fix deleted val problem caused by move assignment to itself (emscri…

…pten-core#20072)

The implementation of `val& operator=(val&& v)`
should not directly call `_emval_decref(handle)`.
According to the semantics of move & assignment,
the input argument(v) need to be reset first,
and then the internal handle is judged to be
a valid handle before calling `_emval_decref`.
This avoids the loss of reference problem that
occurs when moving the assignment to itself (this).

system/include/emscripten/val.h

@@ -420,9 +420,12 @@ class val {
   }
 
   val& operator=(val&& v) & {
-    internal::_emval_decref(handle);
-    handle = v.handle;
+    auto v_handle = v.handle;
     v.handle = 0;
+    if (handle) {
+      internal::_emval_decref(handle);
+    }
+    handle = v_handle;
     return *this;
   }
 

test/embind/test_val.cpp

@@ -225,6 +225,17 @@ int main() {
   ensure_not(val::global("c").isArray());
   ensure_not(val::global("d").isArray());
 
+  test("val& operator=(val&& v)");
+  val source(val::object());
+  val target(val::object());
+  source.set("val", 1);
+  target = std::move(source);
+  ensure(target["val"].as<int>() == 1);
+  // move and assign to itself
+  target.set("val", 2);
+  target = std::move(target);
+  ensure(target["val"].as<int>() == 2);
+
   test("bool equals(const val& v)");
   EM_ASM(
     a = 1;

test/embind/test_val.out

@@ -15,6 +15,7 @@ test: bool isFalse()...
 test: bool isNumber()...
 test: bool isString()...
 test: bool isArray()...
+test: val& operator=(val&& v)...
 test: bool equals(const val& v)...
 test: bool operator==(const val& v)...
 test: bool operator!=(const val& v)...