Re: [PATCH] RFC: A new type of symbolic refs

>A symbolic ref can currently only point at (another symbolic) ref.
> However are use cases for symbolic refs to point at other things
> representing a commit-ish, such as gitlink entries in other
> repositories.  In this use case we can use such a symbolic link
> to strengthen the relationship between a submodule and a superproject.
> Examples:

>
> 1) It makes it easier to explain when we recurse into submodules and
>   to which sha1 the submodule is updated.
>
>   Currently a submodule (or any repo) is either on a branch (i.e.
>   HEAD is a symbolic ref) or is in 'detached HEAD' state (HEAD is
>   a direct ref).
>   For submodules it is wrong to be on its own branch if it wants to be
>   controlled by the superproject as being on its own branch signals that
>   the submodule is independent and can move the HEAD freely.
>   Being in 'detached HEAD' state is the alternative to that and was
>   chosen when git-submodule was implemented, but it is equally wrong;
>   the lesser of two evils.
>
>   Semantically the correct way to state a submodule is part of the
>   superproject is by pointing its HEAD to the superproject.
>
>   We do have "reset/checkout --recurse-submodules" now, but it is
>   hard to explain what actually happens there (Which submodules are
>   updated? All of them! -- But I want a subset only!)
>
>   With this new mode of symbolic refs, any submodule that tracks the
>   superproject, would 'automatically follow' the superproject as the
>   submodules HEAD moves when the superproject changes.
>
> 2) "git -C submodule commit" would behave the same as it would on branch
>   nowadays: It would make the commit in the submodule and then change
>   the target of the symbolic ref which would be the index of the
>   superproject! That implies that you do not need to 'git add' the
>   submodule to the superproject, but have it done automatically.
>
> Signed-off-by: Stefan Beller <sbeller@google.com>
> ---
> cache.h              |  2 ++
> refs/files-backend.c | 10 ++++++++++
> submodule.c          | 40 ++++++++++++++++++++++++++++++++++++++++
> 3 files changed, 52 insertions(+)
>
> diff --git a/cache.h b/cache.h
> index 71fe092644..4f79d23202 100644
> --- a/cache.h
> +++ b/cache.h
> @@ -2029,4 +2029,6 @@ void sleep_millisec(int millisec);
>  */
> void safe_create_dir(const char *dir, int share);
>
> +extern int read_external_symref(struct strbuf *from, struct strbuf *out);
> +
> #endif /* CACHE_H */
> diff --git a/refs/files-backend.c b/refs/files-backend.c
> index 0404f2c233..f56f7b87ce 100644
> --- a/refs/files-backend.c
> +++ b/refs/files-backend.c
> @@ -713,6 +713,16 @@ static int files_read_raw_ref(struct ref_store 
> *ref_store,
>  goto out;
>  }
>
> + if (starts_with(buf, "repo:")) {
> + if (read_external_symref(&sb_contents, referent)) {
> + *type |= REF_ISBROKEN;
> + ret = -1;
> + goto out;
> + }
> + *type |= REF_ISSYMREF;
> + ret = 0;
> + }
> +
>  /*
>  * Please note that FETCH_HEAD has additional
>  * data after the sha.
> diff --git a/submodule.c b/submodule.c
> index da2b484879..7297f90485 100644
> --- a/submodule.c
> +++ b/submodule.c
> @@ -2037,3 +2037,43 @@ int submodule_to_gitdir(struct strbuf *buf, const 
> char *submodule)
> cleanup:
>  return ret;
> }
> +
> +int read_external_symref(struct strbuf *from, struct strbuf *out)
> +{
> + struct child_process cp = CHILD_PROCESS_INIT;
> + const char *repo, *gitlink;
> + int hint, code;
> + struct strbuf **split = strbuf_split(from, 0);
> + struct strbuf cmd_out = STRBUF_INIT;
> +
> + if (!split[0] || !split[1])
> + return -1;
> +
> + repo = split[0]->buf + 5; /* skip 'repo:' */
> + gitlink = split[1]->buf;
> +
> + argv_array_pushl(&cp.args,
> + "ignored-first-arg",
> + "-C", repo,
> + "ls-tree", "-z", "HEAD", "--", gitlink, NULL);
> +
> + /*
> + * 17 accounts for '160000 commit ',
> + * the \t before path and trailing \0.
> + */
> + hint = 17 + GIT_SHA1_HEXSZ + split[1]->len;
> + code = capture_command(&cp, &cmd_out, hint);
> +
> + strbuf_release(split[0]);
> + strbuf_release(split[1]);
> +
> + if (!code) {
> + strbuf_reset(out);
> + strbuf_add(out, cmd_out.buf + strlen("160000 commit "),
> +    GIT_SHA1_HEXSZ);
> + } else
> + return -1;
> +
> + return 0;
> +}
> +
> -- 
> 2.13.2.695.g117ddefdb4
>