Add a 256-bit reserved field to the block header

This field has no defined semantics. While it was added as a result of
discussions about merged mining in #724, this field will not necessarily ever be
used for that purpose.

src/chain.h

@@ -141,6 +141,7 @@ public:
     //! block header
     int nVersion;
     uint256 hashMerkleRoot;
+    uint256 hashReserved;
     unsigned int nTime;
     unsigned int nBits;
     uint256 nNonce;
@@ -166,6 +167,7 @@ public:
 
         nVersion       = 0;
         hashMerkleRoot = uint256();
+        hashReserved   = uint256();
         nTime          = 0;
         nBits          = 0;
         nNonce         = uint256();
@@ -183,6 +185,7 @@ public:
 
         nVersion       = block.nVersion;
         hashMerkleRoot = block.hashMerkleRoot;
+        hashReserved   = block.hashReserved;
         nTime          = block.nTime;
         nBits          = block.nBits;
         nNonce         = block.nNonce;
@@ -214,6 +217,7 @@ public:
         if (pprev)
             block.hashPrevBlock = pprev->GetBlockHash();
         block.hashMerkleRoot = hashMerkleRoot;
+        block.hashReserved   = hashReserved;
         block.nTime          = nTime;
         block.nBits          = nBits;
         block.nNonce         = nNonce;
@@ -321,6 +325,7 @@ public:
         READWRITE(this->nVersion);
         READWRITE(hashPrev);
         READWRITE(hashMerkleRoot);
+        READWRITE(hashReserved);
         READWRITE(nTime);
         READWRITE(nBits);
         READWRITE(nNonce);
@@ -333,6 +338,7 @@ public:
         block.nVersion        = nVersion;
         block.hashPrevBlock   = hashPrev;
         block.hashMerkleRoot  = hashMerkleRoot;
+        block.hashReserved    = hashReserved;
         block.nTime           = nTime;
         block.nBits           = nBits;
         block.nNonce          = nNonce;

src/miner.cpp

@@ -354,6 +354,7 @@ CBlockTemplate* CreateNewBlock(const CScript& scriptPubKeyIn)
 
         // Fill in header
         pblock->hashPrevBlock  = pindexPrev->GetBlockHash();
+        pblock->hashReserved   = uint256();
         UpdateTime(pblock, Params().GetConsensus(), pindexPrev);
         pblock->nBits          = GetNextWorkRequired(pindexPrev, pblock, Params().GetConsensus());
         pblock->nNonce         = uint256();

src/primitives/block.cpp

@@ -112,11 +112,12 @@ uint256 CBlock::CheckMerkleBranch(uint256 hash, const std::vector<uint256>& vMer
 std::string CBlock::ToString() const
 {
     std::stringstream s;
-    s << strprintf("CBlock(hash=%s, ver=%d, hashPrevBlock=%s, hashMerkleRoot=%s, nTime=%u, nBits=%08x, nNonce=%s, vtx=%u)\n",
+    s << strprintf("CBlock(hash=%s, ver=%d, hashPrevBlock=%s, hashMerkleRoot=%s, hashReserved=%s, nTime=%u, nBits=%08x, nNonce=%s, vtx=%u)\n",
         GetHash().ToString(),
         nVersion,
         hashPrevBlock.ToString(),
         hashMerkleRoot.ToString(),
+        hashReserved.ToString(),
         nTime, nBits, nNonce.ToString(),
         vtx.size());
     for (unsigned int i = 0; i < vtx.size(); i++)

src/primitives/block.h

@@ -25,6 +25,7 @@ public:
     int32_t nVersion;
     uint256 hashPrevBlock;
     uint256 hashMerkleRoot;
+    uint256 hashReserved;
     uint32_t nTime;
     uint32_t nBits;
     uint256 nNonce;
@@ -43,6 +44,7 @@ public:
         nVersion = this->nVersion;
         READWRITE(hashPrevBlock);
         READWRITE(hashMerkleRoot);
+        READWRITE(hashReserved);
         READWRITE(nTime);
         READWRITE(nBits);
         READWRITE(nNonce);
@@ -54,6 +56,7 @@ public:
         nVersion = CBlockHeader::CURRENT_VERSION;
         hashPrevBlock.SetNull();
         hashMerkleRoot.SetNull();
+        hashReserved.SetNull();
         nTime = 0;
         nBits = 0;
         nNonce = uint256();
@@ -115,6 +118,7 @@ public:
         block.nVersion       = nVersion;
         block.hashPrevBlock  = hashPrevBlock;
         block.hashMerkleRoot = hashMerkleRoot;
+        block.hashReserved   = hashReserved;
         block.nTime          = nTime;
         block.nBits          = nBits;
         block.nNonce         = nNonce;
@@ -155,6 +159,7 @@ public:
         nVersion = this->nVersion;
         READWRITE(hashPrevBlock);
         READWRITE(hashMerkleRoot);
+        READWRITE(hashReserved);
         READWRITE(nTime);
         READWRITE(nBits);
     }