Subject: Re: bug#20890: master 1233bcb: Work around GC+Cairo bug

From: Paul Eggert <[email-masked]>
Date: Wed, 4 Apr 2018 11:00:55 -0700

On 04/04/2018 06:44 AM, Eli Zaretskii wrote:
> Then I think we should install it.

OK, I installed the attached into master.

>From 81f2010c21849e758a53f1d0c58c465146c1efee Mon Sep 17 00:00:00 2001
From: Paul Eggert <[email-masked]>
Date: Wed, 4 Apr 2018 10:59:09 -0700
Subject: [PATCH] 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.
---
 src/font.h     | 16 ++++++++++++++++
 src/ftcrfont.c |  3 +++
 src/ftfont.c   |  9 +--------
 3 files changed, 20 insertions(+), 8 deletions(-)

diff --git a/src/font.h b/src/font.h
index d88c8eb4f6..469431fee6 100644
--- a/src/font.h
+++ b/src/font.h
@@ -945,6 +945,22 @@ 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 614ef08370..425250e229 100644
--- a/src/ftcrfont.c
+++ b/src/ftcrfont.c
@@ -164,6 +164,9 @@ 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 51b04a8682..9a8777ef07 100644
--- a/src/ftfont.c
+++ b/src/ftfont.c
@@ -1242,15 +1242,8 @@ 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;
-- 
2.14.3