Do not store Merkle branches in the wallet.

Assume that when a wallet transaction has a valid block hash and transaction position
in it, the transaction is actually there. We're already trusting wallet data in a
much more fundamental way anyway.

To prevent backward compatibility issues, a new record is used for storing the
block locator in the wallet. Old wallets will see a wallet file synchronized up
to the genesis block, and rescan automatically.

src/wallet/wallet.cpp

@@ -702,9 +702,8 @@ bool CWallet::AddToWallet(const CWalletTx& wtxIn, bool fFromLoadWallet, CWalletD
                 wtx.hashBlock = wtxIn.hashBlock;
                 fUpdated = true;
             }
-            if (wtxIn.nIndex != -1 && (wtxIn.vMerkleBranch != wtx.vMerkleBranch || wtxIn.nIndex != wtx.nIndex))
+            if (wtxIn.nIndex != -1 && (wtxIn.nIndex != wtx.nIndex))
             {
-                wtx.vMerkleBranch = wtxIn.vMerkleBranch;
                 wtx.nIndex = wtxIn.nIndex;
                 fUpdated = true;
             }
@@ -2812,15 +2811,11 @@ int CMerkleTx::SetMerkleBranch(const CBlock& block)
             break;
     if (nIndex == (int)block.vtx.size())
     {
-        vMerkleBranch.clear();
         nIndex = -1;
         LogPrintf("ERROR: SetMerkleBranch(): couldn't find tx in block\n");
         return 0;
     }
 
-    // Fill in merkle branch
-    vMerkleBranch = block.GetMerkleBranch(nIndex);
-
     // Is the tx in a block that's in the main chain
     BlockMap::iterator mi = mapBlockIndex.find(hashBlock);
     if (mi == mapBlockIndex.end())
@@ -2846,14 +2841,6 @@ int CMerkleTx::GetDepthInMainChainINTERNAL(const CBlockIndex* &pindexRet) const
     if (!pindex || !chainActive.Contains(pindex))
         return 0;
 
-    // Make sure the merkle branch connects to this block
-    if (!fMerkleVerified)
-    {
-        if (CBlock::CheckMerkleBranch(GetHash(), vMerkleBranch, nIndex) != pindex->hashMerkleRoot)
-            return 0;
-        fMerkleVerified = true;
-    }
-
     pindexRet = pindex;
     return chainActive.Height() - pindex->nHeight + 1;
 }

src/wallet/wallet.h

@@ -151,13 +151,8 @@ private:
 
 public:
     uint256 hashBlock;
-    std::vector<uint256> vMerkleBranch;
     int nIndex;
 
-    // memory only
-    mutable bool fMerkleVerified;
-
-
     CMerkleTx()
     {
         Init();
@@ -172,13 +167,13 @@ public:
     {
         hashBlock = uint256();
         nIndex = -1;
-        fMerkleVerified = false;
     }
 
     ADD_SERIALIZE_METHODS;
 
     template <typename Stream, typename Operation>
     inline void SerializationOp(Stream& s, Operation ser_action, int nType, int nVersion) {
+        std::vector<uint256> vMerkleBranch; // For compatibility with older versions.
         READWRITE(*(CTransaction*)this);
         nVersion = this->nVersion;
         READWRITE(hashBlock);

src/wallet/walletdb.cpp

@@ -131,12 +131,14 @@ bool CWalletDB::EraseWatchOnly(const CScript &dest)
 bool CWalletDB::WriteBestBlock(const CBlockLocator& locator)
 {
     nWalletDBUpdated++;
-    return Write(std::string("bestblock"), locator);
+    Write(std::string("bestblock"), CBlockLocator()); // Write empty block locator so versions that require a merkle branch automatically rescan
+    return Write(std::string("bestblock_nomerkle"), locator);
 }
 
 bool CWalletDB::ReadBestBlock(CBlockLocator& locator)
 {
-    return Read(std::string("bestblock"), locator);
+    if (Read(std::string("bestblock"), locator) && !locator.vHave.empty()) return true;
+    return Read(std::string("bestblock_nomerkle"), locator);
 }
 
 bool CWalletDB::WriteOrderPosNext(int64_t nOrderPosNext)