figure out whether something would be a root ref or not.

This change of course changes semantics for our callers. As there are
only three of them we can assess each of them individually:

  - "ref-filter.c:ref_kind_from_refname()" uses it to classify refs.
    It's clear that the intent is to classify based on the ref name,
    only.

  - "refs/reftable_backend.c:reftable_ref_iterator_advance()" uses it to
    filter root refs. Again, using existence checks is pointless here as
    the iterator has just surfaced the ref, so we know it does exist.

  - "refs/files_backend.c:add_pseudoref_and_head_entries()" uses it to
    determine whether it should add a ref to the root directory of its
    iterator. This had the effect that we skipped over any files that
    are either a symbolic ref, or which are not a ref at all.

    The new behaviour is to include symbolic refs know, which aligns us
    with the adapted terminology. Furthermore, files which look like
    root refs but aren't are now mark those as "broken". As broken refs
    are not surfaced by our tooling, this should not lead to a change in
    user-visible behaviour, but may cause us to emit warnings. This
    feels like the right thing to do as we would otherwise just silently
    ignore corrupted root refs completely.

So in all cases the existence check was either superfluous, not in line
with the adapted terminology or masked potential issues. This commit
thus changes the behaviour as proposed and drops the existence check
altogether.

Add a test that verifies that this does not change user-visible
behaviour. Namely, we still don't want to show broken refs to the user
by default in git-for-each-ref(1). What this does allow though is for
internal callers to surface dangling root refs when they pass in the
`DO_FOR_EACH_INCLUDE_BROKEN` flag.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 3 insertions, 2 deletions

diff --git a/refs.h b/refs.h
index d0374c3275..8a574a22c7 100644
--- a/refs.h
+++ b/refs.h
@@ -1052,7 +1052,8 @@ extern struct ref_namespace_info ref_namespace[NAMESPACE__COUNT];
 void update_ref_namespace(enum ref_namespace namespace, char *ref);
 
 /*
- * Check whether the reference is an existing root reference.
+ * Check whether the provided name names a root reference. This function only
+ * performs a syntactic check.
  *
  * A root ref is a reference that lives in the root of the reference hierarchy.
  * These references must conform to special syntax:
@@ -1076,7 +1077,7 @@ void update_ref_namespace(enum ref_namespace namespace, char *ref);
  *
  *   - MERGE_AUTOSTASH
  */
-int is_root_ref(struct ref_store *refs, const char *refname);
+int is_root_ref(const char *refname);
 
 int is_headref(struct ref_store *refs, const char *refname);