refs: implement function to read references from file

Currently, we only provide functions to read references directly from a
repository's reference store via e.g. `git_reference_lookup`. But in
some cases, we may want to read files not connected to the current
repository, e.g. when looking up HEAD of connected work trees. This
commit implements `git_reference__read_head`, which will read out and
allocate a reference at an arbitrary path.

1 files changed, 34 insertions, 0 deletions

diff --git a/src/refs.c b/src/refs.c
index 0837dc4a8..a53d094fc 100644
--- a/src/refs.c
+++ b/src/refs.c
@@ -249,6 +249,40 @@ int git_reference_lookup_resolved(
 	return 0;
 }
 
+int git_reference__read_head(
+	git_reference **out,
+	git_repository *repo,
+	const char *path)
+{
+	git_buf reference = GIT_BUF_INIT;
+	char *name = NULL;
+	int error;
+
+	if ((error = git_futils_readbuffer(&reference, path)) < 0)
+		goto out;
+	git_buf_rtrim(&reference);
+
+	if (git__strncmp(reference.ptr, GIT_SYMREF, strlen(GIT_SYMREF)) == 0) {
+		git_buf_consume(&reference, reference.ptr + strlen(GIT_SYMREF));
+
+		name = git_path_basename(path);
+
+		if ((*out = git_reference__alloc_symbolic(name, reference.ptr)) == NULL) {
+			error = -1;
+			goto out;
+		}
+	} else {
+		if ((error = git_reference_lookup(out, repo, reference.ptr)) < 0)
+			goto out;
+	}
+
+out:
+	free(name);
+	git_buf_clear(&reference);
+
+	return error;
+}
+
 int git_reference_dwim(git_reference **out, git_repository *repo, const char *refname)
 {
 	int error = 0, i;