Re: [PATCH 12/17] update submodules: add submodule_move_head

> In later patches we introduce the options and flag for commands
> that modify the working directory, e.g. git-checkout.
> 
> This piece of code will be used universally for
> all these working tree modifications as it
> * supports dry run to answer the question:
>   "Is it safe to change the submodule to this new state?"
>   e.g. is it overwriting untracked files or are there local
>   changes that would be overwritten?
> * supports a force flag that can be used for resetting
>   the tree.
> 
> Signed-off-by: Stefan Beller <sbeller@google.com>
> ---
>  submodule.c | 135 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
>  submodule.h |   7 ++++
>  2 files changed, 142 insertions(+)
> 
> diff --git a/submodule.c b/submodule.c
> index 0b2596e88a..bc5fecf8c5 100644
> --- a/submodule.c
> +++ b/submodule.c
> @@ -1239,6 +1239,141 @@ int bad_to_remove_submodule(const char *path, unsigned flags)
>  	return ret;
>  }
>  
> +static int submodule_has_dirty_index(const struct submodule *sub)
> +{
> +	struct child_process cp = CHILD_PROCESS_INIT;
> +
> +	prepare_submodule_repo_env_no_git_dir(&cp.env_array);
> +
> +	cp.git_cmd = 1;
> +	argv_array_pushl(&cp.args, "diff-index", "--quiet", \
> +					"--cached", "HEAD", NULL);

> +	cp.no_stdin = 1;
> +	cp.no_stdout = 1;
> +	cp.dir = sub->path;
> +	if (start_command(&cp))
> +		die("could not recurse into submodule '%s'", sub->path);
> +
> +	return finish_command(&cp);
> +}
> +
> +static void submodule_reset_index(const char *path)
> +{
> +	struct child_process cp = CHILD_PROCESS_INIT;
> +	prepare_submodule_repo_env_no_git_dir(&cp.env_array);
> +
> +	cp.git_cmd = 1;
> +	cp.no_stdin = 1;
> +	cp.dir = path;
> +
> +	argv_array_pushf(&cp.args, "--super-prefix=%s/", path);
> +	argv_array_pushl(&cp.args, "read-tree", "-u", "--reset", NULL);
> +
> +	argv_array_push(&cp.args, EMPTY_TREE_SHA1_HEX);
> +
> +	if (run_command(&cp))
> +		die("could not reset submodule index");
> +}
> +
> +/**
> + * Moves a submodule at a given path from a given head to another new head.
> + * For edge cases (a submodule coming into existence or removing a submodule)
> + * pass NULL for old or new respectively.
> + */
> +int submodule_move_head(const char *path,
> +			 const char *old,
> +			 const char *new,
> +			 unsigned flags)
> +{
> +	int ret = 0;
> +	struct child_process cp = CHILD_PROCESS_INIT;
> +	const struct submodule *sub;
> +
> +	sub = submodule_from_path(null_sha1, path);
> +
> +	if (!sub)
> +		die("BUG: could not get submodule information for '%s'", path);
> +
> +	if (old && !(flags & SUBMODULE_MOVE_HEAD_FORCE)) {
> +		/* Check if the submodule has a dirty index. */
> +		if (submodule_has_dirty_index(sub))
> +			return error(_("submodule '%s' has dirty index"), path);
> +	}
> +
> +	if (!(flags & SUBMODULE_MOVE_HEAD_DRY_RUN)) {
> +		if (old) {
> +			if (!submodule_uses_gitfile(path))
> +				absorb_git_dir_into_superproject("", path,
> +					ABSORB_GITDIR_RECURSE_SUBMODULES);
> +		} else {
> +			struct strbuf sb = STRBUF_INIT;
> +			strbuf_addf(&sb, "%s/modules/%s",
> +				    get_git_common_dir(), sub->name);
> +			connect_work_tree_and_git_dir(path, sb.buf);
> +			strbuf_release(&sb);
> +
> +			/* make sure the index is clean as well */
> +			submodule_reset_index(path);
> +		}
> +	}
> +
> +	prepare_submodule_repo_env_no_git_dir(&cp.env_array);
> +
> +	cp.git_cmd = 1;
> +	cp.no_stdin = 1;
> +	cp.dir = path;
> +
> +	argv_array_pushf(&cp.args, "--super-prefix=%s/", path);
> +	argv_array_pushl(&cp.args, "read-tree", NULL);
> +
> +	if (flags & SUBMODULE_MOVE_HEAD_DRY_RUN)
> +		argv_array_push(&cp.args, "-n");
> +	else
> +		argv_array_push(&cp.args, "-u");
> +
> +	if (flags & SUBMODULE_MOVE_HEAD_FORCE)
> +		argv_array_push(&cp.args, "--reset");
> +	else
> +		argv_array_push(&cp.args, "-m");
> +
> +	argv_array_push(&cp.args, old ? old : EMPTY_TREE_SHA1_HEX);
> +	argv_array_push(&cp.args, new ? new : EMPTY_TREE_SHA1_HEX);
> +
> +	if (run_command(&cp)) {
> +		ret = -1;
> +		goto out;
> +	}
> +
> +	if (!(flags & SUBMODULE_MOVE_HEAD_DRY_RUN)) {
> +		if (new) {
> +			struct child_process cp1 = CHILD_PROCESS_INIT;
> +			/* also set the HEAD accordingly */
> +			cp1.git_cmd = 1;
> +			cp1.no_stdin = 1;
> +			cp1.dir = path;
> +
> +			argv_array_pushl(&cp1.args, "update-ref", "HEAD",
> +					 new ? new : EMPTY_TREE_SHA1_HEX, NULL);
> +
> +			if (run_command(&cp1)) {
> +				ret = -1;
> +				goto out;
> +			}
> +		} else {
> +			struct strbuf sb = STRBUF_INIT;
> +
> +			strbuf_addf(&sb, "%s/.git", path);
> +			unlink_or_warn(sb.buf);
> +			strbuf_release(&sb);
> +
> +			if (is_empty_dir(path))
> +				rmdir_or_warn(path);
> +		}
> +	}
> +out:
> +	return ret;
> +}
> +
>  static int find_first_merges(struct object_array *result, const char *path,
>  		struct commit *a, struct commit *b)
>  {
> diff --git a/submodule.h b/submodule.h
> index 6f3fe85c7c..4cdf6445f7 100644
> --- a/submodule.h
> +++ b/submodule.h
> @@ -96,6 +96,13 @@ extern int push_unpushed_submodules(struct sha1_array *commits,
>  extern void connect_work_tree_and_git_dir(const char *work_tree, const char *git_dir);
>  extern int parallel_submodules(void);
>  
> +#define SUBMODULE_MOVE_HEAD_DRY_RUN (1<<0)
> +#define SUBMODULE_MOVE_HEAD_FORCE   (1<<1)
> +extern int submodule_move_head(const char *path,
> +			       const char *old,
> +			       const char *new,
> +			       unsigned flags);
> +
>  /*
>   * Prepare the "env_array" parameter of a "struct child_process" for executing
>   * a submodule by clearing any repo-specific envirionment variables, but
> -- 
> 2.12.0.rc1.45.g207f5fbb2b
>