Ignore problematic blocks in DisconnectBlock

When using checklevel=4, block verification fails because of duplicate coinbase transactions
involving blocks 91812 and 91722. There was already a check in place for ConnectBlock to
ignore the problematic blocks, but DisconnectBlock did not contain a similar check.

This change ignores the blocks where these inconsistencies surface so
that block verification will succeed at checklevel=4.

Author: seejee

src/validation.cpp

@@ -1837,6 +1837,15 @@ DisconnectResult CChainState::DisconnectBlock(const CBlock& block, const CBlockI
         return DISCONNECT_FAILED;
     }
 
+    // Ignore blocks that contain transactions which are 'overwritten' by later transactions,
+    // unless those are already completely spent.
+    // See https://github.com/bitcoin/bitcoin/issues/22596 for additional information.
+    // Note: the blocks specified here are different than the ones used in ConnectBlock because DisconnectBlock
+    // unwinds the blocks in reverse. As a result, the inconsistency is not discovered until the earlier
+    // blocks with the duplicate coinbase transactions are disconnected.
+    bool fEnforceBIP30 = !((pindex->nHeight==91722 && pindex->GetBlockHash() == uint256S("0x00000000000271a2dc26e7667f8419f2e15416dc6955e5a6c6cdf3f2574dd08e")) ||
+                           (pindex->nHeight==91812 && pindex->GetBlockHash() == uint256S("0x00000000000af0aed4792b1acee3d966af36cf5def14935db8de83d6f9306f2f")));
+
     // undo transactions in reverse order
     for (int i = block.vtx.size() - 1; i >= 0; i--) {
         const CTransaction &tx = *(block.vtx[i]);
@@ -1850,7 +1859,7 @@ DisconnectResult CChainState::DisconnectBlock(const CBlock& block, const CBlockI
                 COutPoint out(hash, o);
                 Coin coin;
                 bool is_spent = view.SpendCoin(out, &coin);
-                if (!is_spent || tx.vout[o] != coin.out || pindex->nHeight != coin.nHeight || is_coinbase != coin.fCoinBase) {
+                if (fEnforceBIP30 && (!is_spent || tx.vout[o] != coin.out || pindex->nHeight != coin.nHeight || is_coinbase != coin.fCoinBase)) {
                     fClean = false; // transaction output mismatch
                 }
             }