[BUG] Silent data loss on merge with uncommited changes + renames

[BUG] Silent data loss on merge with uncommited changes + renames

[Matthieu Moy]

Hi,

Following the discussion on "merge with uncommited changes" inside the
"git pull --autostash" thread, I did a bit of testing, and encountered a
case with silent data loss. In short: merge a branch introducing changes
to a file. If the file has been renamed in the current branch, then "git
merge" follows the rename and brings changes to the renamed file, but
uncommited changes in this file are overriden silently.

I could have expected "git merge --abort" to fail, but the problem is
really more serious here: data loss is done silently before giving me an
opportunity to do or abort anything.

Reproduction script below:

#! /bin/sh

# Create repo
git init git.$$
cd git.$$
echo init > test.txt
git add test.txt
git commit -m init

# Make a branch changing test.txt
git checkout -b branch
echo new > test.txt
git commit -am new

# Move test.txt on master
git checkout master
git mv test.txt moved.txt
git commit -m move

# Make uncommited changes to moved.txt
echo precious > moved.txt

# Merge loses uncommited content "precious" in "moved.txt" silently
git merge --no-edit branch
ls # lists just moved.txt
git status # nothing to commit, working directory clean
cat moved.txt # Says "new".

-- 
Matthieu Moy
http://www-verimag.imag.fr/~moy/

Re: [BUG] Silent data loss on merge with uncommited changes + renames

[Johannes Sixt]

Am 4/22/2013 11:24, schrieb Matthieu Moy:
> Following the discussion on "merge with uncommited changes" inside the
> "git pull --autostash" thread, I did a bit of testing, and encountered a
> case with silent data loss. In short: merge a branch introducing changes
> to a file. If the file has been renamed in the current branch, then "git
> merge" follows the rename and brings changes to the renamed file, but
> uncommited changes in this file are overriden silently.

Can you check whether your case is already covered by one of:

  git grep expect_failure t/*merge*

and if not, contribute a test case?

-- Hannes

Re: [BUG] Silent data loss on merge with uncommited changes + renames

[Matthieu Moy]

Johannes Sixt <j.sixt@viscovery.net> writes:

> Am 4/22/2013 11:24, schrieb Matthieu Moy:
>> Following the discussion on "merge with uncommited changes" inside the
>> "git pull --autostash" thread, I did a bit of testing, and encountered a
>> case with silent data loss. In short: merge a branch introducing changes
>> to a file. If the file has been renamed in the current branch, then "git
>> merge" follows the rename and brings changes to the renamed file, but
>> uncommited changes in this file are overriden silently.
>
> Can you check whether your case is already covered by one of:
>
>   git grep expect_failure t/*merge*

Indeed, it's already here:

test_expect_failure 'will not overwrite unstaged changes in renamed file' '
	git reset --hard c1 &&
	git mv c1.c other.c &&
	git commit -m rename &&
	cp important other.c &&
	git merge c1a &&
	test_cmp important other.c
'

-- 
Matthieu Moy
http://www-verimag.imag.fr/~moy/

Re: [BUG] Silent data loss on merge with uncommited changes + renames

[Junio C Hamano]

Matthieu Moy <Matthieu.Moy@grenoble-inp.fr> writes:

> I could have expected "git merge --abort" to fail, but the problem is
> really more serious here: data loss is done silently before giving me an
> opportunity to do or abort anything.

I think this is a well known and longstanding failure case in the
recursive merge.  As it does not perform its internal operation
while handling renames in clean and distinct steps (i.e. figure out
what goes to where before touching any index entry or working tree,
then check if a proposed change to the index or the working tree
conflicts with local changes, and finally carry out the proposed
change), it is somewhat hard to fix it correctly in the current
implementation, even though you probably could patch these up case
by case basis.