Unwinding continuations via "poison" value.

* unwind.c (sys_cont_poison_s): New symbol variable.
(revive_cont): If the argument is the poison symbol, then
unwind through the continuation all the way back to
revive_cont's own block, instead of to the continuation's very
top block. Thus the continuation is not restarted but
completely unwound.
(uw_late_init): Initialize sys_cont_poison_s.

* txr.1: Documented sys:cont-poison.

2 files changed, 11 insertions, 2 deletions

diff --git a/txr.1 b/txr.1
index 23958b86..71c8f5e8 100644
--- a/txr.1
+++ b/txr.1
@@ -27512,6 +27512,14 @@ delimiting block named by
 then the result value of that block will appear as the return value
 of the continuation function.
 
+If the symbol
+.code sys:cont-poison
+is passed to the continuation function, the continuation will be
+resumed in a different manner: its context will be restored as in the
+ordinary resume case, whereupon it will be immediately abandoned by
+a nonlocal exit, causing unwinding to take place across all of the
+evaluation's continuation frames.
+
 .TP* Note:
 
 The continuation function may be used any time after it is produced, and may be
diff --git a/unwind.c b/unwind.c
index c679c54e..3f04c800 100644
--- a/unwind.c
+++ b/unwind.c
@@ -50,7 +50,7 @@ static uw_frame_t *uw_env_stack;
 static uw_frame_t *uw_exit_point;
 static uw_frame_t toplevel_env;
 
-static val unhandled_hook_s, types_s, jump_s, sys_cont_s;
+static val unhandled_hook_s, types_s, jump_s, sys_cont_s, sys_cont_poison_s;
 static val sys_capture_cont_s;
 
 static val frame_type, catch_frame_type, handle_frame_type;
@@ -748,7 +748,7 @@ static val revive_cont(val dc, val arg)
   bug_unless (uw_stack->uw.type == UW_BLOCK);
 
   uw_stack->bl.result = cons(nil, arg);
-  uw_exit_point = uw_stack;
+  uw_exit_point = if3(arg == sys_cont_poison_s, &uw_blk, uw_stack);
   uw_unwind_to_exit_point();
   abort();
 
@@ -843,6 +843,7 @@ void uw_late_init(void)
   types_s = intern(lit("types"), user_package);
   jump_s = intern(lit("jump"), user_package);
   sys_cont_s = intern(lit("cont"), system_package);
+  sys_cont_poison_s = intern(lit("cont-poison"), system_package);
   frame_type = make_struct_type(intern(lit("frame"), user_package),
                                 nil, nil, nil, nil, nil, nil);
   catch_frame_type = make_struct_type(intern(lit("catch-frame"),