ncd: Reverse the immediate effect logic of backtrack_point::rgo().

Turns out we want the backtrack triggering effects to have priority. Consider the following test case.

process main {
    backtrack_point() pointA;
    println("A");
    sleep("1");
    If ("true") {
        backtrack_point() pointB;
        println("B");
        pointA->rgo();
        pointB->go();
    };
}

The result of this would originally have been "A B B B B...". The reason is that the If process would be caught in an infinite loop, and the main process would never get a chance to backtrack past the If statement. With this change, the result is "A B A B...", since the rgo() deinitialization immediately causes backtracking in main.

This is actually a nice example about a design bug regarding the subtle ways in which a process call (If, in this case) is not equivalent to embedding the statements in place. Without the If, there would not be an infinite loop in any case.

ncd/modules/backtrack.c

@@ -40,8 +40,8 @@
  *   itself goes up immedietely, but side effects of triggering backtracking have
  *   priority.
  *   The rgo() method triggers backtracking when it deinitializes. In this case,
- *   the immediate effects of rgo() deinitialization happen before the immediate
- *   effects of triggering backtracking in the backtrack_point().
+ *   the immediate effects of triggering backtracking in the backtrack_point() have
+ *   priority over the immediate effects of rgo() deinitialization completion.
  */
 
 #include <ncd/module_common.h>
@@ -121,11 +121,11 @@ static void rgo_func_die (void *vo)
 {
     struct rgo_instance *o = vo;
     
-    // toggle backtrack point
+    // deref backtrack_point
+    NCDModuleInst *backtrack_point_inst = NULL;
     NCDObject rgo_obj;
     if (NCDObjRef_Deref(&o->bp_ref, &rgo_obj)) {
-        NCDModuleInst *backtrack_point_inst = NCDObject_MethodUser(&rgo_obj);
-        NCDModuleInst_Backend_DownUp(backtrack_point_inst);
+        backtrack_point_inst = NCDObject_MethodUser(&rgo_obj);
     }
     
     // free object reference
@@ -133,6 +133,11 @@ static void rgo_func_die (void *vo)
     
     // die
     NCDModuleInst_Backend_Dead(o->i);
+    
+    // toggle backtrack_point
+    if (backtrack_point_inst) {
+        NCDModuleInst_Backend_DownUp(backtrack_point_inst);
+    }
 }
 
 static struct NCDModule modules[] = {

ncd/tests/backtracking.ncd

@@ -52,10 +52,12 @@ template helper1 {
 }
 
 template helper2 {
-    _caller._caller.flag->set("true");
+    val_equal(_caller._caller.flag, "true") a;
+    assert(a);
 }
 
 template helper3 {
-    val_equal(_caller.flag, "true") a;
+    val_equal(_caller.flag, "false") a;
     assert(a);
+    _caller.flag->set("true");
 }