diff: avoid segfault with freed entries

When computing a diff in a partial clone, there is a chance that we
could trigger a prefetch of missing objects at the same time as we are
freeing entries from the global diff queue. This is difficult to
reproduce, as we need to have some objects be freed from the queue
before triggering the prefetch of missing objects. There is a new test
in t4067 that does trigger the segmentation fault that results in this
case.

The fix is to set the queue pointer to NULL after it is freed, and then
to be careful about NULL values in the prefetch.

The more elaborate explanation is that within diffcore_std(), we may
skip the initial prefetch due to the output format (--name-only in the
test) and go straight to diffcore_skip_stat_unmatch(). In that method,
the index entries that have been invalidated by path changes show up as
entries but may be deleted because they are not actually content diffs
and only newer timestamps than expected. As those entries are deleted,
later entries are checked with diff_filespec_check_stat_unmatch(), which
uses diff_queued_diff_prefetch() as the missing_object_cb in its diff
options. That can trigger downloading missing objects if the appropriate
scenario occurs to trigger a call to diff_popoulate_filespec(). It's
finally within that callback to diff_queued_diff_prefetch() that the
segfault occurs.

The test was hard to find because it required some real differences,
some not-different files that had a newer modified time, and the order
of those files alphabetically was important to trigger the deletion
before the prefetch was triggered.

I briefly considered a "lock" member for the diff queue, but it was a
much larger diff and introduced many more possible error scenarios.

Signed-off-by: Derrick Stolee <stolee@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff.c

@@ -7098,6 +7098,7 @@ static void diffcore_skip_stat_unmatch(struct diff_options *diffopt)
 			if (!diffopt->flags.no_index)
 				diffopt->skip_stat_unmatch++;
 			diff_free_filepair(p);
+			q->queue[i] = NULL;
 		}
 	}
 	free(q->queue);
@@ -7141,6 +7142,10 @@ void diff_queued_diff_prefetch(void *repository)
 
 	for (i = 0; i < q->nr; i++) {
 		struct diff_filepair *p = q->queue[i];
+
+		if (!p)
+			continue;
+
 		diff_add_if_missing(repo, &to_fetch, p->one);
 		diff_add_if_missing(repo, &to_fetch, p->two);
 	}