refs: classify HEAD as a root ref

Root refs are those refs that live in the root of the ref hierarchy.
Our old and venerable "HEAD" reference falls into this category, but we
don't yet classify it as such in `is_root_ref()`.

Adapt the function to also treat "HEAD" as a root ref. This change is
safe to do for all current callers:

  - `ref_kind_from_refname()` already handles "HEAD" explicitly before
    calling `is_root_ref()`.

  - The "files" and "reftable" backends explicitly call both
    `is_root_ref()` and `is_headref()` together.

This also aligns behaviour or `is_root_ref()` and `is_headref()` such
that we stop checking for ref existence. This changes semantics for our
backends:

  - In the reftable backend we already know that the ref must exist
    because `is_headref()` is called as part of the ref iterator. The
    existence check is thus redundant, and the change is safe to do.

  - In the files backend we use it when populating root refs, where we
    would skip adding the "HEAD" file if it was not possible to resolve
    it. The new behaviour is to instead mark "HEAD" as broken, which
    will cause us to emit warnings in various places.

As there are no callers of `is_headref()` left afer the refactoring, we
can absorb it completely into `is_root_ref()`.

Signed-off-by: Patrick Steinhardt <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: pks-t

refs.c

@@ -859,6 +859,7 @@ static int is_root_ref_syntax(const char *refname)
 int is_root_ref(const char *refname)
 {
 	static const char *const irregular_root_refs[] = {
+		"HEAD",
 		"AUTO_MERGE",
 		"BISECT_EXPECTED_REV",
 		"NOTES_MERGE_PARTIAL",
@@ -880,14 +881,6 @@ int is_root_ref(const char *refname)
 	return 0;
 }
 
-int is_headref(struct ref_store *refs, const char *refname)
-{
-	if (!strcmp(refname, "HEAD"))
-		return refs_ref_exists(refs, refname);
-
-	return 0;
-}
-
 static int is_current_worktree_ref(const char *ref) {
 	return is_root_ref_syntax(ref) || is_per_worktree_ref(ref);
 }

refs.h

@@ -1060,7 +1060,8 @@ void update_ref_namespace(enum ref_namespace namespace, char *ref);
  *
  *   - Their name must be all-uppercase or underscores ("_").
  *
- *   - Their name must end with "_HEAD".
+ *   - Their name must end with "_HEAD". As a special rule, "HEAD" is a root
+ *     ref, as well.
  *
  *   - Their name may not contain a slash.
  *
@@ -1079,6 +1080,4 @@ void update_ref_namespace(enum ref_namespace namespace, char *ref);
  */
 int is_root_ref(const char *refname);
 
-int is_headref(struct ref_store *refs, const char *refname);
-
 #endif /* REFS_H */

refs/files-backend.c

@@ -351,8 +351,7 @@ static void add_pseudoref_and_head_entries(struct ref_store *ref_store,
 		strbuf_addstr(&refname, de->d_name);
 
 		dtype = get_dtype(de, &path, 1);
-		if (dtype == DT_REG && (is_root_ref(de->d_name) ||
-					is_headref(ref_store, de->d_name)))
+		if (dtype == DT_REG && is_root_ref(de->d_name))
 			loose_fill_ref_dir_regular_file(refs, refname.buf, dir);
 
 		strbuf_setlen(&refname, dirnamelen);

refs/reftable-backend.c

@@ -356,8 +356,7 @@ static int reftable_ref_iterator_advance(struct ref_iterator *ref_iterator)
 		 */
 		if (!starts_with(iter->ref.refname, "refs/") &&
 		    !(iter->flags & DO_FOR_EACH_INCLUDE_ROOT_REFS &&
-		     (is_root_ref(iter->ref.refname) ||
-		      is_headref(&iter->refs->base, iter->ref.refname)))) {
+		      is_root_ref(iter->ref.refname))) {
 			continue;
 		}