+#define GIT_REFLOG_ACTION_ENVIRONMENT "GIT_REFLOG_ACTION"
+
+#define RESET_HEAD_DETACH (1<<0)
+#define RESET_HEAD_HARD (1<<1)
+
+static int reset_head(struct object_id *oid, const char *action,
+ const char *switch_to_branch, unsigned flags,
+ const char *reflog_orig_head, const char *reflog_head)
+{
+ unsigned detach_head = flags & RESET_HEAD_DETACH;
+ unsigned reset_hard = flags & RESET_HEAD_HARD;
+ struct object_id head_oid;
+ struct tree_desc desc[2] = { { NULL }, { NULL } };
+ struct lock_file lock = LOCK_INIT;
+ struct unpack_trees_options unpack_tree_opts;
+ struct tree *tree;
+ const char *reflog_action;
+ struct strbuf msg = STRBUF_INIT;
+ size_t prefix_len;
+ struct object_id *orig = NULL, oid_orig,
+ *old_orig = NULL, oid_old_orig;
+ int ret = 0, nr = 0;
+
+ if (switch_to_branch && !starts_with(switch_to_branch, "refs/"))
+ BUG("Not a fully qualified branch: '%s'", switch_to_branch);
+
+ if (hold_locked_index(&lock, LOCK_REPORT_ON_ERROR) < 0) {
+ ret = -1;
+ goto leave_reset_head;
+ }
+
+ if ((!oid || !reset_hard) && get_oid("HEAD", &head_oid)) {
+ ret = error(_("could not determine HEAD revision"));
+ goto leave_reset_head;
+ }
+
+ if (!oid)
+ oid = &head_oid;
+
+ memset(&unpack_tree_opts, 0, sizeof(unpack_tree_opts));
+ setup_unpack_trees_porcelain(&unpack_tree_opts, action);
+ unpack_tree_opts.head_idx = 1;
+ unpack_tree_opts.src_index = the_repository->index;
+ unpack_tree_opts.dst_index = the_repository->index;
+ unpack_tree_opts.fn = reset_hard ? oneway_merge : twoway_merge;
+ unpack_tree_opts.update = 1;
+ unpack_tree_opts.merge = 1;
+ if (!detach_head)
+ unpack_tree_opts.reset = 1;
+
+ if (read_index_unmerged(the_repository->index) < 0) {
+ ret = error(_("could not read index"));
+ goto leave_reset_head;
+ }
+
+ if (!reset_hard && !fill_tree_descriptor(&desc[nr++], &head_oid)) {
+ ret = error(_("failed to find tree of %s"),
+ oid_to_hex(&head_oid));
+ goto leave_reset_head;
+ }
+
+ if (!fill_tree_descriptor(&desc[nr++], oid)) {
+ ret = error(_("failed to find tree of %s"), oid_to_hex(oid));
+ goto leave_reset_head;
+ }
+
+ if (unpack_trees(nr, desc, &unpack_tree_opts)) {
+ ret = -1;
+ goto leave_reset_head;
+ }
+
+ tree = parse_tree_indirect(oid);
+ prime_cache_tree(the_repository->index, tree);
+
+ if (write_locked_index(the_repository->index, &lock, COMMIT_LOCK) < 0) {
+ ret = error(_("could not write index"));
+ goto leave_reset_head;
+ }
+
+ reflog_action = getenv(GIT_REFLOG_ACTION_ENVIRONMENT);
+ strbuf_addf(&msg, "%s: ", reflog_action ? reflog_action : "rebase");
+ prefix_len = msg.len;
+
+ if (!get_oid("ORIG_HEAD", &oid_old_orig))
+ old_orig = &oid_old_orig;
+ if (!get_oid("HEAD", &oid_orig)) {
+ orig = &oid_orig;
+ if (!reflog_orig_head) {
+ strbuf_addstr(&msg, "updating ORIG_HEAD");
+ reflog_orig_head = msg.buf;
+ }
+ update_ref(reflog_orig_head, "ORIG_HEAD", orig, old_orig, 0,
+ UPDATE_REFS_MSG_ON_ERR);
+ } else if (old_orig)
+ delete_ref(NULL, "ORIG_HEAD", old_orig, 0);
+ if (!reflog_head) {
+ strbuf_setlen(&msg, prefix_len);
+ strbuf_addstr(&msg, "updating HEAD");
+ reflog_head = msg.buf;
+ }
+ if (!switch_to_branch)
+ ret = update_ref(reflog_head, "HEAD", oid, orig,
+ detach_head ? REF_NO_DEREF : 0,
+ UPDATE_REFS_MSG_ON_ERR);
+ else {
+ ret = create_symref("HEAD", switch_to_branch, msg.buf);
+ if (!ret)
+ ret = update_ref(reflog_head, "HEAD", oid, NULL, 0,
+ UPDATE_REFS_MSG_ON_ERR);
+ }
+
+leave_reset_head:
+ strbuf_release(&msg);
+ rollback_lock_file(&lock);
+ while (nr)
+ free((void *)desc[--nr].buffer);
+ return ret;
+}
+