bug#29151: [PATCH] ia64: fix crash in thread context switch

[Sergei Trofimovich <slyfox@gentoo.org>]

Backtrace looks like that:

  Program terminated with signal SIGSEGV, Segmentation fault.
  #0  0x200000000014a5c0 in scm_ia64_longjmp (JB=0x6000000000817020, VAL=1) at continuations.c:372
  372                   t->pending_rbs_continuation->backing_store,
  [Current thread is 1 (Thread 0x2000000000049340 (LWP 8190))]
  (gdb) bt
  #0  0x200000000014a5c0 in scm_ia64_longjmp (JB=0x6000000000817020, VAL=1) at continuations.c:372
  #1  0x2000000000148e00 in scm_c_abort (vm=0x60000000000edea0, tag=0x6000000000795ba0, n=0, argv=0x60000fffff7f0ce0, cookie=-1) at control.c:239
  #2  0x2000000000149070 in scm_at_abort (tag=0x6000000000795ba0, args=0x304) at control.c:258
  (gdb) print t
  $2 = (scm_i_thread *) 0x6000000000068000
  (gdb) print t->pending_rbs_continuation
  $3 = (scm_t_contregs *) 0xffeb

The problem here is the value of 't->pending_rbs_continuation' pointer.
It's supposed to poin to a register stack pointer or be NULL if not yet
backed up.

The problem is it is never initialized to NULL at creation time and
contained garbage on stack. Sometimes people are lucky and have zeros
on stack and guile works. But sometimes there is something and guile
crashes.

The fix is trivial: initialize 'pending_rbs_continuation = NULL'
at thread registration time (the same way other threads are registered).

Reported-by: Matt Turner
Bug: https://bugs.gentoo.org/613986

* libguile/threads.c(guilify_self_1): initialize pending_rbs_continuation
  to avoid guile crash on ia64.

Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>
---
 libguile/threads.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/libguile/threads.c b/libguile/threads.c
index 9ceb5b88a..770f62c44 100644
--- a/libguile/threads.c
+++ b/libguile/threads.c
@@ -395,6 +395,7 @@ guilify_self_1 (struct GC_stack_base *base, int needs_unregister)
   t.base = base->mem_base;
 #ifdef __ia64__
   t.register_backing_store_base = base->reg_base;
+  t.pending_rbs_continuation = 0;
 #endif
   t.continuation_root = SCM_EOL;
   t.continuation_base = t.base;
-- 
2.15.0