webassembly: Reuse PyProxy objects when they are the same Python object.

This commit makes it so that PyProxy objects are reused (on the JavaScript
side) when they correspond to an existing Python object that is the same
object.

For example, proxying the same Python function to JavaScript, the same
PyProxy instance is now used.  This means that if `foo` is a Python
function then accessing it on the JavaScript side such as
`api.globals().get("foo")` has the property that:

    api.globals().get("foo") === api.globals().get("foo")

Prior to this commit the above was not true because new PyProxy instances
were created each time `foo` was accessed.

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

ports/webassembly/proxy_js.js

@@ -40,6 +40,7 @@ const PROXY_KIND_MP_CALLABLE = 6;
 const PROXY_KIND_MP_GENERATOR = 7;
 const PROXY_KIND_MP_OBJECT = 8;
 const PROXY_KIND_MP_JSPROXY = 9;
+const PROXY_KIND_MP_EXISTING = 10;
 
 const PROXY_KIND_JS_UNDEFINED = 0;
 const PROXY_KIND_JS_NULL = 1;
@@ -61,13 +62,39 @@ class PythonError extends Error {
 function proxy_js_init() {
     globalThis.proxy_js_ref = [globalThis, undefined];
     globalThis.proxy_js_ref_next = PROXY_JS_REF_NUM_STATIC;
+    globalThis.proxy_js_map = new Map();
+    globalThis.proxy_js_existing = [undefined];
     globalThis.pyProxyFinalizationRegistry = new FinalizationRegistry(
         (cRef) => {
+            globalThis.proxy_js_map.delete(cRef);
             Module.ccall("proxy_c_free_obj", "null", ["number"], [cRef]);
         },
     );
 }
 
+// Check if the c_ref (Python proxy index) has a corresponding JavaScript-side PyProxy
+// associated with it.  If so, take a concrete reference to this PyProxy from the WeakRef
+// and put it in proxy_js_existing, to be referenced and reused by PROXY_KIND_MP_EXISTING.
+function proxy_js_check_existing(c_ref) {
+    const existing_obj = globalThis.proxy_js_map.get(c_ref)?.deref();
+    if (existing_obj === undefined) {
+        return -1;
+    }
+
+    // Search for a free slot in proxy_js_existing.
+    for (let i = 0; i < globalThis.proxy_js_existing.length; ++i) {
+        if (globalThis.proxy_js_existing[i] === undefined) {
+            // Free slot found, put existing_obj here and return the index.
+            globalThis.proxy_js_existing[i] = existing_obj;
+            return i;
+        }
+    }
+
+    // No free slot, so append to proxy_js_existing and return the new index.
+    globalThis.proxy_js_existing.push(existing_obj);
+    return globalThis.proxy_js_existing.length - 1;
+}
+
 // js_obj cannot be undefined
 function proxy_js_add_obj(js_obj) {
     // Search for the first free slot in proxy_js_ref.
@@ -241,6 +268,10 @@ function proxy_convert_mp_to_js_obj_jsside(value) {
         // js proxy
         const id = Module.getValue(value + 4, "i32");
         obj = proxy_js_ref[id];
+    } else if (kind === PROXY_KIND_MP_EXISTING) {
+        const id = Module.getValue(value + 4, "i32");
+        obj = globalThis.proxy_js_existing[id];
+        globalThis.proxy_js_existing[id] = undefined;
     } else {
         // obj
         const id = Module.getValue(value + 4, "i32");
@@ -257,6 +288,7 @@ function proxy_convert_mp_to_js_obj_jsside(value) {
             obj = new Proxy(target, py_proxy_handler);
         }
         globalThis.pyProxyFinalizationRegistry.register(obj, id);
+        globalThis.proxy_js_map.set(id, new WeakRef(obj));
     }
     return obj;
 }