xcb: Correct handling of index 0 glyphs

Glyph caches (with direct glyph index matching) cannot be completely
initialized with zeroes, otherwise the code will incorrectly believe
that the lookup for the 0-index glyph has already been performed.

1 files changed, 7 insertions, 0 deletions

diff --git a/src/cairo-xcb-surface-render.c b/src/cairo-xcb-surface-render.c
index 760c90d9..0aaab2dc 100644
--- a/src/cairo-xcb-surface-render.c
+++ b/src/cairo-xcb-surface-render.c
@@ -3730,7 +3730,14 @@ _can_composite_glyphs (cairo_xcb_surface_t *dst,
     const int max_glyph_size = dst->connection->maximum_request_length - 64;
     int i;
 
+    /* We must initialize the cache with values that cannot match the
+     * "hash" to guarantee that when compared for the first time they
+     * will result in a mismatch. The hash function is simply modulus,
+     * so we cannot use 0 in glyph_cache[0], but we can use it in all
+     * other array cells.
+     */
     memset (glyph_cache, 0, sizeof (glyph_cache));
+    glyph_cache[0] = 1;
 
     /* first scan for oversized glyphs, and fallback in that case */
     for (i = 0; i < num_glyphs; i++) {