Introduce CHashVerifier to hash read data

This is necessary later, when we drop the nVersion field from the undo
data. At that point deserializing and reserializing the data won't
roundtrip anymore, and thus that approach can't be used to verify
checksums anymore.

With this CHashVerifier approach, we can deserialize while hashing the
exact serialized form that was used. This is both more efficient and
more correct in that case.

2 files changed, 39 insertions, 5 deletions

diff --git a/src/hash.h b/src/hash.h
index eacb8f04fe..b8de19c0fd 100644
--- a/src/hash.h
+++ b/src/hash.h
@@ -160,6 +160,41 @@ public:
     }
 };
 
+/** Reads data from an underlying stream, while hashing the read data. */
+template<typename Source>
+class CHashVerifier : public CHashWriter
+{
+private:
+    Source* source;
+
+public:
+    CHashVerifier(Source* source_) : CHashWriter(source_->GetType(), source_->GetVersion()), source(source_) {}
+
+    void read(char* pch, size_t nSize)
+    {
+        source->read(pch, nSize);
+        this->write(pch, nSize);
+    }
+
+    void ignore(size_t nSize)
+    {
+        char data[1024];
+        while (nSize > 0) {
+            size_t now = std::min<size_t>(nSize, 1024);
+            read(data, now);
+            nSize -= now;
+        }
+    }
+
+    template<typename T>
+    CHashVerifier<Source>& operator>>(T& obj)
+    {
+        // Unserialize from this stream
+        ::Unserialize(*this, obj);
+        return (*this);
+    }
+};
+
 /** Compute the 256-bit hash of an object's serialization. */
 template<typename T>
 uint256 SerializeHash(const T& obj, int nType=SER_GETHASH, int nVersion=PROTOCOL_VERSION)
diff --git a/src/validation.cpp b/src/validation.cpp
index ed94be5c26..fc7e129c05 100644
--- a/src/validation.cpp
+++ b/src/validation.cpp
@@ -1210,8 +1210,10 @@ bool UndoReadFromDisk(CBlockUndo& blockundo, const CDiskBlockPos& pos, const uin
 
     // Read block
     uint256 hashChecksum;
+    CHashVerifier<CAutoFile> verifier(&filein); // We need a CHashVerifier as reserializing may lose data
     try {
-        filein >> blockundo;
+        verifier << hashBlock;
+        verifier >> blockundo;
         filein >> hashChecksum;
     }
     catch (const std::exception& e) {
@@ -1219,10 +1221,7 @@ bool UndoReadFromDisk(CBlockUndo& blockundo, const CDiskBlockPos& pos, const uin
     }
 
     // Verify checksum
-    CHashWriter hasher(SER_GETHASH, PROTOCOL_VERSION);
-    hasher << hashBlock;
-    hasher << blockundo;
-    if (hashChecksum != hasher.GetHash())
+    if (hashChecksum != verifier.GetHash())
         return error("%s: Checksum mismatch", __func__);
 
     return true;