[PATCH 5/8] xdl_change_compact(): fix compaction heuristic to adjust io

[Michael Haggerty <mhagger@alum.mit.edu>]

The code branch used for the compaction heuristic incorrectly forgot to
keep io in sync while the group was shifted. I think that could have
led to reading past the end of the rchgo array.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
---
I didn't actually try to verify the presence of a bug, because it
seems like more work than worthwhile. But here is my reasoning:

If io is not decremented correctly during one iteration of the outer
`while` loop, then it will loose sync with the `end` counter. In
particular it will be too large.

Suppose that the next iterations of the outer `while` loop (i.e.,
processing the next block of add/delete lines) don't have any sliders.
Then the `io` counter would be incremented by the number of
non-changed lines in xdf, which is the same as the number of
non-changed lines in xdfo that *should have* followed the group that
experienced the malfunction. But since `io` was too large at the end
of that iteration, it will be incremented past the end of the
xdfo->rchg array, and will try to read that memory illegally.

 xdiff/xdiffi.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/xdiff/xdiffi.c b/xdiff/xdiffi.c
index c67cfe3..66129db 100644
--- a/xdiff/xdiffi.c
+++ b/xdiff/xdiffi.c
@@ -562,6 +562,10 @@ int xdl_change_compact(xdfile_t *xdf, xdfile_t *xdfo, long flags) {
 			       recs_match(recs, start - 1, end - 1, flags)) {
 				rchg[--start] = 1;
 				rchg[--end] = 0;
+
+				io--;
+				while (rchgo[io])
+					io--;
 			}
 		} else if (end_matching_other != -1) {
 			/*
-- 
2.8.1