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/main.cpp

@@ -650,10 +650,35 @@ bool IsFinalTx(const CTransaction &tx, int nBlockHeight, int64_t nBlockTime)
     return true;
 }
 
-bool CheckFinalTx(const CTransaction &tx)
+bool CheckFinalTx(const CTransaction &tx, int flags)
 {
     AssertLockHeld(cs_main);
-    return IsFinalTx(tx, chainActive.Height() + 1, GetAdjustedTime());
+
+    // By convention a negative value for flags indicates that the
+    // current network-enforced consensus rules should be used. In
+    // a future soft-fork scenario that would mean checking which
+    // rules would be enforced for the next block and setting the
+    // appropriate flags. At the present time no soft-forks are
+    // scheduled, so no flags are set.
+    flags = std::max(flags, 0);
+
+    // CheckFinalTx() uses chainActive.Height()+1 to evaluate
+    // nLockTime because when IsFinalTx() is called within
+    // CBlock::AcceptBlock(), the height of the block *being*
+    // evaluated is what is used. Thus if we want to know if a
+    // transaction can be part of the *next* block, we need to call
+    // IsFinalTx() with one more than chainActive.Height().
+    const int nBlockHeight = chainActive.Height() + 1;
+
+    // Timestamps on the other hand don't get any special treatment,
+    // because we can't know what timestamp the next block will have,
+    // and there aren't timestamp applications where it matters.
+    // However this changes once median past time-locks are enforced:
+    const int64_t nBlockTime = (flags & LOCKTIME_MEDIAN_TIME_PAST)
+                             ? chainActive.Tip()->GetMedianTimePast()
+                             : GetAdjustedTime();
+
+    return IsFinalTx(tx, nBlockHeight, nBlockTime);
 }
 
 unsigned int GetLegacySigOpCount(const CTransaction& tx)
@@ -797,7 +822,7 @@ bool AcceptToMemoryPool(CTxMemPool& pool, CValidationState &state, const CTransa
     // Only accept nLockTime-using transactions that can be mined in the next
     // block; we don't want our mempool filled up with transactions that can't
     // be mined yet.
-    if (!CheckFinalTx(tx))
+    if (!CheckFinalTx(tx, STANDARD_LOCKTIME_VERIFY_FLAGS))
         return state.DoS(0, false, REJECT_NONSTANDARD, "non-final");
 
     // is it already in the memory pool?
@@ -2723,10 +2748,15 @@ bool ContextualCheckBlock(const CBlock& block, CValidationState& state, CBlockIn
     const Consensus::Params& consensusParams = Params().GetConsensus();
 
     // Check that all transactions are finalized
-    BOOST_FOREACH(const CTransaction& tx, block.vtx)
-        if (!IsFinalTx(tx, nHeight, block.GetBlockTime())) {
+    BOOST_FOREACH(const CTransaction& tx, block.vtx) {
+        int nLockTimeFlags = 0;
+        int64_t nLockTimeCutoff = (nLockTimeFlags & LOCKTIME_MEDIAN_TIME_PAST)
+                                ? pindexPrev->GetMedianTimePast()
+                                : block.GetBlockTime();
+        if (!IsFinalTx(tx, nHeight, nLockTimeCutoff)) {
             return state.DoS(10, error("%s: contains a non-final transaction", __func__), REJECT_INVALID, "bad-txns-nonfinal");
         }
+    }
 
     // Enforce block.nVersion=2 rule that the coinbase starts with serialized block height
     // if 750 of the last 1,000 blocks are version 2 or greater (51/100 if testnet):