[snapshot] support multiple contexts in the same snapshot.

src/snapshot/snapshot.h

Index: src/snapshot/snapshot.h
diff --git a/src/snapshot/snapshot.h b/src/snapshot/snapshot.h
index 479265c4d2471f14746afb07710a31ed067582ff..e332967eeebd37403688e8fd69444ad2ff57fcd2 100644
--- a/src/snapshot/snapshot.h
+++ b/src/snapshot/snapshot.h
@@ -16,6 +16,41 @@ class Isolate;
 class PartialSerializer;
 class StartupSerializer;
 
+// Wrapper around reservation sizes and the serialization payload.
+class SnapshotData : public SerializedData {
+ public:
+  // Used when producing.
+  explicit SnapshotData(const Serializer* serializer);
+
+  // Used when consuming.
+  explicit SnapshotData(const Vector<const byte> snapshot)
+      : SerializedData(const_cast<byte*>(snapshot.begin()), snapshot.length()) {
+    CHECK(IsSane());
+  }
+
+  Vector<const Reservation> Reservations() const;
+  Vector<const byte> Payload() const;
+
+  Vector<const byte> RawData() const {
+    return Vector<const byte>(data_, size_);
+  }
+
+ private:
+  bool IsSane();
+
+  // The data header consists of uint32_t-sized entries:
+  // [0] magic number and external reference count
+  // [1] version hash
+  // [2] number of reservation size entries
+  // [3] payload length
+  // ... reservations
+  // ... serialized payload
+  static const int kCheckSumOffset = kMagicNumberOffset + kInt32Size;
+  static const int kNumReservationsOffset = kCheckSumOffset + kInt32Size;
+  static const int kPayloadLengthOffset = kNumReservationsOffset + kInt32Size;
+  static const int kHeaderSize = kPayloadLengthOffset + kInt32Size;
+};
+
 class Snapshot : public AllStatic {
  public:
   // Initialize the Isolate from the internal snapshot. Returns false if no
@@ -23,7 +58,8 @@ class Snapshot : public AllStatic {
   static bool Initialize(Isolate* isolate);
   // Create a new context using the internal partial snapshot.
   static MaybeHandle<Context> NewContextFromSnapshot(
-      Isolate* isolate, Handle<JSGlobalProxy> global_proxy);
+      Isolate* isolate, Handle<JSGlobalProxy> global_proxy,
+      size_t context_index);
 
   static bool HaveASnapshotToStartFrom(Isolate* isolate);
 
@@ -36,32 +72,44 @@ class Snapshot : public AllStatic {
   static const v8::StartupData* DefaultSnapshotBlob();
 
   static v8::StartupData CreateSnapshotBlob(
-      const StartupSerializer* startup_serializer,
-      const PartialSerializer* context_serializer);
+      const SnapshotData* startup_snapshot,
+      const List<SnapshotData*>* context_snapshots);
 
 #ifdef DEBUG
   static bool SnapshotIsValid(v8::StartupData* snapshot_blob);
 #endif  // DEBUG
 
  private:
+  static int ExtractNumContexts(const v8::StartupData* data);
   static Vector<const byte> ExtractStartupData(const v8::StartupData* data);
-  static Vector<const byte> ExtractContextData(const v8::StartupData* data);
+  static Vector<const byte> ExtractContextData(const v8::StartupData* data,
+                                               int index);
 
   // Snapshot blob layout:
   // [0 - 5] pre-calculated first page sizes for paged spaces
-  // [6] serialized start up data length
-  // ... serialized start up data
-  // ... serialized context data
+  // [6] number of contexts N
+  // [7] offset to context 0
+  // [8] offset to context 1
+  // ...
+  // ... offset to context N - 1
+  // ... startup snapshot data
+  // ... context 0 snapshot data
+  // ... context 1 snapshot data
 
   static const int kNumPagedSpaces = LAST_PAGED_SPACE - FIRST_PAGED_SPACE + 1;
 
   static const int kFirstPageSizesOffset = 0;
-  static const int kStartupLengthOffset =
+  static const int kNumberOfContextsOffset =
       kFirstPageSizesOffset + kNumPagedSpaces * kInt32Size;
-  static const int kStartupDataOffset = kStartupLengthOffset + kInt32Size;
+  static const int kFirstContextOffsetOffset =
+      kNumberOfContextsOffset + kInt32Size;
+
+  static int StartupSnapshotOffset(int num_contexts) {
+    return kFirstContextOffsetOffset + num_contexts * kInt32Size;
+  }
 
-  static int ContextOffset(int startup_length) {
-    return kStartupDataOffset + startup_length;
+  static int ContextSnapshotOffsetOffset(int index) {
+    return kFirstContextOffsetOffset + index * kInt32Size;
   }
 
   DISALLOW_IMPLICIT_CONSTRUCTORS(Snapshot);
@@ -71,41 +119,6 @@ class Snapshot : public AllStatic {
 void SetSnapshotFromFile(StartupData* snapshot_blob);
 #endif
 
-// Wrapper around reservation sizes and the serialization payload.
-class SnapshotData : public SerializedData {
- public:
-  // Used when producing.
-  explicit SnapshotData(const Serializer* serializer);
-
-  // Used when consuming.
-  explicit SnapshotData(const Vector<const byte> snapshot)
-      : SerializedData(const_cast<byte*>(snapshot.begin()), snapshot.length()) {
-    CHECK(IsSane());
-  }
-
-  Vector<const Reservation> Reservations() const;
-  Vector<const byte> Payload() const;
-
-  Vector<const byte> RawData() const {
-    return Vector<const byte>(data_, size_);
-  }
-
- private:
-  bool IsSane();
-
-  // The data header consists of uint32_t-sized entries:
-  // [0] magic number and external reference count
-  // [1] version hash
-  // [2] number of reservation size entries
-  // [3] payload length
-  // ... reservations
-  // ... serialized payload
-  static const int kCheckSumOffset = kMagicNumberOffset + kInt32Size;
-  static const int kNumReservationsOffset = kCheckSumOffset + kInt32Size;
-  static const int kPayloadLengthOffset = kNumReservationsOffset + kInt32Size;
-  static const int kHeaderSize = kPayloadLengthOffset + kInt32Size;
-};
-
 }  // namespace internal
 }  // namespace v8