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

> In the upcoming builtin rebase, we will have to start by detaching
> the HEAD, just like shell script version does. Essentially, we have
> to do the same thing as `git checkout -q <revision>^0 --`, in pure C.
>
> The aforementioned functionality was already present in `sequencer.c`
> in `do_reset()` function. But `do_reset()` performs more than detaching
> the HEAD, and performs action specific to `sequencer.c`.
>
> So this commit refactors out that part from `do_reset()`, and moves it
> to a new function called `detach_head_to()`. As this function has
> nothing to do with the sequencer, and everything to do with what `git
> checkout -q <revision>^0 --` does, we move that function to checkout.c.
>
> This refactoring actually introduces a slight change in behavior:
> previously, the index was locked before parsing the argument to the
> todo command `reset`, while it now gets locked *after* that, in the
> `detach_head_to()` function.
>
> It does not make a huge difference, and the upside is that this closes
> a few (unlikely) code paths where the index would not be unlocked upon
> error.
>
> Signed-off-by: Pratik Karki <predatoramigo@gmail.com>
> ---

>  checkout.c  | 64 +++++++++++++++++++++++++++++++++++++++++++++++++++++
>  checkout.h  |  3 +++
>  sequencer.c | 58 +++++-------------------------------------------
>  3 files changed, 72 insertions(+), 53 deletions(-)
>
> 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);
> +
> +	strbuf_release(&ref_name);
> +	return ret;
> +
> +}
> diff --git a/checkout.h b/checkout.h
> index 9980711179..6ce46cf4e8 100644
> --- a/checkout.h
> +++ b/checkout.h
> @@ -10,4 +10,7 @@
>   */
>  extern const char *unique_tracking_name(const char *name, struct object_id *oid);
>  
> +int detach_head_to(struct object_id *oid, const char *action,
> +		   const char *reflog_message);
> +
>  #endif /* CHECKOUT_H */
> diff --git a/sequencer.c b/sequencer.c
> index 5354d4d51e..106d518f4d 100644
> --- a/sequencer.c
> +++ b/sequencer.c
> @@ -29,6 +29,7 @@
>  #include "oidset.h"
>  #include "commit-slab.h"
>  #include "alias.h"
> +#include "checkout.h"
>  
>  #define GIT_REFLOG_ACTION "GIT_REFLOG_ACTION"
>  
> @@ -2756,14 +2757,7 @@ static int do_reset(const char *name, int len, struct replay_opts *opts)
>  {
>  	struct strbuf ref_name = STRBUF_INIT;
>  	struct object_id oid;
> -	struct lock_file lock = LOCK_INIT;
> -	struct tree_desc desc;
> -	struct tree *tree;
> -	struct unpack_trees_options unpack_tree_opts;
> -	int ret = 0, i;
> -
> -	if (hold_locked_index(&lock, LOCK_REPORT_ON_ERROR) < 0)
> -		return -1;
> +	int i;
>  
>  	if (len == 10 && !strncmp("[new root]", name, len)) {
>  		if (!opts->have_squash_onto) {
> @@ -2789,56 +2783,14 @@ static int do_reset(const char *name, int len, struct replay_opts *opts)
>  		if (get_oid(ref_name.buf, &oid) &&
>  		    get_oid(ref_name.buf + strlen("refs/rewritten/"), &oid)) {
>  			error(_("could not read '%s'"), ref_name.buf);
> -			rollback_lock_file(&lock);
>  			strbuf_release(&ref_name);
>  			return -1;
>  		}
>  	}
>  
> -	memset(&unpack_tree_opts, 0, sizeof(unpack_tree_opts));
> -	setup_unpack_trees_porcelain(&unpack_tree_opts, "reset");
> -	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_name(opts)));
> -	}
> -
> -	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(opts, "reset", "'%.*s'",
> -						len, name), "HEAD", &oid,
> -				 NULL, 0, UPDATE_REFS_MSG_ON_ERR);
> -
> -	strbuf_release(&ref_name);
> -	return ret;
> +	return detach_head_to(&oid, action_name(opts),
> +			      reflog_message(opts, "reset", "'%.*s'",
> +					     len, name));
>  }
>  
>  static int do_merge(struct commit *commit, const char *arg, int arg_len,