Avoid segfaults due to using fonts that were closed

* src/composite.c (composition_gstring_cache_clear_font): New
function.
* src/composite.h (composition_gstring_cache_clear_font): Add
prototype.
* src/font.c (font_clear_cache): When we are about to close a
font, remove from the gstring cache any lgstring that uses this
font.  (Bug#42943)

3 files changed, 28 insertions, 0 deletions

diff --git a/src/composite.c b/src/composite.c
index 90f8536b2de..66c1e86aae1 100644
--- a/src/composite.c
+++ b/src/composite.c
@@ -677,6 +677,27 @@ composition_gstring_from_id (ptrdiff_t id)
   return HASH_VALUE (h, id);
 }
 
+/* Remove from the composition hash table every lgstring that
+   references the given FONT_OBJECT.  */
+void
+composition_gstring_cache_clear_font (Lisp_Object font_object)
+{
+  struct Lisp_Hash_Table *h = XHASH_TABLE (gstring_hash_table);
+
+  for (ptrdiff_t i = 0; i < HASH_TABLE_SIZE (h); ++i)
+    {
+      Lisp_Object k = HASH_KEY (h, i);
+
+      if (!EQ (k, Qunbound))
+        {
+          Lisp_Object gstring = HASH_VALUE (h, i);
+
+          if (EQ (LGSTRING_FONT (gstring), font_object))
+            hash_remove_from_table (h, k);
+        }
+    }
+}
+
 DEFUN ("clear-composition-cache", Fclear_composition_cache,
        Sclear_composition_cache, 0, 0, 0,
        doc: /* Internal use only.
diff --git a/src/composite.h b/src/composite.h
index d39fdbaae05..bdf63fed10e 100644
--- a/src/composite.h
+++ b/src/composite.h
@@ -332,6 +332,8 @@ extern int composition_update_it (struct composition_it *,
 extern ptrdiff_t composition_adjust_point (ptrdiff_t, ptrdiff_t);
 extern Lisp_Object composition_gstring_lookup_cache (Lisp_Object);
 
+extern void composition_gstring_cache_clear_font (Lisp_Object);
+
 INLINE_HEADER_END
 
 #endif /* not EMACS_COMPOSITE_H */
diff --git a/src/font.c b/src/font.c
index f7c4c816b5d..8dbf8cb8999 100644
--- a/src/font.c
+++ b/src/font.c
@@ -2645,6 +2645,11 @@ font_clear_cache (struct frame *f, Lisp_Object cache,
                       if (! NILP (AREF (val, FONT_TYPE_INDEX)))
                         {
                           eassert (font && driver == font->driver);
+                          /* We are going to close the font, so make
+                             sure we don't have any lgstrings lying
+                             around in lgstring cache that reference
+                             the font.  */
+                          composition_gstring_cache_clear_font (val);
                           driver->close_font (font);
                         }
                     }