feat(xsnap): isReady() eliminates need for .evaluate('null')

this lets us ensure that snapshot loading is finished without
affecting the XS engine heap state.

packages/xsnap/src/xsnap.c

@@ -320,8 +320,6 @@ ExitCode main(int argc, char* argv[])
 		if (snapshot.stream) {
 			machine = fxReadSnapshot(&snapshot, "xsnap", NULL);
 			fclose(snapshot.stream);
-			// collect after restore from snapshots for consistency
-			fx_gc(machine);
 		}
 		else
 			snapshot.error = errno;
@@ -372,6 +370,9 @@ ExitCode main(int argc, char* argv[])
 			char command = *nsbuf;
 			// fprintf(stderr, "command: len %d %c arg: %s\n", nslen, command, nsbuf + 1);
 			switch(command) {
+			case 'R': // isReady
+				fxWriteNetString(toParent, ".", "", 0);
+				break;
 			case '?':
 			case 'e':
 				xsBeginCrank(machine, gxCrankMeteringLimit);

packages/xsnap/src/xsnap.js

@@ -238,6 +238,18 @@ export function xsnap(options) {
     return Promise.race([vatCancelled, result]);
   }
 
+  /**
+   * @returns {Promise<void>}
+   */
+  async function isReady() {
+    const result = baton.then(async () => {
+      await messagesToXsnap.next(encoder.encode(`R`));
+      await runToIdle();
+    });
+    baton = result.catch(() => {});
+    return Promise.race([vatCancelled, result]);
+  }
+
   /**
    * @param {Uint8Array} message
    * @returns {Promise<RunResult<Uint8Array>>}
@@ -305,6 +317,7 @@ export function xsnap(options) {
   return freeze({
     issueCommand,
     issueStringCommand,
+    isReady,
     close,
     terminate,
     evaluate,

packages/xsnap/test/test-snapstore.js

@@ -163,7 +163,7 @@ test('create SES worker, save, restore, resume', async t => {
 
   const worker = await store.load(h, async snapshot => {
     const xs = xsnap({ name: 'ses-resume', snapshot, os: osType(), spawn });
-    await xs.evaluate('0');
+    await xs.isReady();
     return xs;
   });
   t.teardown(() => worker.close());