Cache tid in STTx:

The digest for a transaction (its transaction ID, or tid) is
computed once upon constructed when the STTx is deserialized.
Subsequent calls to retrieve the digest use the cached value.

Any code which modifies the STTx and then attempts to
retrieve the digest will terminate the process with a
logic error contract violation.

* Nested types removed
* All STTx are contained as const
  (Except in transaction sign, which must modify)
* tid in STTx is computed once on deserialization

src/ripple/protocol/impl/STTx.cpp

@@ -27,6 +27,7 @@
 #include <ripple/protocol/STArray.h>
 #include <ripple/protocol/TxFlags.h>
 #include <ripple/protocol/types.h>
+#include <ripple/basics/contract.h>
 #include <ripple/basics/Log.h>
 #include <ripple/basics/StringUtilities.h>
 #include <ripple/json/to_string.h>
@@ -34,6 +35,7 @@
 #include <beast/cxx14/memory.h> // <memory>
 #include <boost/format.hpp>
 #include <array>
+#include <utility>
 
 namespace ripple {
 
@@ -74,6 +76,8 @@ STTx::STTx (STObject&& object)
             "Transaction not legal for format";
         throw std::runtime_error ("transaction not valid");
     }
+
+    tid_ = getHash(HashPrefix::transactionID);
 }
 
 STTx::STTx (SerialIter& sit)
@@ -106,6 +110,8 @@ STTx::STTx (SerialIter& sit)
             "Transaction not legal for format";
         throw std::runtime_error ("transaction not valid");
     }
+
+    tid_ = getHash(HashPrefix::transactionID);
 }
 
 std::string
@@ -161,7 +167,10 @@ STTx::getSigningHash () const
 uint256
 STTx::getTransactionID () const
 {
-    return getHash (HashPrefix::transactionID);
+    assert(tid_);
+    if (! tid_)
+        LogicError("digest is undefined");
+    return *tid_;
 }
 
 Blob STTx::getSignature () const
@@ -180,6 +189,7 @@ void STTx::sign (RippleAddress const& private_key)
 {
     Blob const signature = private_key.accountPrivateSign (getSigningData (*this));
     setFieldVL (sfTxnSignature, signature);
+    tid_ = getHash(HashPrefix::transactionID);
 }
 
 bool STTx::checkSign(bool allowMultiSign) const
@@ -499,4 +509,13 @@ bool passesLocalChecks (STObject const& st, std::string& reason)
     return true;
 }
 
+std::shared_ptr<STTx const>
+sterilize (STTx const& stx)
+{
+    Serializer s;
+    stx.add(s);
+    SerialIter sit(s.slice());
+    return std::make_shared<STTx const>(std::ref(sit));
+}
+
 } // ripple