Bug with "git mv" and submodules, and with "git submodule add something --force"

[Stefan Beller <sbeller@google.com>]

On Sun, Oct 21, 2018 at 7:52 PM Junio C Hamano <gitster@pobox.com> wrote:
>
> Jonathan Nieder <jrnieder@gmail.com> writes:
>
> > Stefan Beller wrote:
> >
> >> Maybe for now we can do with just an update of the documentation/bugs
> >> section and say we cannot move files in and out of submodules?
> >
> > I think we have some existing logic to prevent "git add"-ing a file
> > within a submodule to the superproject, for example.
>
> There is die_path_inside_submodule() that sanity-checks the pathspec
> and rejects.  But I think that is done primarily to give an error
> message and not strictly necesary for correctness.

c08397e3aa (pathspec: remove PATHSPEC_STRIP_SUBMODULE_SLASH_EXPENSIVE
flag, 2017-05-11) seems to be relevant here, as that factors out the
warning.

> The real safety of "git add" is its call to dir.c::fill_directory();
> it collects untracked paths that match the pathspec so that they can
> be added as new paths, but because it won't cross the module
> boundary, you won't get such a path in the index to begin with.

It would not cross the boundary and fail silently as it would
treat a path into the submodule as a no-op.

> > So "git mv" should learn the same trick.  And perhaps the trick needs
> > to be moved down a layer (e.g. into the index API).  Hints?

Yeah, I think we'd want to teach git-mv about that trick.

Unfortunately git-mv is one of the last remainders of not
properly using pathspecs, and die_path_inside_submodule
expects a pathspec. :-/

> You would want to be able to remove a submodule and replace it with
> a directory, but you can probably do it in two steps, i.e.
>
>         git reset --hard
>         git rm --cached sha1collisiondetection
>         echo Now a regular dir >sha1collisiondetection/READ.ME
>         find sha1collisiondetection ! -type d -print0 |
>         git update-index --add --stdin -z

    "Ignoring path sha1collisiondetection/.git"

Nice!

>
> So from that point of view, forbidding (starting from the same state
> of our project) this sequence:
>
>         git reset --hard
>         echo Now a regular dir >sha1collisiondetection/READ.ME
>         find sha1collisiondetection ! -type d -print0 |
>         git update-index --add --remove --stdin -z
>
> that would nuke the submodule and replace it with a directory within
> which there are files would be OK.  Making the latter's default
> rejection overridable with ADD_CACHE_OK_TO_REPLACE would also be
> fine.

I am having trouble of relating these commands to the original git-mv
across submodule boundaries.

Moving files from the submodule out to the superproject, seems
to fail properly, though having a less-than-optimal error message:

$ git mv sha1collisiondetection/LICENSE.txt .
fatal: not under version control, source=sha1collisiondetection/LICENSE.txt, destination=LICENSE.txt

and moving things inside was the original report, below is a proof of concept
that would yield

./git mv -v cache.h sha1collisiondetection/
fatal: moving across submodule boundaries not supported, source=cache.h, destination=sha1collisiondetection/cache.h

--8<--
Subject: [WIP/PATCH] builtin/mv.c: disallow moving across submodule boundaries

Signed-off-by: Stefan Beller <sbeller@google.com>
---
 builtin/mv.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/builtin/mv.c b/builtin/mv.c
index 80bb967a63..9ec4b2f0a3 100644
--- a/builtin/mv.c
+++ b/builtin/mv.c
@@ -172,7 +172,7 @@ int cmd_mv(int argc, const char **argv, const char *prefix)
 	/* Checking */
 	for (i = 0; i < argc; i++) {
 		const char *src = source[i], *dst = destination[i];
-		int length, src_is_dir;
+		int length, src_is_dir, pos;
 		const char *bad = NULL;
 
 		if (show_only)
@@ -243,6 +243,13 @@ int cmd_mv(int argc, const char **argv, const char *prefix)
 		else
 			string_list_insert(&src_for_dst, dst);
 
+		pos = cache_name_pos(dst, strlen(dst));
+		if (pos < 0) {
+			pos = -(pos + 1);
+			if (!S_ISGITLINK(active_cache[pos]->ce_mode))
+				bad = _("moving across submodule boundaries not supported");
+		}
+
 		if (!bad)
 			continue;
 		if (!ignore_errors)
-- 
2.19.0