Improve GC+Cairo workaround

Suggested by Eli Zaretskii (Bug#20890#31).
* src/font.h (font_data_structures_may_be_ill_formed): New function.
* src/ftfont.c (ftfont_close):
* src/ftcrfont.c (ftcrfont_close): Use it.

(cherry picked from commit d02fd482fbeaf6ed551e78223b538495cb0c3541)

3 files changed, 20 insertions, 8 deletions

diff --git a/src/font.h b/src/font.h
index bb468333e85..b6e43b0c9ca 100644
--- a/src/font.h
+++ b/src/font.h
@@ -948,6 +948,22 @@ extern void font_deferred_log (const char *, Lisp_Object, Lisp_Object);
       font_deferred_log ((ACTION), (ARG), (RESULT));    \
   } while (false)
 
+/* FIXME: This is for use in functions that can be called while
+   garbage-collecting, but which assume that Lisp data structures are
+   properly-formed.  This invalid assumption can lead to core dumps
+   (Bug#20890).  */
+INLINE bool
+font_data_structures_may_be_ill_formed (void)
+{
+#ifdef USE_CAIRO
+  /* Although this works around Bug#20890, it is probably not the
+     right thing to do.  */
+  return gc_in_progress;
+#else
+  return false;
+#endif
+}
+
 INLINE_HEADER_END
 
 #endif  /* not EMACS_FONT_H */
diff --git a/src/ftcrfont.c b/src/ftcrfont.c
index 0e3490c570e..62f44573a86 100644
--- a/src/ftcrfont.c
+++ b/src/ftcrfont.c
@@ -165,6 +165,9 @@ ftcrfont_open (struct frame *f, Lisp_Object entity, int pixel_size)
 static void
 ftcrfont_close (struct font *font)
 {
+  if (font_data_structures_may_be_ill_formed ())
+    return;
+
   struct ftcrfont_info *ftcrfont_info = (struct ftcrfont_info *) font;
   int i;
 
diff --git a/src/ftfont.c b/src/ftfont.c
index bdb1dff8cb9..823fb2095ce 100644
--- a/src/ftfont.c
+++ b/src/ftfont.c
@@ -1243,15 +1243,8 @@ ftfont_open (struct frame *f, Lisp_Object entity, int pixel_size)
 void
 ftfont_close (struct font *font)
 {
-  /* FIXME: Although this function can be called while garbage-collecting,
-     the function assumes that Lisp data structures are properly-formed.
-     This invalid assumption can lead to core dumps (Bug#20890).  */
-#ifdef USE_CAIRO
-  /* Although this works around Bug#20890, it is probably not the
-     right thing to do.  */
-  if (gc_in_progress)
+  if (font_data_structures_may_be_ill_formed ())
     return;
-#endif
 
   struct ftfont_info *ftfont_info = (struct ftfont_info *) font;
   Lisp_Object val, cache;