Provide a way to free the continuation stacks.

* share/txr/stdlib/yield.tl (sys:obtain-impl): Pass
sys:cont-free symbol to each abandoned continuation
to release its stack buffer.
(obtain): Handle the sys:cont-free symbol in the
lambda, so the initial lambda can be treated
uniformly with continuation functions.

* txr.1: Documented sys:obtain-impl.

* unwind.c (sys_cont_free_s): New symbol variable.
(cont_mark): Check for null stack pointer and avoid marking.
(revive_cont): If arg is sys:cont-free, then free the
continuation and return nil. If the continuation has a null
stack buffer, throw an error.
(uw_late_init): Initialize sys_cont_free_s.

1 files changed, 5 insertions, 2 deletions

diff --git a/share/txr/stdlib/yield.tl b/share/txr/stdlib/yield.tl
index ea9bb4d2..236faaa8 100644
--- a/share/txr/stdlib/yield.tl
+++ b/share/txr/stdlib/yield.tl
@@ -31,6 +31,7 @@
       (let ((yi (call fun reply)))
         (cond
           ((eq (typeof yi) 'sys:yld-item)
+           (call fun 'sys:cont-free)
            (set fun yi.cont)
            yi.val)
           (t yi))))
@@ -44,8 +45,10 @@
       (cdr cont))))
 
 (defmacro obtain (. body)
-  (let ((ignored (gensym "ign")))
-    ^(sys:obtain-impl (lambda (,ignored) ,*body))))
+  (let ((arg (gensym "arg")))
+    ^(sys:obtain-impl (lambda (,arg)
+                        (unless (eq ,arg 'sys:cont-free)
+                          ,*body)))))
 
 (defmacro obtain-block (name . body)
   ^(obtain (block ,name ,*body)))