commit: fix erroneous BUG, 'multiple renames on the same target? how?'

builtin/commit.c:prepare_to_commit() can call run_status() twice if
using the editor, including status, and the user attempts to record a
non-merge empty commit without explicit --allow-empty.  If there is also
a rename involved as well (due to using 'git add -N'), then a BUG in
wt-status.c is triggered:

  BUG: wt-status.c:476: multiple renames on the same target? how?

The reason we hit this bug is that both run_status() calls use the same
struct wt_status * (named s), and s->change is not freed between runs.
Changes are inserted into s with string_list_insert, which usually means
that the second run just recomputes all the same results and overwrites
what was computed the first time.  However, ever since commit
176ea7479309 ("wt-status.c: handle worktree renames", 2017-12-27),
wt-status started checking for renames and copies but also added a
preventative check that d->rename_status wasn't already set and output a
BUG message if it was.  The problem isn't that there are multiple rename
targets to a single path as the error implies, the problem is that 's'
is not freed/cleared between the two run_status() calls.

Ever since commit dc6b1d92ca9c ("wt-status: use settings from
git_diff_ui_config", 2018-05-04), which stopped hardcoding
DIFF_DETECT_RENAME and allowed users to ask for copy detection, this bug
has also been triggerable with a copy instead of a rename.

Fix the bug by clearing s->change.  A better change might be to clean up
all of s between the two run_status() calls.  A good first step towards
such a goal might be writing a function to free the necessary fields in
the wt_status * struct; a cursory glance at the code suggests all of its
allocated data is probably leaked.  However, doing all that cleanup is a
bigger task for someone else interested to tackle; just fix the bug for
now.

Reported-by: Andrea Stacchiotti <andreastacchiotti@gmail.com>
Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 1 insertions, 0 deletions

diff --git a/builtin/commit.c b/builtin/commit.c
index 0d9828e..83233ca 100644
--- a/builtin/commit.c
+++ b/builtin/commit.c
@@ -872,6 +872,7 @@ static int prepare_to_commit(const char *index_file, const char *prefix,
 		s->use_color = 0;
 		commitable = run_status(s->fp, index_file, prefix, 1, s);
 		s->use_color = saved_color_setting;
+		string_list_clear(&s->change, 1);
 	} else {
 		struct object_id oid;
 		const char *parent = "HEAD";