Refactor GetTxid() into UpdateTxid() to match coding style of hash member variable.

UpdateTxid() is called alongside UpdateHash() when a CTransaction is
deserialized or constructed.  GetTxid() now returns a const reference.

src/primitives/transaction.cpp

@@ -132,6 +132,7 @@ CTransaction::CTransaction(const CMutableTransaction &tx) : nVersion(tx.nVersion
                                                             joinSplitPubKey(tx.joinSplitPubKey), joinSplitSig(tx.joinSplitSig)
 {
     UpdateHash();
+    UpdateTxid();
 }
 
 CTransaction& CTransaction::operator=(const CTransaction &tx) {
@@ -143,6 +144,7 @@ CTransaction& CTransaction::operator=(const CTransaction &tx) {
     *const_cast<uint256*>(&joinSplitPubKey) = tx.joinSplitPubKey;
     *const_cast<joinsplit_sig_t*>(&joinSplitSig) = tx.joinSplitSig;
     *const_cast<uint256*>(&hash) = tx.hash;
+    *const_cast<uint256*>(&txid) = tx.txid;
     return *this;
 }
 
@@ -227,7 +229,7 @@ std::string CTransaction::ToString() const
 
 // Return a txid which is non-malleable.
 // Signature data is cleared before the transaction is serialized and hashed.
-uint256 CTransaction::GetTxid() const
+void CTransaction::UpdateTxid() const
 {
     // Create a deep copy of this transaction
     CMutableTransaction tx(*this);
@@ -241,10 +243,9 @@ uint256 CTransaction::GetTxid() const
     tx.joinSplitSig.assign(0);
 
     // Return double SHA256 hash
-    return tx.GetSerializeHash();
+    *const_cast<uint256*>(&txid) = tx.GetSerializeHash();
 }
 
-
 // Return a txid which is non-malleable.
 uint256 CMutableTransaction::GetTxid() const
 {

src/primitives/transaction.h

@@ -288,6 +288,8 @@ private:
     /** Memory only. */
     const uint256 hash;
     void UpdateHash() const;
+    uint256 txid;
+    void UpdateTxid() const;
 
 public:
     typedef boost::array<unsigned char, 64> joinsplit_sig_t;
@@ -331,8 +333,10 @@ public:
                 READWRITE(*const_cast<joinsplit_sig_t*>(&joinSplitSig));
             }
         }
-        if (ser_action.ForRead())
+        if (ser_action.ForRead()) {
             UpdateHash();
+            UpdateTxid();
+        }
     }
 
     bool IsNull() const {
@@ -374,9 +378,10 @@ public:
 
     std::string ToString() const;
 
-    // Return the txid which is the double SHA256 hash of the transaction.
-    uint256 GetTxid() const;
-
+    // Return the txid, which is the double SHA256 hash over portions of the transaction.
+    const uint256& GetTxid() const {
+        return txid;
+    }
 };
 
 /** A mutable version of CTransaction. */