Avoid crashes when X fonts are erroneously freed on reused X

'Display *' connection data (Bug#16069).  Note that X font
resources still may be leaked, but currently there is no way
to completely avoid it.
* xterm.h (struct x_display_info): New member x_id.  Add comments.
* xterm.c (x_display_id): New variable.
(x_term_init): Assign identifier to each opened X connection.
* xfont.c (struct xfont): New member x_display_id.
(xfont_open): Initialize it with frame's display id.
(xfont_close): Check whether font's display id matches the one
recorded for the given display.  Adjust comment.
* xftfont.c (struct xftfont_info):
(xftfont_open, xftfont_close): Exactly as above with xfont stuff.

5 files changed, 40 insertions, 3 deletions

diff --git a/src/ChangeLog b/src/ChangeLog
index 280077c3844..7bc69b2894e 100644
--- a/src/ChangeLog
+++ b/src/ChangeLog
@@ -6,6 +6,20 @@
 	(Fframe_font_cache) [FONT_DEBUG]: New function.
 	(syms_of_font) [FONT_DEBUG]: Defsubr it.
 
+	Avoid crashes when X fonts are erroneously freed on reused X
+	'Display *' connection data (Bug#16069).  Note that X font
+	resources still may be leaked, but currently there is no way
+	to completely avoid it.
+	* xterm.h (struct x_display_info): New member x_id.  Add comments.
+	* xterm.c (x_display_id): New variable.
+	(x_term_init): Assign identifier to each opened X connection.
+	* xfont.c (struct xfont): New member x_display_id.
+	(xfont_open): Initialize it with frame's display id.
+	(xfont_close): Check whether font's display id matches the one
+	recorded for the given display.  Adjust comment.
+	* xftfont.c (struct xftfont_info):
+	(xftfont_open, xftfont_close): Exactly as above with xfont stuff.
+
 2014-03-01  Martin Rudalics  <rudalics@gmx.at>
 
 	Consider Vother_window_scroll_buffer valid iff it's a live buffer.
diff --git a/src/xfont.c b/src/xfont.c
index 3295a62bbb0..525f00031c5 100644
--- a/src/xfont.c
+++ b/src/xfont.c
@@ -42,6 +42,7 @@ struct xfont_info
   struct font font;
   Display *display;
   XFontStruct *xfont;
+  unsigned x_display_id;
 };
 
 /* Prototypes of support functions.  */
@@ -808,6 +809,7 @@ xfont_open (struct frame *f, Lisp_Object entity, int pixel_size)
   font = XFONT_OBJECT (font_object);
   ((struct xfont_info *) font)->xfont = xfont;
   ((struct xfont_info *) font)->display = FRAME_X_DISPLAY (f);
+  ((struct xfont_info *) font)->x_display_id = FRAME_DISPLAY_INFO (f)->x_id;
   font->pixel_size = pixel_size;
   font->driver = &xfont_driver;
   font->encoding_charset = encoding->id;
@@ -892,12 +894,20 @@ xfont_open (struct frame *f, Lisp_Object entity, int pixel_size)
 static void
 xfont_close (struct font *font)
 {
+  struct x_display_info *xdi;
   struct xfont_info *xfi = (struct xfont_info *) font;
 
   /* This function may be called from GC when X connection is gone
      (Bug#16093), and an attempt to free font resources on invalid
-     display may lead to X protocol errors or segfaults.  */
-  if (xfi->xfont && x_display_info_for_display (xfi->display))
+     display may lead to X protocol errors or segfaults.  Moreover,
+     the memory referenced by 'Display *' pointer may be reused for
+     the logically different X connection after the previous display
+     connection was closed.  That's why we also check whether font's
+     ID matches the one recorded in x_display_info for this display.
+     See http://debbugs.gnu.org/cgi/bugreport.cgi?bug=16069.  */
+  if (xfi->xfont
+      && ((xdi = x_display_info_for_display (xfi->display))
+	  && xfi->x_display_id == xdi->x_id))
     {
       block_input ();
       XFreeFont (xfi->display, xfi->xfont);
diff --git a/src/xftfont.c b/src/xftfont.c
index 32816b97f23..18c180f906a 100644
--- a/src/xftfont.c
+++ b/src/xftfont.c
@@ -59,6 +59,7 @@ struct xftfont_info
   FT_Matrix matrix;
   Display *display;
   XftFont *xftfont;
+  unsigned x_display_id;
 };
 
 /* Structure pointed by (struct face *)->extra  */
@@ -372,6 +373,7 @@ xftfont_open (struct frame *f, Lisp_Object entity, int pixel_size)
   xftfont_info = (struct xftfont_info *) font;
   xftfont_info->display = display;
   xftfont_info->xftfont = xftfont;
+  xftfont_info->x_display_id = FRAME_DISPLAY_INFO (f)->x_id;
   /* This means that there's no need of transformation.  */
   xftfont_info->matrix.xx = 0;
   if (FcPatternGetMatrix (xftfont->pattern, FC_MATRIX, 0, &matrix)
@@ -481,6 +483,7 @@ xftfont_open (struct frame *f, Lisp_Object entity, int pixel_size)
 static void
 xftfont_close (struct font *font)
 {
+  struct x_display_info *xdi;
   struct xftfont_info *xftfont_info = (struct xftfont_info *) font;
 
 #ifdef HAVE_LIBOTF
@@ -493,7 +496,8 @@ xftfont_close (struct font *font)
 
   /* See comment in xfont_close.  */
   if (xftfont_info->xftfont
-      && x_display_info_for_display (xftfont_info->display))
+      && ((xdi = x_display_info_for_display (xftfont_info->display))
+	  && xftfont_info->x_display_id == xdi->x_id))
     {
       block_input ();
       XftUnlockFace (xftfont_info->xftfont);
diff --git a/src/xterm.c b/src/xterm.c
index e1873127276..f06225002a7 100644
--- a/src/xterm.c
+++ b/src/xterm.c
@@ -9679,6 +9679,10 @@ my_log_handler (const gchar *log_domain, GLogLevelFlags log_level,
 }
 #endif
 
+/* Current X display connection identifier.  Incremented for each next
+   connection established.  */
+static unsigned x_display_id;
+
 /* Open a connection to X display DISPLAY_NAME, and return
    the structure that describes the open display.
    If we cannot contact the display, return null.  */
@@ -9896,6 +9900,7 @@ x_term_init (Lisp_Object display_name, char *xrm_option, char *resource_name)
   lim = min (PTRDIFF_MAX, SIZE_MAX) - sizeof "@";
   if (lim - SBYTES (Vinvocation_name) < SBYTES (Vsystem_name))
     memory_full (SIZE_MAX);
+  dpyinfo->x_id = ++x_display_id;
   dpyinfo->x_id_name = xmalloc (SBYTES (Vinvocation_name)
 				+ SBYTES (Vsystem_name) + 2);
   strcat (strcat (strcpy (dpyinfo->x_id_name, SSDATA (Vinvocation_name)), "@"),
diff --git a/src/xterm.h b/src/xterm.h
index 3e79d46702c..50df88cb592 100644
--- a/src/xterm.h
+++ b/src/xterm.h
@@ -198,6 +198,10 @@ struct x_display_info
      mouse-face.  */
   Mouse_HLInfo mouse_highlight;
 
+  /* Logical identifier of this display.  */
+  unsigned x_id;
+
+  /* Default name for all frames on this display.  */
   char *x_id_name;
 
   /* The number of fonts opened for this display.  */