SnapshotCreator: start from existing snapshot if we have one

src/snapshot/code-serializer.cc

Index: src/snapshot/code-serializer.cc
diff --git a/src/snapshot/code-serializer.cc b/src/snapshot/code-serializer.cc
index 7f57f0aa645bab4419a173ad13477789d894010f..1205991c9a4f6c9dd7dacb6e509227e621508b09 100644
--- a/src/snapshot/code-serializer.cc
+++ b/src/snapshot/code-serializer.cc
@@ -387,6 +387,9 @@ SerializedCodeData::SerializedCodeData(const List<byte>* payload,
   SetHeaderValue(kNumCodeStubKeysOffset, num_stub_keys);
   SetHeaderValue(kPayloadLengthOffset, payload->length());
 
+  // Zero out any padding in the header.
+  memset(data_ + kUnalignedHeaderSize, 0, kHeaderSize - kUnalignedHeaderSize);
+
   // Copy reservation chunk sizes.
   CopyBytes(data_ + kHeaderSize, reinterpret_cast<byte*>(reservations.begin()),
             reservation_size);
@@ -395,6 +398,7 @@ SerializedCodeData::SerializedCodeData(const List<byte>* payload,
   CopyBytes(data_ + kHeaderSize + reservation_size,
             reinterpret_cast<byte*>(stub_keys->begin()), stub_keys_size);
 
+  // Zero out any padding before the payload.
   memset(data_ + payload_offset, 0, padded_payload_offset - payload_offset);
 
   // Copy serialized data.
@@ -411,10 +415,14 @@ SerializedCodeData::SanityCheckResult SerializedCodeData::SanityCheck(
   if (this->size_ < kHeaderSize) return INVALID_HEADER;
   uint32_t magic_number = GetMagicNumber();
   if (magic_number != ComputeMagicNumber(isolate)) return MAGIC_NUMBER_MISMATCH;
+  if (GetExtraReferences() > GetExtraReferences(isolate)) {
+    return MAGIC_NUMBER_MISMATCH;
+  }
   uint32_t version_hash = GetHeaderValue(kVersionHashOffset);
   uint32_t source_hash = GetHeaderValue(kSourceHashOffset);
   uint32_t cpu_features = GetHeaderValue(kCpuFeaturesOffset);
   uint32_t flags_hash = GetHeaderValue(kFlagHashOffset);
+  uint32_t payload_length = GetHeaderValue(kPayloadLengthOffset);
   uint32_t c1 = GetHeaderValue(kChecksum1Offset);
   uint32_t c2 = GetHeaderValue(kChecksum2Offset);
   if (version_hash != Version::Hash()) return VERSION_MISMATCH;
@@ -423,6 +431,12 @@ SerializedCodeData::SanityCheckResult SerializedCodeData::SanityCheck(
     return CPU_FEATURES_MISMATCH;
   }
   if (flags_hash != FlagList::Hash()) return FLAGS_MISMATCH;
+  uint32_t max_payload_length =
+      this->size_ -
+      POINTER_SIZE_ALIGN(kHeaderSize +
+                         GetHeaderValue(kNumReservationsOffset) * kInt32Size +
+                         GetHeaderValue(kNumCodeStubKeysOffset) * kInt32Size);
+  if (payload_length > max_payload_length) return LENGTH_MISMATCH;
   if (!Checksum(DataWithoutHeader()).Check(c1, c2)) return CHECKSUM_MISMATCH;
   return CHECK_SUCCESS;
 }