If emacs is compiled for xwindows --with-cairo, then started with --daemon,
and any packages that define a fringe bitmap are used (including builtin
flymake), then 'define-fringe-bitmap' will be called from lisp during
startup and attempt to define dynamic bitmaps when emacs has no active
frame.

This is a problem because 'init_fringe_bitmap' (in src/fringe.c) relies on
the SELECTED_FRAME macro in order to access a redisplay_interface structure
and call 'rif->define_fringe_bitmap'.  Until a frame is created for a
client session, this process will fail, causing emacs to skip calling
'rif->define_fringe_bitmap' (which should initialize 'fringe_bmp[which]'
for cairo builds), but otherwise proceed as if the bitmap in question
exists.  (note the code in question already contains the following comment:
'/* XXX Is SELECTED_FRAME OK here? */' ... it is not.)

The upshot from there is, if[/when] emacs tries to *draw* these bitmaps, it
pulls a null pointer out of 'fringe_bmp[which]' and hands it straight to
libcairo.  Predictably, libcairo retaliates with the SEGV, and it's all
over but the coredumping.


As best as I can tell, the following diff resolves the issue (at least as
it manifests when using X; I haven't experimented with other
configurations).  Style-wise it's a touch crude and hamfisted, but tbh
trying to figure out the right approach for this codebase seemed to mean a
whole lot of stumbling through giant briar patches of macro indirection,
and I bailed.  Hopefully someone familiar with all that stuff can translate
it readily enough.
[git diff against commit d5622eb6fff94714c5d5a64c98c5e02bc1be478c]

diff --git a/src/frame.h b/src/frame.h
index fa45a32d6b..94a880f4eb 100644
--- a/src/frame.h
+++ b/src/frame.h
@@ -1587,6 +1587,9 @@ #define EMACS_CLASS "Emacs"

 #if defined HAVE_X_WINDOWS
 extern void x_wm_set_icon_position (struct frame *, int, int);
+  #if defined USE_CAIRO
+  extern struct redisplay_interface x_redisplay_interface;
+  #endif
 #if !defined USE_X_TOOLKIT
 extern const char *x_get_resource_string (const char *, const char *);
 #endif
diff --git a/src/fringe.c b/src/fringe.c
index d0d599223d..7a93afd418 100644
--- a/src/fringe.c
+++ b/src/fringe.c
@@ -1482,6 +1482,10 @@ init_fringe_bitmap (int which, struct fringe_bitmap
*fb, int once_p)

       if (rif && rif->define_fringe_bitmap)
        rif->define_fringe_bitmap (which, fb->bits, fb->height, fb->width);
+#if (defined HAVE_X_WINDOWS) && (defined USE_CAIRO)
+      else
+        x_redisplay_interface.define_fringe_bitmap(which, fb->bits,
fb->height, fb->width);
+#endif

       fringe_bitmaps[which] = fb;
       if (which >= max_used_fringe_bitmap)
diff --git a/src/xterm.c b/src/xterm.c
index bbe68ef622..76fc5c21ac 100644
--- a/src/xterm.c
+++ b/src/xterm.c
@@ -13298,7 +13298,7 @@ x_activate_timeout_atimer (void)

 extern frame_parm_handler x_frame_parm_handlers[];

-static struct redisplay_interface x_redisplay_interface =
+struct redisplay_interface x_redisplay_interface =
   {
     x_frame_parm_handlers,
     gui_produce_glyphs,