[PATCH 21.5] Bind print-circle when printing backtraces

PATCH 21.5

Yesterday, I received a bug report filed against the Fedora XFT build
of XEmacs.  (The default build is non-XFT, for the curious.)  This bug
report shows at least two bugs.  This patch addresses the first.

The XFT build segfaults (looks like a NULL pointer gets passed to
FCPatternObjectAddWithBinding), and tries to print a backtrace.  But
the backtrace printing gets into an infinite loop and blows the stack,
like this (line numbers translated to current Mercurial head):

1. debug_backtrace: print.c, line 3089
2. Fbacktrace: eval.c, line 7216
3. Fprin1: print.c, line 1128
4. print_internal: print.c, line 2507
5. print_compiled_function: bytecode.c, line 2339
6. print_internal: print.c, line 2507
7. print_vector_internal: print.c, line 1600
8. goto #4

Apparently, the way we print compiled functions includes printing some
internal vector, which includes a pointer back to the compiled
function.  Here is a portion of the output that XEmacs generates
before it explodes, with some whitespace changes for readability:

#<compiled-function (from "/builddir/build/BUILD/xemacs-21.5.34/lisp/faces.elc")
  (locale inst-list prop devtype-spec) "...(264)"
  [#:G23465 devtype #:G23464 devtype-spec tag-set x nil
   derive-device-type-from-locale-and-tag-set functionp plist-get
error unimplemented "mapper"
   vectorp assoc t 1 #<compiled-function ...

I don't understand that part, but it seems like binding print-circle
while printing a backtrace would be a good idea in any case.  That is
what this patch does.

diff -r 2d20d57d4e7b src/ChangeLog
--- a/src/ChangeLog Wed May 07 13:33:50 2014 -0600
+++ b/src/ChangeLog Thu May 08 08:51:32 2014 -0600
@@ -1,3 +1,9 @@
+2014-05-08  Jerry James  <[email protected]>
+
+ * print.c (struct debug_bindings): Add print_circle field.
+ (debug_print_exit): Restore print_circle.
+ (debug_print_enter): Save print_circle binding and set it.
+
 2014-01-27  Michael Sperber  <[email protected]>

  * symbols.c (Fdefine_function): Allow optional `docstring'
diff -r 2d20d57d4e7b src/print.c
--- a/src/print.c Wed May 07 13:33:50 2014 -0600
+++ b/src/print.c Thu May 08 08:51:32 2014 -0600
@@ -164,6 +164,7 @@
   int print_depth;
   int print_readably;
   int print_unbuffered;
+  int print_circle;
   int in_debug_print;
   int gc_currently_forbidden;
   Lisp_Object Vprint_length;
@@ -2847,6 +2848,7 @@
   print_depth = bindings->print_depth;
   print_readably = bindings->print_readably;
   print_unbuffered = bindings->print_unbuffered;
+  print_circle = bindings->print_circle;
   in_debug_print = bindings->in_debug_print;
   gc_currently_forbidden = bindings->gc_currently_forbidden;
   Vprint_length = bindings->Vprint_length;
@@ -2873,6 +2875,7 @@
   bindings->print_depth = print_depth;
   bindings->print_readably = print_readably;
   bindings->print_unbuffered = print_unbuffered;
+  bindings->print_circle = print_circle;
   bindings->in_debug_print = in_debug_print;
   bindings->gc_currently_forbidden = gc_currently_forbidden;
   bindings->Vprint_length = Vprint_length;
@@ -2885,6 +2888,7 @@
   print_depth = 0;
   print_readably = debug_print_readably != -1 ? debug_print_readably : 0;
   print_unbuffered++;
+  print_circle = 1;
   in_debug_print = 1;
   gc_currently_forbidden = 1;
   if (debug_print_length > 0)

-- 
Jerry James
http://www.jamezone.org/