Add rules--presently disabled--for using GetMedianTimePast as endpoint for lock-time calculations

The lock-time code currently uses CBlock::nTime as the cutoff point for time based locked transactions. This has the unfortunate outcome of creating a perverse incentive for miners to lie about the time of a block in order to collect more fees by including transactions that by wall clock determination have not yet matured. By using CBlockIndex::GetMedianTimePast from the prior block instead, the self-interested miner no longer gains from generating blocks with fraudulent timestamps. Users can compensate for this change by simply adding an hour (3600 seconds) to their time-based lock times.

If enforced, this would be a soft-fork change. This commit only adds the functionality on an unexecuted code path, without changing the behaviour of Bitcoin Core.

src/miner.cpp

@@ -148,6 +148,7 @@ CBlockTemplate* CreateNewBlock(const CScript& scriptPubKeyIn)
         CBlockIndex* pindexPrev = chainActive.Tip();
         const int nHeight = pindexPrev->nHeight + 1;
         pblock->nTime = GetAdjustedTime();
+        const int64_t nMedianTimePast = pindexPrev->GetMedianTimePast();
         CCoinsViewCache view(pcoinsTip);
 
         // Priority order to process transactions
@@ -162,7 +163,12 @@ CBlockTemplate* CreateNewBlock(const CScript& scriptPubKeyIn)
              mi != mempool.mapTx.end(); ++mi)
         {
             const CTransaction& tx = mi->GetTx();
-            if (tx.IsCoinBase() || !IsFinalTx(tx, nHeight, pblock->nTime))
+
+            int64_t nLockTimeCutoff = (STANDARD_LOCKTIME_VERIFY_FLAGS & LOCKTIME_MEDIAN_TIME_PAST)
+                                    ? nMedianTimePast
+                                    : pblock->GetBlockTime();
+
+            if (tx.IsCoinBase() || !IsFinalTx(tx, nHeight, nLockTimeCutoff))
                 continue;
 
             COrphan* porphan = NULL;