From 79cc13c586603e940c3d88ad480bb8510f271359 Mon Sep 17 00:00:00 2001
From: Mathieu Hofman <mathieu@agoric.com>
Date: Mon, 12 Dec 2022 23:50:37 +0000
Subject: [PATCH] feat(swingset): handle vc syscalls during transcript replay

---
 .../src/kernel/vat-loader/manager-helper.js   |  8 +-
 .../vat-loader/manager-subprocess-xsnap.js    |  3 +-
 .../src/kernel/vat-loader/transcript.js       | 74 +++++++++++++++++--
 3 files changed, 75 insertions(+), 10 deletions(-)

diff --git a/packages/SwingSet/src/kernel/vat-loader/manager-helper.js b/packages/SwingSet/src/kernel/vat-loader/manager-helper.js
index 51f79e24ed40..e12bbee44e21 100644
--- a/packages/SwingSet/src/kernel/vat-loader/manager-helper.js
+++ b/packages/SwingSet/src/kernel/vat-loader/manager-helper.js
@@ -103,7 +103,7 @@ import { makeTranscriptManager } from './transcript.js';
  * @param {KernelSlog} kernelSlog
  * @param {(vso: VatSyscallObject) => VatSyscallResult} vatSyscallHandler
  * @param {boolean} workerCanBlock
- * @param {(vatID: any, originalSyscall: any, newSyscall: any) => Error | undefined} [compareSyscalls]
+ * @param {(vatID: any, originalSyscall: any, newSyscall: any) => import('./transcript.js').CompareSyscallsResult} [compareSyscalls]
  * @param {boolean} [useTranscript]
  * @returns {ManagerKit}
  */
@@ -247,7 +247,9 @@ function makeManagerKit(
       // but if the puppy deviates one inch from previous twitches, explode
       kernelSlog.syscall(vatID, undefined, vso);
       const vres = transcriptManager.simulateSyscall(vso);
-      return vres;
+      if (vres) {
+        return vres;
+      }
     }
 
     const vres = vatSyscallHandler(vso);
@@ -256,7 +258,7 @@ function makeManagerKit(
     if (successFlag === 'ok' && data && !workerCanBlock) {
       console.log(`warning: syscall returns data, but worker cannot get it`);
     }
-    if (transcriptManager) {
+    if (transcriptManager && !transcriptManager.inReplay()) {
       transcriptManager.addSyscall(vso, vres);
     }
     return vres;
diff --git a/packages/SwingSet/src/kernel/vat-loader/manager-subprocess-xsnap.js b/packages/SwingSet/src/kernel/vat-loader/manager-subprocess-xsnap.js
index 9ffd730e917b..6ef10e60ed0f 100644
--- a/packages/SwingSet/src/kernel/vat-loader/manager-subprocess-xsnap.js
+++ b/packages/SwingSet/src/kernel/vat-loader/manager-subprocess-xsnap.js
@@ -2,6 +2,7 @@
 import { assert, details as X, q } from '@agoric/assert';
 import { ExitCode } from '@agoric/xsnap/api.js';
 import { makeManagerKit } from './manager-helper.js';
+import { requireIdenticalExceptStableVCSyscalls } from './transcript.js';
 
 import {
   insistVatSyscallObject,
@@ -55,7 +56,7 @@ export function makeXsSubprocessFactory({
     const {
       name: vatName,
       metered,
-      compareSyscalls,
+      compareSyscalls = requireIdenticalExceptStableVCSyscalls,
       useTranscript,
       sourcedConsole,
     } = managerOptions;
diff --git a/packages/SwingSet/src/kernel/vat-loader/transcript.js b/packages/SwingSet/src/kernel/vat-loader/transcript.js
index e90100fea3c4..66b83885e3c1 100644
--- a/packages/SwingSet/src/kernel/vat-loader/transcript.js
+++ b/packages/SwingSet/src/kernel/vat-loader/transcript.js
@@ -1,5 +1,16 @@
 import djson from '../../lib/djson.js';
 
+const missingSyscall = Symbol('missing transcript syscall');
+const extraSyscall = Symbol('extra transcript syscall');
+
+/** @typedef {typeof missingSyscall | typeof extraSyscall | Error | undefined} CompareSyscallsResult */
+
+/**
+ * @param {any} vatID
+ * @param {object} originalSyscall
+ * @param {object} newSyscall
+ * @returns {CompareSyscallsResult}
+ */
 export function requireIdentical(vatID, originalSyscall, newSyscall) {
   if (djson.stringify(originalSyscall) !== djson.stringify(newSyscall)) {
     console.error(`anachrophobia strikes vat ${vatID}`);
@@ -10,6 +21,39 @@ export function requireIdentical(vatID, originalSyscall, newSyscall) {
   return undefined;
 }
 
+const vcSyscallRE = /^vc\.\d+\.\|(?:schemata|label)$/;
+
+/**
+ * @param {any} vatID
+ * @param {object} originalSyscall
+ * @param {object} newSyscall
+ * @returns {CompareSyscallsResult}
+ */
+export function requireIdenticalExceptStableVCSyscalls(
+  vatID,
+  originalSyscall,
+  newSyscall,
+) {
+  const error = requireIdentical(vatID, originalSyscall, newSyscall);
+
+  if (error) {
+    if (
+      originalSyscall[0] === 'vatstoreGet' &&
+      vcSyscallRE.test(originalSyscall[1])
+    ) {
+      console.warn(`  mitigation: ignoring extra vc syscall`);
+      return extraSyscall;
+    }
+
+    if (newSyscall[0] === 'vatstoreGet' && vcSyscallRE.test(newSyscall[1])) {
+      console.warn(`  mitigation: falling through to syscall handler`);
+      return missingSyscall;
+    }
+  }
+
+  return error;
+}
+
 export function makeTranscriptManager(
   vatKeeper,
   vatID,
@@ -59,13 +103,31 @@ export function makeTranscriptManager(
   let replayError;
 
   function simulateSyscall(newSyscall) {
-    const s = playbackSyscalls.shift();
-    const newReplayError = compareSyscalls(vatID, s.d, newSyscall);
-    if (newReplayError) {
-      replayError = newReplayError;
-      throw replayError;
+    while (playbackSyscalls.length) {
+      const compareError = compareSyscalls(
+        vatID,
+        playbackSyscalls[0].d,
+        newSyscall,
+      );
+
+      if (compareError === missingSyscall) {
+        return undefined;
+      }
+
+      const s = playbackSyscalls.shift();
+
+      if (!compareError) {
+        return s.response;
+      } else if (compareError !== extraSyscall) {
+        replayError = compareError;
+        break;
+      }
+    }
+
+    if (!replayError) {
+      replayError = new Error(`historical inaccuracy in replay of ${vatID}`);
     }
-    return s.response;
+    throw replayError;
   }
 
   function finishReplayDelivery(dnum) {