On Wed, Jul 11, 2018 at 01:00:57PM +0200, Ævar Arnfjörð Bjarmason wrote:
> This segfaults, but should print an error instead, have a repo with a
> corrupt HEAD:
>
> (
> rm -rf /tmp/git &&
> git clone --single-branch --branch todo g...@github.com:git/git.git
> /tmp/git &&
> echo 1111111111111111111111111111111111111111
> >/tmp/git/.git/refs/heads/todo &&
> git -C /tmp/git pull
> )
It took me a minute to reproduce this. It needs "pull --rebase" if you
don't have that setup in your config.
> The immediate reason is that in run_diff_index() we have this:
>
> ent = revs->pending.objects;
>
> And that in this case that's NULL:
>
> (gdb) bt
> #0 0x000055555565993f in run_diff_index (revs=0x7fffffffcb90, cached=1)
> at diff-lib.c:524
> #1 0x00005555557633da in has_uncommitted_changes (ignore_submodules=1)
> at wt-status.c:2345
These two are the interesting functions. has_uncommitted_changes() calls
add_head_to_pending(). So it could realize then that there is no valid
HEAD to compare against.
But as you note, it's run_diff_index() that blindly dereferences
revs->pending.objects without seeing if it's non-empty. Normally
setup_revisions() would barf on a bad object, but the manual
add_head_to_pending() quietly returns (as it must for some cases, like
unborn branches).
So I feel like the right answer here is probably this:
diff --git a/wt-status.c b/wt-status.c
index d1c05145a4..5fcaa3d0f8 100644
--- a/wt-status.c
+++ b/wt-status.c
@@ -2340,7 +2340,16 @@ int has_uncommitted_changes(int ignore_submodules)
if (ignore_submodules)
rev_info.diffopt.flags.ignore_submodules = 1;
rev_info.diffopt.flags.quick = 1;
+
add_head_to_pending(&rev_info);
+ if (!rev_info.pending.nr) {
+ /*
+ * We have no head (or it's corrupt), but the index is not
+ * unborn; declare it as uncommitted changes.
+ */
+ return 1;
+ }
+
diff_setup_done(&rev_info.diffopt);
result = run_diff_index(&rev_info, 1);
return diff_result_code(&rev_info.diffopt, result);
That does quietly paper over the corruption, but it does the
conservative thing, and a follow-up "git status" would yield "bad
object: HEAD".
I do worry that other callers of run_diff_index() might have similar
problems, though. Grepping around, the other callers seem to fall into
one of three categories:
- they resolve the object themselves and put it in the pending list
(and often fallback to the empty tree, which is more or less what the
patch above is doing)
- they resolve the object themselves and avoid calling run_diff_index()
if it's not valid
- they use setup_revisions(), which will barf on the broken object
So I think this may be sufficient. We probably should also add an
assertion to run_diff_index(), since that's better than segfaulting.
-Peff
