[Bindings] Create and use V8 context snapshots

gin/isolate_holder.cc

Index: gin/isolate_holder.cc
diff --git a/gin/isolate_holder.cc b/gin/isolate_holder.cc
index 2e707359ca70f51778e179a74c26c95060ed52a1..e996fb97c163a699ea8dd7845fc0f95a434e7846 100644
--- a/gin/isolate_holder.cc
+++ b/gin/isolate_holder.cc
@@ -35,23 +35,53 @@ IsolateHolder::IsolateHolder(
 IsolateHolder::IsolateHolder(
     scoped_refptr<base::SingleThreadTaskRunner> task_runner,
     AccessMode access_mode)
-    : IsolateHolder(std::move(task_runner), access_mode, kAllowAtomicsWait) {}
+    : IsolateHolder(std::move(task_runner),
+                    access_mode,
+                    kAllowAtomicsWait,
+                    nullptr,
+                    kDefault) {}
 
 IsolateHolder::IsolateHolder(
     scoped_refptr<base::SingleThreadTaskRunner> task_runner,
     AccessMode access_mode,
-    AllowAtomicsWaitMode atomics_wait_mode)
-    : access_mode_(access_mode) {
+    AllowAtomicsWaitMode atomics_wait_mode,
+    intptr_t* reference,
+    V8ContextMode context_mode)
+    : access_mode_(access_mode), v8_context_mode_(context_mode) {
   v8::ArrayBuffer::Allocator* allocator = g_array_buffer_allocator;
-  CHECK(allocator) << "You need to invoke gin::IsolateHolder::Initialize first";
-  v8::Isolate::CreateParams params;
-  params.entry_hook = DebugImpl::GetFunctionEntryHook();
-  params.code_event_handler = DebugImpl::GetJitCodeEventHandler();
-  params.constraints.ConfigureDefaults(base::SysInfo::AmountOfPhysicalMemory(),
-                                       base::SysInfo::AmountOfVirtualMemory());
-  params.array_buffer_allocator = allocator;
-  params.allow_atomics_wait = atomics_wait_mode == kAllowAtomicsWait;
-  isolate_ = v8::Isolate::New(params);
+
+  if (v8_context_mode_ == kTakeSnapshot) {
+    DCHECK(reference);
+    snapshot_creator_.reset(new v8::SnapshotCreator(reference));

[haraken, 2017/05/20 19:10:02]

It looks strange that we have to create the snapshot when instantiating the
IsolateHolder. Can we delay creating the snapshot to when
blink::SnapshotCreator::TakeSnapshot gets called?

I think it will simplify the code. e.g., I think we can remove
blink::SnapshotCreator::SetTakingSnapshot(true) from gin_prepare_main.cc.

[peria, 2017/05/30 08:25:43]

Done.

+    isolate_ = snapshot_creator_->GetIsolate();
+    isolate_->Exit();

[haraken, 2017/05/20 19:10:02]

Just to confirm: Does "new v8::SnapshotCreator(reference)" do all the work to
take the snapshot and thus we no longer need the isolate (and can exit
immediately) here, right?

Also is there any reason you have to call isolate->Exit()? It might be okay but
looks a bit strange because the isolate is used afterwards.

[peria, 2017/05/30 08:25:43]

Acknowledged.

+  } else {
+    CHECK(allocator)
+        << "You need to invoke gin::IsolateHolder::Initialize first";
+    v8::Isolate::CreateParams params;
+    params.entry_hook = DebugImpl::GetFunctionEntryHook();
+    params.code_event_handler = DebugImpl::GetJitCodeEventHandler();
+    params.constraints.ConfigureDefaults(
+        base::SysInfo::AmountOfPhysicalMemory(),
+        base::SysInfo::AmountOfVirtualMemory());
+    params.array_buffer_allocator = allocator;
+    params.allow_atomics_wait = atomics_wait_mode == kAllowAtomicsWait;
+
+    params.external_references = reference;
+    if (v8_context_mode_ == kUseSnapshot) {
+      CHECK(reference);
+      V8Initializer::GetV8ContextData(&startup_data_.data,
+                                      &startup_data_.raw_size);

[haraken, 2017/05/20 19:10:02]

Can we move this code to when LocalWindowProxy uses the snapshot?

In general it looks strange to do context-snapshot-related things when
instantiating IsolateHolder. I hope all the changes to this file are moved to
other places.

[peria, 2017/05/30 08:25:43]

Function name seems wrong, but this actually loads the snapshot blob file, and
does not create contexts.

+      if (startup_data_.data) {
+        params.snapshot_blob = &startup_data_;
+      } else {
+        params.snapshot_blob = nullptr;
+        v8_context_mode_ = kDefault;

[haraken, 2017/05/20 19:10:02]

Why do we need to overwrite v8_context_mode_ here?

[peria, 2017/05/30 08:25:43]

it is used to handle context creation (FromSnapshot() or New())
If it fails to load Snapshot file, we can't do Context::FromSnapshot()

+      }
+    }
+    isolate_ = v8::Isolate::New(params);
+  }
+
   isolate_data_.reset(
       new PerIsolateData(isolate_, allocator, access_mode, task_runner));
   isolate_memory_dump_provider_.reset(new V8IsolateMemoryDumpProvider(this));
@@ -85,7 +115,7 @@ IsolateHolder::~IsolateHolder() {
   isolate_memory_dump_provider_.reset();
   isolate_data_.reset();
   isolate_->Dispose();
-  isolate_ = NULL;
+  isolate_ = nullptr;
 }
 
 // static