[snapshot] revisit snapshot API.

src/snapshot/snapshot-common.cc

 // Copyright 2006-2008 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // The common functionality when building with or without snapshots.
 
 #include "src/snapshot/snapshot.h"
 
 #include "src/api.h"
 #include "src/base/platform/platform.h"
 #include "src/full-codegen/full-codegen.h"
 #include "src/snapshot/deserializer.h"
 #include "src/snapshot/snapshot-source-sink.h"
 #include "src/version.h"
 
 namespace v8 {
 namespace internal {
 
 #ifdef DEBUG
 bool Snapshot::SnapshotIsValid(v8::StartupData* snapshot_blob) {
   return Snapshot::ExtractNumContexts(snapshot_blob) > 0;
 }
 #endif  // DEBUG
 
-
-bool Snapshot::HaveASnapshotToStartFrom(Isolate* isolate) {
+bool Snapshot::HasContextSnapshot(Isolate* isolate, size_t index) {
   // Do not use snapshots if the isolate is used to create snapshots.
-  return isolate->snapshot_blob() != NULL &&
-         isolate->snapshot_blob()->data != NULL;
+  const v8::StartupData* blob = isolate->snapshot_blob();
+  if (blob == nullptr) return false;
+  if (blob->data == nullptr) return false;
+  size_t num_contexts = static_cast<size_t>(ExtractNumContexts(blob));
+  return index < num_contexts;
 }
 
 
 uint32_t Snapshot::SizeOfFirstPage(Isolate* isolate, AllocationSpace space) {
   DCHECK(space >= FIRST_PAGED_SPACE && space <= LAST_PAGED_SPACE);
   if (!isolate->snapshot_available()) {
     return static_cast<uint32_t>(MemoryAllocator::PageAreaSize(space));
   }
   uint32_t size;
   int offset = kFirstPageSizesOffset + (space - FIRST_PAGED_SPACE) * kInt32Size;
@@ skipping 242 matching lines @@
 Vector<const byte> SnapshotData::Payload() const {
   int reservations_size = GetHeaderValue(kNumReservationsOffset) * kInt32Size;
   const byte* payload = data_ + kHeaderSize + reservations_size;
   int length = GetHeaderValue(kPayloadLengthOffset);
   DCHECK_EQ(data_ + size_, payload + length);
   return Vector<const byte>(payload, length);
 }
 
 }  // namespace internal
 }  // namespace v8