refs: introduce is_pseudoref() and is_headref()

Introduce two new functions `is_pseudoref()` and `is_headref()`. This
provides the necessary functionality for us to add pseudorefs and HEAD
to the loose ref cache in the files backend, allowing us to build
tooling to print these refs.

The `is_pseudoref()` function internally calls `is_pseudoref_syntax()`
but adds onto it by also checking to ensure that the pseudoref either
ends with a "_HEAD" suffix or matches a list of exceptions. After which
we also parse the contents of the pseudoref to ensure that it conforms
to the ref format.

We cannot directly add the new syntax checks to `is_pseudoref_syntax()`
because the function is also used by `is_current_worktree_ref()` and
making it stricter to match only known pseudorefs might have unintended
consequences due to files like 'BISECT_START' which isn't a pseudoref
but sometimes contains object ID.

Keeping this in mind, we leave `is_pseudoref_syntax()` as is and create
`is_pseudoref()` which is stricter. Ideally we'd want to move the new
syntax checks to `is_pseudoref_syntax()` but a prerequisite for this
would be to actually remove the exception list by converting those
pseudorefs to also contain a '_HEAD' suffix and perhaps move bisect
related files like 'BISECT_START' to a new directory similar to the
'rebase-merge' directory.

Helped-by: Jeff King <[email protected]>
Signed-off-by: Karthik Nayak <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: KarthikNayak

refs.c

@@ -860,6 +860,47 @@ static int is_pseudoref_syntax(const char *refname)
 	return 1;
 }
 
+int is_pseudoref(struct ref_store *refs, const char *refname)
+{
+	static const char *const irregular_pseudorefs[] = {
+		"AUTO_MERGE",
+		"BISECT_EXPECTED_REV",
+		"NOTES_MERGE_PARTIAL",
+		"NOTES_MERGE_REF",
+		"MERGE_AUTOSTASH",
+	};
+	struct object_id oid;
+	size_t i;
+
+	if (!is_pseudoref_syntax(refname))
+		return 0;
+
+	if (ends_with(refname, "_HEAD")) {
+		refs_resolve_ref_unsafe(refs, refname,
+					RESOLVE_REF_READING | RESOLVE_REF_NO_RECURSE,
+					&oid, NULL);
+		return !is_null_oid(&oid);
+	}
+
+	for (i = 0; i < ARRAY_SIZE(irregular_pseudorefs); i++)
+		if (!strcmp(refname, irregular_pseudorefs[i])) {
+			refs_resolve_ref_unsafe(refs, refname,
+						RESOLVE_REF_READING | RESOLVE_REF_NO_RECURSE,
+						&oid, NULL);
+			return !is_null_oid(&oid);
+		}
+
+	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_pseudoref_syntax(ref) || is_per_worktree_ref(ref);
 }

refs.h

@@ -1023,4 +1023,7 @@ extern struct ref_namespace_info ref_namespace[NAMESPACE__COUNT];
  */
 void update_ref_namespace(enum ref_namespace namespace, char *ref);
 
+int is_pseudoref(struct ref_store *refs, const char *refname);
+int is_headref(struct ref_store *refs, const char *refname);
+
 #endif /* REFS_H */