From 355e626643d44568915bf1a8a61e5423ac6bd2cf Mon Sep 17 00:00:00 2001
From: Dan Connolly <connolly@agoric.com>
Date: Tue, 29 Jun 2021 16:55:47 -0500
Subject: [PATCH] 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            |  5 +++--
 packages/xsnap/src/xsnap.js           | 13 +++++++++++++
 packages/xsnap/test/test-snapstore.js |  2 +-
 3 files changed, 17 insertions(+), 3 deletions(-)

diff --git a/packages/xsnap/src/xsnap.c b/packages/xsnap/src/xsnap.c
index 356abef43b7..42c6fdd9e52 100644
--- a/packages/xsnap/src/xsnap.c
+++ b/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);
diff --git a/packages/xsnap/src/xsnap.js b/packages/xsnap/src/xsnap.js
index 9fc55f63a5a..814c05990a7 100644
--- a/packages/xsnap/src/xsnap.js
+++ b/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,
diff --git a/packages/xsnap/test/test-snapstore.js b/packages/xsnap/test/test-snapstore.js
index c2dd8dc0d23..65e826971df 100644
--- a/packages/xsnap/test/test-snapstore.js
+++ b/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());