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bootstrap: make snapshot reproducible
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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.
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joyeecheung committed Mar 22, 2024
1 parent e96272f commit 8dc86b6
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Showing 3 changed files with 68 additions and 3 deletions.
8 changes: 7 additions & 1 deletion src/api/environment.cc
Original file line number Diff line number Diff line change
Expand Up @@ -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());
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56 changes: 54 additions & 2 deletions src/node_snapshotable.cc
Original file line number Diff line number Diff line change
Expand Up @@ -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);
}

Expand Down Expand Up @@ -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,
Expand Down Expand Up @@ -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) {
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7 changes: 7 additions & 0 deletions src/node_snapshotable.h
Original file line number Diff line number Diff line change
Expand Up @@ -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);
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