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Do not allow overwriting unspent transactions (BIP 30)

Introduce the following network rule:
 * a block is not valid if it contains a transaction whose hash
   already exists in the block chain, unless all that transaction's
   outputs were already spent before said block.

Warning: this is effectively a network rule change, with potential
risk for forking the block chain. Leaving this unfixed carries the
same risk however, for attackers that can cause a reorganisation
in part of the network.

Thanks to Russell O'Connor and Ben Reeves.
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commit a206b0ea12eb4606b93323268fc81a4f1f952531 1 parent 50abb55
@sipa sipa authored
Showing with 24 additions and 2 deletions.
  1. +24 −2 src/main.cpp
26 src/main.cpp
@@ -976,8 +976,10 @@ bool CTransaction::DisconnectInputs(CTxDB& txdb)
// Remove transaction from index
- if (!txdb.EraseTxIndex(*this))
- return error("DisconnectInputs() : EraseTxPos failed");
+ // This can fail if a duplicate of this transaction was in a chain that got
+ // reorganized away. This is only possible if this transaction was completely
+ // spent, so erasing it would be a no-op anway.
+ txdb.EraseTxIndex(*this);
return true;
@@ -1256,6 +1258,26 @@ bool CBlock::ConnectBlock(CTxDB& txdb, CBlockIndex* pindex)
if (!CheckBlock())
return false;
+ // Do not allow blocks that contain transactions which 'overwrite' older transactions,
+ // unless those are already completely spent.
+ // If such overwrites are allowed, coinbases and transactions depending upon those
+ // can be duplicated to remove the ability to spend the first instance -- even after
+ // being sent to another address.
+ // See BIP30 and for more information.
+ // This logic is not necessary for memory pool transactions, as AcceptToMemoryPool
+ // already refuses previously-known transaction id's entirely.
+ // This rule applies to all blocks whose timestamp is after March 15, 2012, 0:00 UTC.
+ // On testnet it is enabled as of februari 20, 2012, 0:00 UTC.
+ if (pindex->nTime > 1331769600 || (fTestNet && pindex->nTime > 1329696000))
+ BOOST_FOREACH(CTransaction& tx, vtx)
+ {
+ CTxIndex txindexOld;
+ if (txdb.ReadTxIndex(tx.GetHash(), txindexOld))
+ BOOST_FOREACH(CDiskTxPos &pos, txindexOld.vSpent)
+ if (pos.IsNull())
+ return false;
+ }
// To avoid being on the short end of a block-chain split,
// don't do secondary validation of pay-to-script-hash transactions
// until blocks with timestamps after paytoscripthashtime (see init.cpp for default).
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