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.

1 files changed, 9 insertions, 7 deletions

diff --git a/src/main.cpp b/src/main.cpp
index 4f4926eb40..39da3406a8 100644
--- a/src/main.cpp
+++ b/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.
@@ -2630,13 +2637,8 @@ bool ContextualCheckBlockHeader(const CBlockHeader& block, CValidationState& sta
         return state.Invalid(error("%s: block's timestamp is too early", __func__),
                              REJECT_INVALID, "time-too-old");
 
-    if(fCheckpointsEnabled)
+    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)