[snapshot] include version string in the startup snapshot.

This is to easier diagnose build issues involving the snapshot.
Sample error message for mismatching snapshot:

#
# Fatal error in ../../src/snapshot/snapshot-common.cc, line 286
# Version mismatch between V8 binary and snapshot.
#   V8 binary version: 6.3.1 (candidate)
#    Snapshot version: 6.3.0 (candidate)
# The snapshot consists of 2820444 bytes and contains 1 contexts.
#


R=machenbach@chromium.org

Bug: chromium:764327
Change-Id: Icdc7aeac77819b113985b424feda814a072d5406
Reviewed-on: https://chromium-review.googlesource.com/684295Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48161}

src/snapshot/code-serializer.h

@@ -102,6 +102,7 @@ class SerializedCodeData : public SerializedData {
   // ...  reservations
   // ...  code stub keys
   // ...  serialized payload
+  static const uint32_t kVersionHashOffset = kMagicNumberOffset + kUInt32Size;
   static const uint32_t kSourceHashOffset = kVersionHashOffset + kUInt32Size;
   static const uint32_t kCpuFeaturesOffset = kSourceHashOffset + kUInt32Size;
   static const uint32_t kFlagHashOffset = kCpuFeaturesOffset + kUInt32Size;

src/snapshot/serializer-common.h

@@ -290,7 +290,6 @@ class SerializedData {
   }
 
   static const uint32_t kMagicNumberOffset = 0;
-  static const uint32_t kVersionHashOffset = kMagicNumberOffset + kUInt32Size;
 
  protected:
   void SetHeaderValue(uint32_t offset, uint32_t value) {

src/snapshot/snapshot-common.cc

@@ -41,6 +41,7 @@ bool Snapshot::Initialize(Isolate* isolate) {
   if (FLAG_profile_deserialization) timer.Start();
 
   const v8::StartupData* blob = isolate->snapshot_blob();
+  CheckVersion(blob);
   Vector<const byte> startup_data = ExtractStartupData(blob);
   SnapshotData startup_snapshot_data(startup_data);
   Vector<const byte> builtin_data = ExtractBuiltinData(blob);
@@ -155,6 +156,11 @@ v8::StartupData Snapshot::CreateSnapshotBlob(
   SetHeaderValue(data, kNumberOfContextsOffset, num_contexts);
   SetHeaderValue(data, kRehashabilityOffset, can_be_rehashed ? 1 : 0);
 
+  // Write version string into snapshot data.
+  memset(data + kVersionStringOffset, 0, kVersionStringLength);
+  Version::GetString(
+      Vector<char>(data + kVersionStringOffset, kVersionStringLength));
+
   // Startup snapshot (isolate-specific data).
   uint32_t payload_offset = startup_snapshot_offset;
   uint32_t payload_length =
@@ -269,6 +275,25 @@ Vector<const byte> Snapshot::ExtractContextData(const v8::StartupData* data,
   return Vector<const byte>(context_data, context_length);
 }
 
+void Snapshot::CheckVersion(const v8::StartupData* data) {
+  char version[kVersionStringLength];
+  memset(version, 0, kVersionStringLength);
+  CHECK_LT(kVersionStringOffset + kVersionStringLength,
+           static_cast<uint32_t>(data->raw_size));
+  Version::GetString(Vector<char>(version, kVersionStringLength));
+  if (memcmp(version, data->data + kVersionStringOffset,
+             kVersionStringLength) != 0) {
+    V8_Fatal(__FILE__, __LINE__,
+             "Version mismatch between V8 binary and snapshot.\n"
+             "#   V8 binary version: %.*s\n"
+             "#    Snapshot version: %.*s\n"
+             "# The snapshot consists of %d bytes and contains %d context(s).",
+             kVersionStringLength, version, kVersionStringLength,
+             data->data + kVersionStringOffset, data->raw_size,
+             ExtractNumContexts(data));
+  }
+}
+
 template <class AllocatorT>
 SnapshotData::SnapshotData(const Serializer<AllocatorT>* serializer) {
   DisallowHeapAllocation no_gc;
@@ -286,7 +311,6 @@ SnapshotData::SnapshotData(const Serializer<AllocatorT>* serializer) {
 
   // Set header values.
   SetMagicNumber(serializer->isolate());
-  SetHeaderValue(kVersionHashOffset, Version::Hash());
   SetHeaderValue(kNumReservationsOffset, static_cast<int>(reservations.size()));
   SetHeaderValue(kPayloadLengthOffset, static_cast<int>(payload->size()));
 
@@ -303,10 +327,6 @@ SnapshotData::SnapshotData(const Serializer<AllocatorT>* serializer) {
 template SnapshotData::SnapshotData(
     const Serializer<DefaultSerializerAllocator>* serializer);
 
-bool SnapshotData::IsSane() {
-  return GetHeaderValue(kVersionHashOffset) == Version::Hash();
-}
-
 Vector<const SerializedData::Reservation> SnapshotData::Reservations() const {
   return Vector<const Reservation>(
       reinterpret_cast<const Reservation*>(data_ + kHeaderSize),

src/snapshot/snapshot.h

@@ -29,7 +29,6 @@ class SnapshotData : public SerializedData {
   // 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;
@@ -40,18 +39,14 @@ class SnapshotData : public SerializedData {
   }
 
  protected:
-  bool IsSane();
-
   // The data header consists of uint32_t-sized entries:
   // [0] magic number and (internal) external reference count
-  // [1] API-provided external reference count
-  // [2] version hash
-  // [3] number of reservation size entries
-  // [4] payload length
+  // [1] number of reservation size entries
+  // [2] payload length
   // ... reservations
   // ... serialized payload
   static const uint32_t kNumReservationsOffset =
-      kVersionHashOffset + kUInt32Size;
+      kMagicNumberOffset + kUInt32Size;
   static const uint32_t kPayloadLengthOffset =
       kNumReservationsOffset + kUInt32Size;
   static const uint32_t kHeaderSize = kPayloadLengthOffset + kUInt32Size;
@@ -67,7 +62,6 @@ class BuiltinSnapshotData final : public SnapshotData {
   // Used when consuming.
   explicit BuiltinSnapshotData(const Vector<const byte> snapshot)
       : SnapshotData(snapshot) {
-    CHECK(IsSane());
   }
 
   // Returns the serialized payload without the builtin offsets table.
@@ -141,12 +135,15 @@ class Snapshot : public AllStatic {
     WriteLittleEndianValue(data + offset, value);
   }
 
+  static void CheckVersion(const v8::StartupData* data);
+
   // Snapshot blob layout:
   // [0] number of contexts N
   // [1] rehashability
-  // [2] offset to builtins
-  // [3] offset to context 0
-  // [4] offset to context 1
+  // [2] (128 bytes) version string
+  // [3] offset to builtins
+  // [4] offset to context 0
+  // [5] offset to context 1
   // ...
   // ... offset to context N - 1
   // ... startup snapshot data
@@ -158,7 +155,11 @@ class Snapshot : public AllStatic {
   // TODO(yangguo): generalize rehashing, and remove this flag.
   static const uint32_t kRehashabilityOffset =
       kNumberOfContextsOffset + kUInt32Size;
-  static const int kBuiltinOffsetOffset = kRehashabilityOffset + kUInt32Size;
+  static const uint32_t kVersionStringOffset =
+      kRehashabilityOffset + kUInt32Size;
+  static const uint32_t kVersionStringLength = 64;
+  static const uint32_t kBuiltinOffsetOffset =
+      kVersionStringOffset + kVersionStringLength;
   static const uint32_t kFirstContextOffsetOffset =
       kBuiltinOffsetOffset + kUInt32Size;