refs: make unborn-ness checking available outside of HEAD

src/refs.c

@@ -1430,3 +1430,28 @@ const char *git_reference_shorthand(const git_reference *ref)
 {
 	return git_reference__shorthand(ref->name);
 }
+
+int git_reference__resolve_unborn(git_reference **out_ref, git_repository *repo, const git_reference *ref)
+{
+	git_reference *tmp_ref = NULL;
+	int error;
+
+	if (ref->type == GIT_REF_OID) {
+		giterr_set(GITERR_REFERENCE, "direct references cannot be unborn");
+		return GIT_EINVALID;
+	}
+
+	if (out_ref == NULL) {
+		out_ref = &tmp_ref;
+	}
+
+	error = git_reference_lookup_resolved(out_ref, repo, git_reference_symbolic_target(ref), -1);
+
+	if (error == GIT_ENOTFOUND) {
+		error = GIT_EUNBORNBRANCH;
+	}
+
+	git_reference_free(tmp_ref);
+
+	return error;
+}

src/refs.h

@@ -136,4 +136,16 @@ int git_reference__update_for_commit(
 	const git_oid *id,
 	const char *operation);
 
+/**
+ * Lookup a reference, with unborn semantics.
+ *
+ * @param out_ref Pointer to the resolved reference. Can be null.
+ * @param repo The repository to use for resolution.
+ * @param ref The reference to resolve.
+ * @return 0 on success and out_ref (if provided) will be populated.
+ *         Otherwise, returns GIT_EINVALID if the reference is not symbolic,
+ *         GIT_EUNBORNBRANCH if the reference is unborn, or another error code.
+ */
+int git_reference__resolve_unborn(git_reference **out_ref, git_repository *repo, const git_reference *ref);
+
 #endif

src/repository.c

@@ -2119,6 +2119,8 @@ int git_repository_head(git_reference **head_out, git_repository *repo)
 	git_reference *head;
 	int error;
 
+	assert(head_out);
+
 	if ((error = git_reference_lookup(&head, repo, GIT_HEAD_FILE)) < 0)
 		return error;
 
@@ -2127,10 +2129,10 @@ int git_repository_head(git_reference **head_out, git_repository *repo)
 		return 0;
 	}
 
-	error = git_reference_lookup_resolved(head_out, repo, git_reference_symbolic_target(head), -1);
+	error = git_reference__resolve_unborn(head_out, repo, head);
 	git_reference_free(head);
 
-	return error == GIT_ENOTFOUND ? GIT_EUNBORNBRANCH : error;
+	return error;
 }
 
 int git_repository_head_for_worktree(git_reference **out, git_repository *repo, const char *name)
@@ -2150,7 +2152,7 @@ int git_repository_head_for_worktree(git_reference **out, git_repository *repo,
 	if (git_reference_type(head) != GIT_REF_OID) {
 		git_reference *resolved;
 
-		error = git_reference_lookup_resolved(&resolved, repo, git_reference_symbolic_target(head), -1);
+		error = git_reference__resolve_unborn(&resolved, repo, head);
 		git_reference_free(head);
 		head = resolved;
 	}