Reduce checkpoints' effect on consensus.

Instead of only checking height to decide whether to disable script checks,
actually check whether a block is an ancestor of a checkpoint, up to which
headers have been validated. This means that we don't have to prevent
accepting a side branch anymore - it will be safe, just less fast to
do.

We still need to prevent being fed a multitude of low-difficulty headers
filling up our memory. The mechanism for that is unchanged for now: once
a checkpoint is reached with headers, no headers chain branching off before
that point are allowed anymore.

src/chain.cpp

@@ -82,9 +82,10 @@ CBlockIndex* CBlockIndex::GetAncestor(int height)
     while (heightWalk > height) {
         int heightSkip = GetSkipHeight(heightWalk);
         int heightSkipPrev = GetSkipHeight(heightWalk - 1);
-        if (heightSkip == height ||
+        if (pindexWalk->pskip != NULL &&
+            (heightSkip == height ||
              (heightSkip > height && !(heightSkipPrev < heightSkip - 2 &&
-                                      heightSkipPrev >= height))) {
+                                       heightSkipPrev >= height)))) {
             // Only follow pskip if pprev->pskip isn't better than pskip->pprev.
             pindexWalk = pindexWalk->pskip;
             heightWalk = heightSkip;

src/checkpoints.cpp

@@ -24,15 +24,6 @@ namespace Checkpoints {
      */
     static const double SIGCHECK_VERIFICATION_FACTOR = 5.0;
 
-    bool CheckBlock(const CCheckpointData& data, int nHeight, const uint256& hash)
-    {
-        const MapCheckpoints& checkpoints = data.mapCheckpoints;
-
-        MapCheckpoints::const_iterator i = checkpoints.find(nHeight);
-        if (i == checkpoints.end()) return true;
-        return hash == i->second;
-    }
-
     //! Guess how far we are in the verification process at the given block index
     double GuessVerificationProgress(const CCheckpointData& data, CBlockIndex *pindex, bool fSigchecks) {
         if (pindex==NULL)

src/checkpoints.h

@@ -26,9 +26,6 @@ struct CCheckpointData {
     double fTransactionsPerDay;
 };
 
-//! Returns true if block passes checkpoint checks
-bool CheckBlock(const CCheckpointData& data, int nHeight, const uint256& hash);
-
 //! Return conservative estimate of total number of blocks, 0 if unknown
 int GetTotalBlocksEstimate(const CCheckpointData& data);
 

src/init.cpp

@@ -347,7 +347,7 @@ std::string HelpMessage(HelpMessageMode mode)
     strUsage += HelpMessageGroup(_("Debugging/Testing options:"));
     if (GetBoolArg("-help-debug", false))
     {
-        strUsage += HelpMessageOpt("-checkpoints", strprintf(_("Only accept block chain matching built-in checkpoints (default: %u)"), 1));
+        strUsage += HelpMessageOpt("-checkpoints", strprintf(_("Disable expensive verification for known chain history (default: %u)"), 1));
         strUsage += HelpMessageOpt("-dblogsize=<n>", strprintf(_("Flush database activity from memory pool to disk log every <n> megabytes (default: %u)"), 100));
         strUsage += HelpMessageOpt("-disablesafemode", strprintf(_("Disable safemode, override a real safe mode event (default: %u)"), 0));
         strUsage += HelpMessageOpt("-testsafemode", strprintf(_("Force safe mode (default: %u)"), 0));

src/main.cpp

@@ -1714,7 +1714,14 @@ bool ConnectBlock(const CBlock& block, CValidationState& state, CBlockIndex* pin
         return true;
     }
 
-    bool fScriptChecks = (!fCheckpointsEnabled || pindex->nHeight >= Checkpoints::GetTotalBlocksEstimate(chainparams.Checkpoints()));
+    bool fScriptChecks = true;
+    if (fCheckpointsEnabled) {
+        CBlockIndex *pindexLastCheckpoint = Checkpoints::GetLastCheckpoint(chainparams.Checkpoints());
+        if (pindexLastCheckpoint && pindexLastCheckpoint->GetAncestor(pindex->nHeight) == pindex) {
+            // This block is an ancestor of a checkpoint: disable script checks
+            fScriptChecks = false;
+        }
+    }
 
     // Do not allow blocks that contain transactions which 'overwrite' older transactions,
     // unless those are already completely spent.
@@ -2632,11 +2639,6 @@ bool ContextualCheckBlockHeader(const CBlockHeader& block, CValidationState& sta
 
     if (fCheckpointsEnabled)
     {
-        // Check that the block chain matches the known block chain up to a checkpoint
-        if (!Checkpoints::CheckBlock(chainParams.Checkpoints(), nHeight, hash))
-            return state.DoS(100, error("%s: rejected by checkpoint lock-in at %d", __func__, nHeight),
-                             REJECT_CHECKPOINT, "checkpoint mismatch");
-
         // Don't accept any forks from the main chain prior to last checkpoint
         CBlockIndex* pcheckpoint = Checkpoints::GetLastCheckpoint(chainParams.Checkpoints());
         if (pcheckpoint && nHeight < pcheckpoint->nHeight)

src/test/Checkpoints_tests.cpp

@@ -21,20 +21,6 @@ BOOST_FIXTURE_TEST_SUITE(Checkpoints_tests, BasicTestingSetup)
 BOOST_AUTO_TEST_CASE(sanity)
 {
     const Checkpoints::CCheckpointData& checkpoints = Params(CBaseChainParams::MAIN).Checkpoints();
-    uint256 p11111 = uint256S("0x0000000069e244f73d78e8fd29ba2fd2ed618bd6fa2ee92559f542fdb26e7c1d");
-    uint256 p134444 = uint256S("0x00000000000005b12ffd4cd315cd34ffd4a594f430ac814c91184a0d42d2b0fe");
-    BOOST_CHECK(Checkpoints::CheckBlock(checkpoints, 11111, p11111));
-    BOOST_CHECK(Checkpoints::CheckBlock(checkpoints, 134444, p134444));
-
-    
-    // Wrong hashes at checkpoints should fail:
-    BOOST_CHECK(!Checkpoints::CheckBlock(checkpoints, 11111, p134444));
-    BOOST_CHECK(!Checkpoints::CheckBlock(checkpoints, 134444, p11111));
-
-    // ... but any hash not at a checkpoint should succeed:
-    BOOST_CHECK(Checkpoints::CheckBlock(checkpoints, 11111+1, p134444));
-    BOOST_CHECK(Checkpoints::CheckBlock(checkpoints, 134444+1, p11111));
-
     BOOST_CHECK(Checkpoints::GetTotalBlocksEstimate(checkpoints) >= 134444);
 }