[BUG] merge into unborn branch resets index+WT even if populated

[BUG] merge into unborn branch resets index+WT even if populated

[Thomas Rast]

Hi,

I'm posting this on behalf of Dscho who reported it on IRC, but
apparently doesn't have the time for email.

If you run the following sequence of commands:

  git init
  echo foo > somefile
  git add somefile
  git pull elsewhere master

(where 'elsewhere' is any repo having a master branch) the "merge into
unborn branch" logic silently nukes the index AND worktree copy of
"somefile", thereby losing it.  The same works with fetch+merge, so
it's not pull's fault.

-- 
Thomas Rast
trast@{inf,student}.ethz.ch

[PATCH] Exhibit merge bug that clobbers index&WT

[Thomas Rast]

Running git-merge on an unborn branch is supposed to do an index-level
merge with the other side, and then update the branch name there.  In
the common case where the index was empty at the start, this makes
'git pull otherrepo branch' a convenient way to populate the history
after 'git init'.

However, if the index was *not* empty, git-merge silently discards
*both index and worktree* copies of all files that were tracked,
leading to data loss.  Exhibit this bug.

Reported-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Thomas Rast <trast@student.ethz.ch>
---

Here's a less lazy approach at a bug report.

I'm not too happy that noone went for a fix though; this is a real
data loss scenario and from his complaints on IRC, Dscho actually hit
it and had to reconstruct from the blobs.


 t/t7607-merge-overwrite.sh |   16 ++++++++++++++++
 1 files changed, 16 insertions(+), 0 deletions(-)

diff --git a/t/t7607-merge-overwrite.sh b/t/t7607-merge-overwrite.sh
index d82349a..ff08ebf 100755
--- a/t/t7607-merge-overwrite.sh
+++ b/t/t7607-merge-overwrite.sh
@@ -84,4 +84,20 @@ test_expect_success 'will not overwrite removed file with staged changes' '
 	test_cmp important c1.c
 '
 
+test_expect_success 'set up unborn branch and content' '
+	git symbolic-ref HEAD refs/heads/unborn &&
+	rm -f .git/index &&
+	echo foo > tracked-file &&
+	git add tracked-file &&
+	echo bar > untracked-file
+'
+
+test_expect_failure 'will not clobber WT/index when merging into unborn' '
+	git merge master &&
+	grep foo tracked-file &&
+	git show :tracked-file >expect &&
+	grep foo expect &&
+	grep bar untracked-file
+'
+
 test_done
-- 
1.7.2.2.479.g07acc