Fix #635: Segfault in async result via dangling pointer from optimized for loop (#671)

* Fix #635: Segfault in async result via dangling pointer from optimized for loop


* The optimized for loop (chaiscript_optimizer.hpp) stored the loop
   counter as a stack-local `int` and exposed it to ChaiScript via
   `var(&i)`, creating a reference-type Boxed_Value pointing to the
   stack frame.

CMakeLists.txt

@@ -295,6 +295,7 @@ list(SORT UNIT_TESTS)
 if(NOT MULTITHREAD_SUPPORT_ENABLED)
   list(REMOVE_ITEM UNIT_TESTS
     async_engine_lifetime.chai
+    async_return_value.chai
     future.chai
     list_push_front.chai
     move_async.chai

include/chaiscript/language/chaiscript_optimizer.hpp

@@ -397,8 +397,9 @@ namespace chaiscript {
                                         assert(children.size() == 1);
                                         chaiscript::eval::detail::Scope_Push_Pop spp(t_ss);
 
-                                        int i = start_int;
+                                        Boxed_Value bv_i(start_int);
-                                        t_ss.add_object(id, var(&i));
+                                        auto &i = *static_cast<int *>(bv_i.get_ptr());
+                                        t_ss.add_object(id, bv_i);
 
                                         try {
                                           for (; i < end_int; ++i) {

unittests/assign_no_aliasing.chai

@@ -0,0 +1,41 @@
+// Issue #635: optimized for loop with := must not produce dangling values
+// := is reference-assign, so ret aliases i and sees the loop exit value
+
+// Basic: capture loop variable via :=
+var ret = 0
+for (var i = 0; i < 100; ++i) {
+  ret := i
+}
+assert_equal(100, ret)
+
+// Return from function containing optimized for loop
+def loop_result() {
+  var ret = 0
+  for (var i = 0; i < 200; ++i) {
+    ret := i
+  }
+  return ret
+}
+assert_equal(200, loop_result())
+
+// Multiple calls return consistent results
+assert_equal(loop_result(), loop_result())
+
+// Nested optimized for loops
+var outer_val = 0
+var inner_val = 0
+for (var i = 0; i < 10; ++i) {
+  for (var j = 0; j < 10; ++j) {
+    inner_val := j
+  }
+  outer_val := i
+}
+assert_equal(10, outer_val)
+assert_equal(10, inner_val)
+
+// Value-assign (=) captures last body-iteration value, not exit value
+var ret2 = 0
+for (var i = 0; i < 100; ++i) {
+  ret2 = i
+}
+assert_equal(99, ret2)

unittests/async_return_value.chai

@@ -0,0 +1,25 @@
+// Issue #635: async + optimized for loop must not segfault from dangling pointer
+// := is reference-assign, so ret aliases i and sees the loop exit value
+
+var func = fun(){
+  var ret = 0;
+  for (var i = 0; i < 1000; ++i) {
+    ret := i;
+  }
+  return ret;
+}
+
+var&fut1 = async(func);
+var fut2 = async(func);
+
+assert_equal(1000, fut1.get())
+assert_equal(1000, fut2.get())
+
+// Multiple concurrent async calls to stress the scenario
+var results = []
+for (var n = 0; n < 5; ++n) {
+  results.push_back(async(func))
+}
+for (var n = 0; n < 5; ++n) {
+  assert_equal(1000, results[n].get())
+}