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/checkpoints.cpp

@@ -25,18 +25,6 @@ namespace Checkpoints {
 
     bool fEnabled = true;
 
-    bool CheckBlock(int nHeight, const uint256& hash)
-    {
-        if (!fEnabled)
-            return true;
-
-        const MapCheckpoints& checkpoints = *Params().Checkpoints().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(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(int nHeight, const uint256& hash);
-
 //! Return conservative estimate of total number of blocks, 0 if unknown
 int GetTotalBlocksEstimate();
 

src/init.cpp

@@ -346,7 +346,7 @@ 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

@@ -1756,7 +1756,12 @@ bool ConnectBlock(const CBlock& block, CValidationState& state, CBlockIndex* pin
         return true;
     }
 
-    bool fScriptChecks = pindex->nHeight >= Checkpoints::GetTotalBlocksEstimate();
+    bool fScriptChecks = true;
+    CBlockIndex *pindexLastCheckpoint = Checkpoints::GetLastCheckpoint();
+    if (pindexLastCheckpoint && pindex->GetAncestor(pindexLastCheckpoint->nHeight) == pindexLastCheckpoint) {
+        // 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.
@@ -2615,11 +2620,6 @@ bool ContextualCheckBlockHeader(const CBlockHeader& block, CValidationState& sta
         return state.Invalid(error("%s: block's timestamp is too early", __func__),
                              REJECT_INVALID, "time-too-old");
 
-    // Check that the block chain matches the known block chain up to a checkpoint
-    if (!Checkpoints::CheckBlock(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();
     if (pcheckpoint && nHeight < pcheckpoint->nHeight)

src/test/Checkpoints_tests.cpp

@@ -21,19 +21,8 @@ BOOST_AUTO_TEST_CASE(sanity)
 {
     uint256 p11111 = uint256S("0x0000000069e244f73d78e8fd29ba2fd2ed618bd6fa2ee92559f542fdb26e7c1d");
     uint256 p134444 = uint256S("0x00000000000005b12ffd4cd315cd34ffd4a594f430ac814c91184a0d42d2b0fe");
-    BOOST_CHECK(Checkpoints::CheckBlock(11111, p11111));
-    BOOST_CHECK(Checkpoints::CheckBlock(134444, p134444));
-
-
-    // Wrong hashes at checkpoints should fail:
-    BOOST_CHECK(!Checkpoints::CheckBlock(11111, p134444));
-    BOOST_CHECK(!Checkpoints::CheckBlock(134444, p11111));
-
-    // ... but any hash not at a checkpoint should succeed:
-    BOOST_CHECK(Checkpoints::CheckBlock(11111+1, p134444));
-    BOOST_CHECK(Checkpoints::CheckBlock(134444+1, p11111));
 
     BOOST_CHECK(Checkpoints::GetTotalBlocksEstimate() >= 134444);
-}    
+}
 
 BOOST_AUTO_TEST_SUITE_END()