Only enable min-difficulty blocks on testnet from a particular height

The min-difficulty change is a bilateral consensus rule change, and so
must be conditionally enabled in order for the earlier section of the
chain to synchronise.

Technically this could be implemented as a network upgrade, but as this will
never be deployed to mainnet, a targeted fork will suffice.

src/chainparams.cpp

@@ -95,7 +95,7 @@ public:
         consensus.nPowMaxAdjustDown = 32; // 32% adjustment down
         consensus.nPowMaxAdjustUp = 16; // 16% adjustment up
         consensus.nPowTargetSpacing = 2.5 * 60;
-        consensus.fPowAllowMinDifficultyBlocks = false;
+        consensus.nPowAllowMinDifficultyBlocksFromHeight = boost::none;
         consensus.vUpgrades[Consensus::BASE_SPROUT].nProtocolVersion = 170002;
         consensus.vUpgrades[Consensus::BASE_SPROUT].nActivationHeight =
             Consensus::NetworkUpgrade::ALWAYS_ACTIVE;
@@ -272,7 +272,7 @@ public:
         consensus.nPowMaxAdjustDown = 32; // 32% adjustment down
         consensus.nPowMaxAdjustUp = 16; // 16% adjustment up
         consensus.nPowTargetSpacing = 2.5 * 60;
-        consensus.fPowAllowMinDifficultyBlocks = true;
+        consensus.nPowAllowMinDifficultyBlocksFromHeight = 299187;
         consensus.vUpgrades[Consensus::BASE_SPROUT].nProtocolVersion = 170002;
         consensus.vUpgrades[Consensus::BASE_SPROUT].nActivationHeight =
             Consensus::NetworkUpgrade::ALWAYS_ACTIVE;
@@ -393,7 +393,7 @@ public:
         consensus.nPowMaxAdjustDown = 0; // Turn off adjustment down
         consensus.nPowMaxAdjustUp = 0; // Turn off adjustment up
         consensus.nPowTargetSpacing = 2.5 * 60;
-        consensus.fPowAllowMinDifficultyBlocks = true;
+        consensus.nPowAllowMinDifficultyBlocksFromHeight = 0;
         consensus.vUpgrades[Consensus::BASE_SPROUT].nProtocolVersion = 170002;
         consensus.vUpgrades[Consensus::BASE_SPROUT].nActivationHeight =
             Consensus::NetworkUpgrade::ALWAYS_ACTIVE;

src/consensus/params.h

@@ -8,6 +8,8 @@
 
 #include "uint256.h"
 
+#include <boost/optional.hpp>
+
 namespace Consensus {
 
 /**
@@ -91,7 +93,7 @@ struct Params {
     NetworkUpgrade vUpgrades[MAX_NETWORK_UPGRADES];
     /** Proof of work parameters */
     uint256 powLimit;
-    bool fPowAllowMinDifficultyBlocks;
+    boost::optional<uint32_t> nPowAllowMinDifficultyBlocksFromHeight;
     int64_t nPowAveragingWindow;
     int64_t nPowMaxAdjustDown;
     int64_t nPowMaxAdjustUp;

src/miner.cpp

@@ -103,8 +103,9 @@ void UpdateTime(CBlockHeader* pblock, const Consensus::Params& consensusParams,
     pblock->nTime = std::max(pindexPrev->GetMedianTimePast()+1, GetAdjustedTime());
 
     // Updating time can change work required on testnet:
-    if (consensusParams.fPowAllowMinDifficultyBlocks)
+    if (consensusParams.nPowAllowMinDifficultyBlocksFromHeight) {
         pblock->nBits = GetNextWorkRequired(pindexPrev, pblock, consensusParams);
+    }
 }
 
 CBlockTemplate* CreateNewBlock(const CScript& scriptPubKeyIn)
@@ -726,7 +727,7 @@ void static BitcoinMiner()
                 // Update nNonce and nTime
                 pblock->nNonce = ArithToUint256(UintToArith256(pblock->nNonce) + 1);
                 UpdateTime(pblock, chainparams.GetConsensus(), pindexPrev);
-                if (chainparams.GetConsensus().fPowAllowMinDifficultyBlocks)
+                if (chainparams.GetConsensus().nPowAllowMinDifficultyBlocksFromHeight)
                 {
                     // Changing pblock->nTime can change work required on testnet:
                     hashTarget.SetCompact(pblock->nBits);

src/pow.cpp

@@ -25,7 +25,8 @@ unsigned int GetNextWorkRequired(const CBlockIndex* pindexLast, const CBlockHead
         return nProofOfWorkLimit;
 
     {
-        if (params.fPowAllowMinDifficultyBlocks)
+        if (params.nPowAllowMinDifficultyBlocksFromHeight &&
+            pindexLast->nHeight >= params.nPowAllowMinDifficultyBlocksFromHeight.get())
         {
             // Special difficulty rule for testnet:
             // If the new block's timestamp is more than 6 * 2.5 minutes