webassembly: Add C-level finaliser to JsProxy object.

And clear the corresponding `proxy_js_ref[js_ref]` entry when the finaliser
runs.  This then allows the JavaScript side to (eventually) free the
corresponding JavaScript object.

Signed-off-by: Damien George <damien@micropython.org>

ports/webassembly/objjsproxy.c

@@ -234,6 +234,22 @@ static mp_obj_t jsproxy_call(mp_obj_t self_in, size_t n_args, size_t n_kw, const
     }
 }
 
+EM_JS(void, proxy_js_free_obj, (int js_ref), {
+    if (js_ref >= PROXY_JS_REF_NUM_STATIC) {
+        proxy_js_ref[js_ref] = undefined;
+        if (js_ref < proxy_js_ref_next) {
+            proxy_js_ref_next = js_ref;
+        }
+    }
+});
+
+static mp_obj_t jsproxy___del__(mp_obj_t self_in) {
+    mp_obj_jsproxy_t *self = MP_OBJ_TO_PTR(self_in);
+    proxy_js_free_obj(self->ref);
+    return mp_const_none;
+}
+static MP_DEFINE_CONST_FUN_OBJ_1(jsproxy___del___obj, jsproxy___del__);
+
 static mp_obj_t jsproxy_reflect_construct(size_t n_args, const mp_obj_t *args) {
     int arg0 = mp_obj_jsproxy_get_ref(args[0]);
     n_args -= 1;
@@ -274,7 +290,11 @@ void mp_obj_jsproxy_attr(mp_obj_t self_in, qstr attr, mp_obj_t *dest) {
     if (dest[0] == MP_OBJ_NULL) {
         // Load attribute.
         uint32_t out[PVN];
-        if (lookup_attr(self->ref, qstr_str(attr), out)) {
+        if (attr == MP_QSTR___del__) {
+            // For finaliser.
+            dest[0] = MP_OBJ_FROM_PTR(&jsproxy___del___obj);
+            dest[1] = self_in;
+        } else if (lookup_attr(self->ref, qstr_str(attr), out)) {
             dest[0] = proxy_convert_js_to_mp_obj_cside(out);
         } else if (attr == MP_QSTR_new) {
             // Special case to handle construction of JS objects.
@@ -515,7 +535,7 @@ MP_DEFINE_CONST_OBJ_TYPE(
     );
 
 mp_obj_t mp_obj_new_jsproxy(int ref) {
-    mp_obj_jsproxy_t *o = mp_obj_malloc(mp_obj_jsproxy_t, &mp_type_jsproxy);
+    mp_obj_jsproxy_t *o = mp_obj_malloc_with_finaliser(mp_obj_jsproxy_t, &mp_type_jsproxy);
     o->ref = ref;
     return MP_OBJ_FROM_PTR(o);
 }

ports/webassembly/proxy_js.js

@@ -24,6 +24,9 @@
  * THE SOFTWARE.
  */
 
+// Number of static entries at the start of proxy_js_ref.
+const PROXY_JS_REF_NUM_STATIC = 2;
+
 // These constants should match the constants in proxy_c.c.
 
 const PROXY_KIND_MP_EXCEPTION = -1;
@@ -57,6 +60,28 @@ class PythonError extends Error {
 
 function proxy_js_init() {
     globalThis.proxy_js_ref = [globalThis, undefined];
+    globalThis.proxy_js_ref_next = PROXY_JS_REF_NUM_STATIC;
+}
+
+// js_obj cannot be undefined
+function proxy_js_add_obj(js_obj) {
+    // Search for the first free slot in proxy_js_ref.
+    while (proxy_js_ref_next < proxy_js_ref.length) {
+        if (proxy_js_ref[proxy_js_ref_next] === undefined) {
+            // Free slot found, reuse it.
+            const id = proxy_js_ref_next;
+            ++proxy_js_ref_next;
+            proxy_js_ref[id] = js_obj;
+            return id;
+        }
+        ++proxy_js_ref_next;
+    }
+
+    // No free slots, so grow proxy_js_ref by one (append at the end of the array).
+    const id = proxy_js_ref.length;
+    proxy_js_ref[id] = js_obj;
+    proxy_js_ref_next = proxy_js_ref.length;
+    return id;
 }
 
 function proxy_call_python(target, argumentsList) {
@@ -147,8 +172,7 @@ function proxy_convert_js_to_mp_obj_jsside(js_obj, out) {
         Module.setValue(out + 4, js_obj._ref, "i32");
     } else {
         kind = PROXY_KIND_JS_OBJECT;
-        const id = proxy_js_ref.length;
+        const id = proxy_js_add_obj(js_obj);
-        proxy_js_ref[id] = js_obj;
         Module.setValue(out + 4, id, "i32");
     }
     Module.setValue(out + 0, kind, "i32");
@@ -161,8 +185,7 @@ function proxy_convert_js_to_mp_obj_jsside_force_double_proxy(js_obj, out) {
         js_obj instanceof PyProxyThenable
     ) {
         const kind = PROXY_KIND_JS_OBJECT;
-        const id = proxy_js_ref.length;
+        const id = proxy_js_add_obj(js_obj);
-        proxy_js_ref[id] = js_obj;
         Module.setValue(out + 4, id, "i32");
         Module.setValue(out + 0, kind, "i32");
     } else {