validation: avoid masking of difficulty adjustment errors

Currently difficulty adjustment violations are not reported for
chains that branch off before the last checkpoint. Change this
by moving the checkpoint check after the difficulty check.

Backported from: 85c82b5
Original Author: Pieter Wuille <pieter.wuille@gmail.com>

src/validation.cpp

@@ -2998,20 +2998,6 @@ bool CheckBlock(const CBlock& block, CValidationState& state, bool fCheckPOW, bo
     return true;
 }
 
-static bool CheckIndexAgainstCheckpoint(const CBlockIndex* pindexPrev, CValidationState& state, const CChainParams& chainparams, const uint256& hash)
-{
-    if (*pindexPrev->phashBlock == chainparams.GetConsensus(0).hashGenesisBlock)
-        return true;
-
-    int nHeight = pindexPrev->nHeight+1;
-    // Don't accept any forks from the main chain prior to last checkpoint
-    CBlockIndex* pcheckpoint = Checkpoints::GetLastCheckpoint(chainparams.Checkpoints());
-    if (pcheckpoint && nHeight < pcheckpoint->nHeight)
-        return state.DoS(100, error("%s: forked chain older than last checkpoint (height %d)", __func__, nHeight));
-
-    return true;
-}
-
 bool IsWitnessEnabled(const CBlockIndex* pindexPrev, const Consensus::Params& params)
 {
     // Dogecoin: Disable SegWit
@@ -3079,7 +3065,8 @@ std::vector<unsigned char> GenerateCoinbaseCommitment(CBlock& block, const CBloc
 bool ContextualCheckBlockHeader(const CBlockHeader& block, CValidationState& state, const CBlockIndex* pindexPrev, int64_t nAdjustedTime)
 {
     const int nHeight = pindexPrev == NULL ? 0 : pindexPrev->nHeight + 1;
-    const Consensus::Params& consensusParams = Params().GetConsensus(nHeight);
+    const CChainParams& params = Params();
+    const Consensus::Params& consensusParams = params.GetConsensus(nHeight);
 
     // Disallow legacy blocks after merge-mining start.
     if (!consensusParams.fAllowLegacyBlocks
@@ -3101,6 +3088,16 @@ bool ContextualCheckBlockHeader(const CBlockHeader& block, CValidationState& sta
     if (block.nBits != GetNextWorkRequired(pindexPrev, &block, consensusParams))
         return state.DoS(100, false, REJECT_INVALID, "bad-diffbits", false, "incorrect proof of work");
 
+    // Check against checkpoints
+    if (fCheckpointsEnabled) {
+        // Don't accept any forks from the main chain prior to last checkpoint.
+        // GetLastCheckpoint finds the last checkpoint in MapCheckpoints that's in our
+        // MapBlockIndex.
+        CBlockIndex* pcheckpoint = Checkpoints::GetLastCheckpoint(params.Checkpoints());
+        if (pcheckpoint && nHeight < pcheckpoint->nHeight)
+            return state.DoS(100, error("%s: forked chain older than last checkpoint (height %d)", __func__, nHeight), REJECT_CHECKPOINT, "bad-fork-prior-to-checkpoint");
+    }
+
     // Check timestamp against prev
     if (block.GetBlockTime() <= pindexPrev->GetMedianTimePast())
         return state.Invalid(false, REJECT_INVALID, "time-too-old", "block's timestamp is too early");
@@ -3236,8 +3233,6 @@ static bool AcceptBlockHeader(const CBlockHeader& block, CValidationState& state
             return state.DoS(100, error("%s: prev block invalid", __func__), REJECT_INVALID, "bad-prevblk");
 
         assert(pindexPrev);
-        if (fCheckpointsEnabled && !CheckIndexAgainstCheckpoint(pindexPrev, state, chainparams, hash))
-            return error("%s: CheckIndexAgainstCheckpoint(): %s", __func__, state.GetRejectReason().c_str());
 
         if (!ContextualCheckBlockHeader(block, state, pindexPrev, GetAdjustedTime()))
             return error("%s: Consensus::ContextualCheckBlockHeader: %s, %s", __func__, hash.ToString(), FormatStateMessage(state));
@@ -3417,9 +3412,6 @@ bool TestBlockValidity(CValidationState& state, const CChainParams& chainparams,
 {
     AssertLockHeld(cs_main);
     assert(pindexPrev && pindexPrev == chainActive.Tip());
-    if (fCheckpointsEnabled && !CheckIndexAgainstCheckpoint(pindexPrev, state, chainparams, block.GetHash()))
-        return error("%s: CheckIndexAgainstCheckpoint(): %s", __func__, state.GetRejectReason().c_str());
-
     CCoinsViewCache viewNew(pcoinsTip);
     CBlockIndex indexDummy(block);
     indexDummy.pprev = pindexPrev;