bootstrap: make snapshot reproducible

This patch uses the new V8 API to {de}serialize context slots for snapshot in order to make the snapshot reproducible. Also added a test for the reproducibility of snapshots.

Author: joyeecheung

src/api/environment.cc

@@ -458,7 +458,13 @@ Environment* CreateEnvironment(
  if (use_snapshot) {
  context = Context::FromSnapshot(isolate,
  SnapshotData::kNodeMainContextIndex,
- {DeserializeNodeInternalFields, env})
+ v8::DeserializeInternalFieldsCallback(
+ DeserializeNodeInternalFields, env),
+ nullptr,
+ MaybeLocal<Value>(),
+ nullptr,
+ v8::DeserializeContextDataCallback(
+ DeserializeNodeContextData, env))
  .ToLocalChecked();
 
  CHECK(!context.IsEmpty());

src/node_snapshotable.cc

@@ -1155,8 +1155,11 @@ ExitCode SnapshotBuilder::CreateSnapshot(SnapshotData* out,
  CHECK_EQ(index, SnapshotData::kNodeVMContextIndex);
  index = creator->AddContext(base_context);
  CHECK_EQ(index, SnapshotData::kNodeBaseContextIndex);
- index = creator->AddContext(main_context,
- {SerializeNodeContextInternalFields, env});
+ index = creator->AddContext(
+ main_context,
+ v8::SerializeInternalFieldsCallback(SerializeNodeContextInternalFields,
+ env),
+ v8::SerializeContextDataCallback(SerializeNodeContextData, env));
  CHECK_EQ(index, SnapshotData::kNodeMainContextIndex);
  }
 
@@ -1255,6 +1258,17 @@ std::string SnapshotableObject::GetTypeName() const {
  }
 }
 
+void DeserializeNodeContextData(Local<Context> holder,
+ int index,
+ StartupData payload,
+ void* callback_data) {
+ DCHECK(index == ContextEmbedderIndex::kEnvironment ||
+ index == ContextEmbedderIndex::kRealm ||
+ index == ContextEmbedderIndex::kContextTag);
+ // This is a no-op for now. We will reset all the pointers in
+ // Environment::AssignToContext() via the realm constructor.
+}
+
 void DeserializeNodeInternalFields(Local<Object> holder,
  int index,
  StartupData payload,
@@ -1320,6 +1334,44 @@ void DeserializeNodeInternalFields(Local<Object> holder,
  }
 }
 
+StartupData SerializeNodeContextData(Local<Context> holder,
+ int index,
+ void* callback_data) {
+ DCHECK(index == ContextEmbedderIndex::kEnvironment ||
+ index == ContextEmbedderIndex::kContextifyContext ||
+ index == ContextEmbedderIndex::kRealm ||
+ index == ContextEmbedderIndex::kContextTag);
+ void* data = holder->GetAlignedPointerFromEmbedderData(index);
+ per_process::Debug(DebugCategory::MKSNAPSHOT,
+ "Serialize context data, index=%d, holder=%p, ptr=%p\n",
+ static_cast<int>(index),
+ *holder,
+ data);
+ // Serialization of contextify context is not yet supported.
+ if (index == ContextEmbedderIndex::kContextifyContext) {
+ DCHECK_NULL(data);
+ return {nullptr, 0};
+ }
+
+ // We need to use use new[] because V8 calls delete[] on the returned data.
+ int size = sizeof(ContextEmbedderIndex);
+ char* result = new char[size];
+ ContextEmbedderIndex* index_data =
+ reinterpret_cast<ContextEmbedderIndex*>(result);
+ *index_data = static_cast<ContextEmbedderIndex>(index);
+
+ // For now we just reset all of them in Environment::AssignToContext()
+ switch (index) {
+ case ContextEmbedderIndex::kEnvironment:
+ case ContextEmbedderIndex::kContextifyContext:
+ case ContextEmbedderIndex::kRealm:
+ case ContextEmbedderIndex::kContextTag:
+ return StartupData{result, size};
+ default:
+ UNREACHABLE();
+ }
+}
+
 StartupData SerializeNodeContextInternalFields(Local<Object> holder,
  int index,
  void* callback_data) {

src/node_snapshotable.h

@@ -126,10 +126,17 @@ class SnapshotableObject : public BaseObject {
 v8::StartupData SerializeNodeContextInternalFields(v8::Local<v8::Object> holder,
  int index,
  void* env);
+v8::StartupData SerializeNodeContextData(v8::Local<v8::Context> holder,
+ int index,
+ void* env);
 void DeserializeNodeInternalFields(v8::Local<v8::Object> holder,
  int index,
  v8::StartupData payload,
  void* env);
+void DeserializeNodeContextData(v8::Local<v8::Context> holder,
+ int index,
+ v8::StartupData payload,
+ void* env);
 void SerializeSnapshotableObjects(Realm* realm,
  v8::SnapshotCreator* creator,
  RealmSerializeInfo* info);

test/parallel/test-snapshot-reproducible.js

@@ -0,0 +1,30 @@
+'use strict';
+
+require('../common');
+const { spawnSyncAndExitWithoutError } = require('../common/child_process');
+const tmpdir = require('../common/tmpdir');
+const fs = require('fs');
+const assert = require('assert');
+
+function generateSnapshot() {
+ tmpdir.refresh();
+
+ spawnSyncAndExitWithoutError(
+ process.execPath,
+ [
+ '--random_seed=42',
+ '--predictable',
+ '--build-snapshot',
+ 'node:generate_default_snapshot',
+ ],
+ {
+ cwd: tmpdir.path
+ }
+ );
+ const blobPath = tmpdir.resolve('snapshot.blob');
+ return fs.readFileSync(blobPath);
+}
+
+const buf1 = generateSnapshot();
+const buf2 = generateSnapshot();
+assert.deepStrictEqual(buf1, buf2);