FW: Bug Report: changes to submodule's files can be silently lost forever

Hi Tim!

Le 2021-01-21 à 08:48, Tim Yorke a écrit :

What did you do before the bug happened? (Steps to reproduce your issue)

- I used the command "git config submodule.recurse true" in my top-level repository
- made a change to a file in a submodule (located within my top-level repository)
- performed a git checkout in the top-level repository to a commit that didn't have the submodule
- performed a git checkout in the top-level repository back to a commit that does have the submodule

What did you expect to happen? (Expected behavior)

When attempting the first checkout (i.e. to a commit without the submodule), I'd expect an warning telling me that the submodule was not up-to-date (uncommited changes). 

That's indeed very sensible.

What happened instead? (Actual behavior)

The changes to the file in the submodule were lost forever without any warning

What's different between what you expected and what actually happened?

My uncommitted changes were silently lost forever, whereas I'd expect either 
- to be warned before I could continue or
- the uncommited changes to be retained.

Thanks for the report. This is a known problem ([1], [2], [3]),
and I'm working on fixing it.

Just to be sure, the changes you lost were to a tracked file in
the submodule, right ? It was not a new, untracked file ?

I'm not quite ready yet to submit my fixes, but if you want to compile
Git from source in the meantime, the heart of the fix is this diff:

diff --git a/unpack-trees.c b/unpack-trees.c index 323280dd48..a3e3d98de1 100644
--- a/unpack-trees.c
+++ b/unpack-trees.c

@@ -1872,7 +1872,7 @@ static int verify_uptodate_1(const struct cache_entry *ce,
   
   		if (submodule_from_ce(ce)) {
   			int r = check_submodule_move_head(ce,
-				"HEAD", oid_to_hex(&ce->oid), o);
+				"HEAD", empty_tree_oid_hex(), o);
   			if (r)
   				return add_rejected_path(o, error_type, ce->name);
   			return 0;

This should prevent Git from switching branches if any tracked files are modified
in the submodule.

Cheers,

Philippe.

[1] https://lore.kernel.org/git/570e77a07f0b4d4ea09307e5fa819d6f@fiveco.ch/t/#u
[2] https://lore.kernel.org/git/20200525094019.22padbzuk7ukr5uv@overdrive.tratt.net/
[3] https://lore.kernel.org/git/CAHsG2VT4YB_nf8PrEmrHwK-iY-AQo0VDcvXGVsf8cEYXws4nig@mail.gmail.com/