Avoid infloop in 'format-mode-line'

* src/xdisp.c (decode_mode_spec): Don't use W->start if it is
outside of the buffer's accessible region.  (Bug#42220)

1 files changed, 16 insertions, 0 deletions

diff --git a/src/xdisp.c b/src/xdisp.c
index 269777a5b15..bc82d0acc0d 100644
--- a/src/xdisp.c
+++ b/src/xdisp.c
@@ -26342,6 +26342,22 @@ decode_mode_spec (struct window *w, register int c, int field_width,
         startpos = marker_position (w->start);
         startpos_byte = marker_byte_position (w->start);
         height = WINDOW_TOTAL_LINES (w);
+        /* We cannot cope with w->start being outside of the
+           accessible portion of the buffer; in particular,
+           display_count_lines call below will infloop if called with
+           startpos_byte outside of the [BEGV_BYTE..ZV_BYTE] region.
+           Such w->start means we were called in some "creative" way
+           when the buffer's restriction was changed, but the window
+           wasn't yet redisplayed after that.  If that happens, we
+           need to determine a new base line.  */
+        if (!(BUF_BEGV_BYTE (b) <= startpos_byte
+              && startpos_byte <= BUF_ZV_BYTE (b)))
+          {
+            startpos = BUF_BEGV (b);
+            startpos_byte = BUF_BEGV_BYTE (b);
+            w->base_line_pos = 0;
+            w->base_line_number = 0;
+          }
 
         /* If we decided that this buffer isn't suitable for line numbers,
            don't forget that too fast.  */