Re: [PATCH v4 3/4] sequencer: refactor the code to detach HEAD to checkout.c

> diff --git a/checkout.c b/checkout.c
> index bdefc888ba..da68915fd7 100644
> --- a/checkout.c
> +++ b/checkout.c
> @@ -2,6 +2,11 @@
>  #include "remote.h"
>  #include "refspec.h"
>  #include "checkout.h"
> +#include "unpack-trees.h"
> +#include "lockfile.h"
> +#include "refs.h"
> +#include "tree.h"
> +#include "cache-tree.h"
>  
>  struct tracking_name_data {
>  	/* const */ char *src_ref;
> @@ -42,3 +47,62 @@ const char *unique_tracking_name(const char *name, struct object_id *oid)
>  	free(cb_data.dst_ref);
>  	return NULL;
>  }
> +
> +int detach_head_to(struct object_id *oid, const char *action,
> +		   const char *reflog_message)
> +{
> +	struct strbuf ref_name = STRBUF_INIT;
> +	struct tree_desc desc;
> +	struct lock_file lock = LOCK_INIT;
> +	struct unpack_trees_options unpack_tree_opts;
> +	struct tree *tree;
> +	int ret = 0;
> +
> +	if (hold_locked_index(&lock, LOCK_REPORT_ON_ERROR) < 0)
> +		return -1;
> +
> +	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_index;
> +	unpack_tree_opts.dst_index = &the_index;
> +	unpack_tree_opts.fn = oneway_merge;
> +	unpack_tree_opts.merge = 1;
> +	unpack_tree_opts.update = 1;
> +
> +	if (read_cache_unmerged()) {
> +		rollback_lock_file(&lock);
> +		strbuf_release(&ref_name);
> +		return error_resolve_conflict(_(action));
> +	}
> +
> +	if (!fill_tree_descriptor(&desc, oid)) {
> +		error(_("failed to find tree of %s"), oid_to_hex(oid));
> +		rollback_lock_file(&lock);
> +		free((void *)desc.buffer);
> +		strbuf_release(&ref_name);
> +		return -1;
> +	}
> +
> +	if (unpack_trees(1, &desc, &unpack_tree_opts)) {
> +		rollback_lock_file(&lock);
> +		free((void *)desc.buffer);
> +		strbuf_release(&ref_name);
> +		return -1;
> +	}
> +
> +	tree = parse_tree_indirect(oid);
> +	prime_cache_tree(&the_index, tree);
> +
> +	if (write_locked_index(&the_index, &lock, COMMIT_LOCK) < 0)
> +		ret = error(_("could not write index"));
> +	free((void *)desc.buffer);
> +
> +	if (!ret)
> +		ret = update_ref(reflog_message, "HEAD", oid,
> +				 NULL, 0, UPDATE_REFS_MSG_ON_ERR);