SnapshotCreator: start from existing snapshot if we have one

src/snapshot/snapshot.h

Index: src/snapshot/snapshot.h
diff --git a/src/snapshot/snapshot.h b/src/snapshot/snapshot.h
index 010072a694f67568f2299452555628d3d26ed30f..7a7274cae8b4b4aa2167b93e3c46fd097440e887 100644
--- a/src/snapshot/snapshot.h
+++ b/src/snapshot/snapshot.h
@@ -39,13 +39,15 @@ class SnapshotData : public SerializedData {
   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
+  // [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
   // ... reservations
   // ... serialized payload
-  static const int kCheckSumOffset = kMagicNumberOffset + kInt32Size;
+  static const int kCheckSumOffset =

[Yang, 2017/03/07 11:28:11]

Now that I see it, this name is a bit misleading. Can we either call this
kVersionHashOffset, or move this and the one in SerializedCodeData into
SerializedData?

[Jakob Kummerow, 2017/03/07 12:39:15]

Done (moved).

+      kExtraExternalReferencesOffset + kInt32Size;
   static const int kNumReservationsOffset = kCheckSumOffset + kInt32Size;
   static const int kPayloadLengthOffset = kNumReservationsOffset + kInt32Size;
   static const int kHeaderSize = kPayloadLengthOffset + kInt32Size;