fix(marshal): serialize empty objects as data, not pass-by-reference

Finally change the serialization of a plain empty object, i.e. `harden({})`,
from a pass-by-reference "marker" to plain pass-by-copy data. We think we've
identified all the places where an empty object was used represent a marker,
and changed them to use `Far(iface, {})` instead, which triggers
pass-by-reference.

refs #2018

packages/marshal/src/marshal.js

@@ -289,15 +289,6 @@ function isPassByCopyRecord(val) {
   const descs = getOwnPropertyDescriptors(val);
   const descKeys = ownKeys(descs);
 
-  // Empty non-array objects must be registered with Far/Remotable, or Data
-  // This causes a warning for now, eventually it will become an
-  // error, then (TODO) it will go back to pass-by-copy.
-  // See https://github.com/Agoric/agoric-sdk/issues/2018
-  if (descKeys.length === 0 && proto === objectPrototype) {
-    // console.log(`--- @@marshal: empty object without Data/Far/Remotable`);
-    // assert.fail(X`empty object without Data/Far/Remotable`);
-    return false;
-  }
   for (const descKey of descKeys) {
     if (typeof descKey === 'symbol') {
       return false;

packages/marshal/test/test-marshal-stringify.js

@@ -45,12 +45,6 @@ test('marshal stringify errors', t => {
   t.throws(() => stringify(Far('y', {})), {
     message: /Marshal's stringify rejects presences and promises .*/,
   });
-
-  // This is due to https://github.com/Agoric/agoric-sdk/issues/2018
-  // and should no longer be an error once this is fixed.
-  t.throws(() => stringify(harden({})), {
-    message: /Marshal's stringify rejects presences and promises .*/,
-  });
 });
 
 test('marshal parse errors', t => {

packages/marshal/test/test-marshal.js

@@ -25,6 +25,7 @@ export const roundTripPairs = harden([
     [1, 2],
   ],
   [{ foo: 1 }, { foo: 1 }],
+  [{}, {}],
   [
     { a: 1, b: 2 },
     { a: 1, b: 2 },
@@ -503,13 +504,7 @@ test('records', t => {
   );
 
   // harden({})
-  // old: pass-by-ref without complaint
-  // interim1: pass-by-ref with warning
-  // interim2: rejected
-  // final: pass-by-copy without complaint
-  t.deepEqual(ser(build()), noIface); // old+interim1
-  // t.throws(() => ser(harden({})), { message: /??/ }, 'unmarked empty object rejected'); // int2
-  // t.deepEqual(ser(build()), emptyData); // final
+  t.deepEqual(ser(build()), emptyData);
 
   // Data({key1: 'data'})
   // old: not applicable, Data() not yet added
@@ -518,8 +513,7 @@ test('records', t => {
   const key1Data = { body: JSON.stringify({ key1: 'data' }), slots: [] };
   t.deepEqual(ser(build('enumStringData', 'data')), key1Data);
 
-  // Serialized data should roundtrip properly. The first case here will
-  // trigger a warning during interim1.
+  // Serialized data should roundtrip properly
   t.deepEqual(unser(ser(harden({}))), {});
   t.deepEqual(unser(ser(harden({ key1: 'data' }))), { key1: 'data' });