Reject very old votes from the memory pool (#168)

Very old votes are now rejected from the memory pool by default.
The feature can be disabled with a runtime flag. A redundant
function that determined the transaction type was also removed
from mempool.go.

config.go

@@ -48,6 +48,7 @@ const (
 	defaultAddrIndex         = false
 	defaultNonAggressive     = false
 	defaultNoMiningStateSync = false
+	defaultAllowOldVotes     = false
 )
 
 var (
@@ -124,6 +125,7 @@ type config struct {
 	DropAddrIndex      bool          `long:"dropaddrindex" description:"Deletes the address-based transaction index from the database on start up, and the exits."`
 	NonAggressive      bool          `long:"nonaggressive" description:"Disable mining off of the parent block of the blockchain if there aren't enough voters"`
 	NoMiningStateSync  bool          `long:"nominingstatesync" description:"Disable synchronizing the mining state with other nodes"`
+	AllowOldVotes      bool          `long:"allowoldvotes" description:"Enable the addition of very old votes to the mempool"`
 	onionlookup        func(string) ([]net.IP, error)
 	lookup             func(string) ([]net.IP, error)
 	oniondial          func(string, string) (net.Conn, error)
@@ -334,6 +336,7 @@ func loadConfig() (*config, []string, error) {
 		Generate:          defaultGenerate,
 		NoAddrIndex:       defaultAddrIndex,
 		NoMiningStateSync: defaultNoMiningStateSync,
+		AllowOldVotes:     defaultAllowOldVotes,
 	}
 
 	// Service options which are only added on Windows.

mempool.go

@@ -109,6 +109,10 @@ const (
 	// of height before SSGen relating to that block are pruned.
 	heightDiffToPruneVotes = 10
 
+	// If a vote is on a block whose height is before tip minus this
+	// amount, reject it from being added to the mempool.
+	maximumVoteAgeDelta = 1440
+
 	// maxNullDataOutputs is the maximum number of OP_RETURN null data
 	// pushes in a transaction, after which it is considered non-standard.
 	maxNullDataOutputs = 4
@@ -1142,29 +1146,6 @@ func (mp *txMemPool) FilterTransactionsByAddress(
 	return nil, fmt.Errorf("address does not have any transactions in the pool")
 }
 
-// This function detects whether or not a transaction is a stake transaction and,
-// if it is, also returns the type of stake transaction.
-func detectTxType(tx *dcrutil.Tx) stake.TxType {
-	// Check to see if it's an SStx
-	if pass, _ := stake.IsSStx(tx); pass {
-		return stake.TxTypeSStx
-	}
-
-	// Check to see if it's an SSGen
-	if pass, _ := stake.IsSSGen(tx); pass {
-		return stake.TxTypeSSGen
-	}
-
-	// Check to see if it's an SSGen
-	if pass, _ := stake.IsSSRtx(tx); pass {
-		return stake.TxTypeSSRtx
-	}
-
-	// If it's none of these things, it's a malformed or non-standard stake tx
-	// which will be rejected during other checks or a regular tx.
-	return stake.TxTypeRegular
-}
-
 // maybeAcceptTransaction is the internal function which implements the public
 // MaybeAcceptTransaction.  See the comment for MaybeAcceptTransaction for
 // more details.
@@ -1229,7 +1210,7 @@ func (mp *txMemPool) maybeAcceptTransaction(tx *dcrutil.Tx, isNew,
 	// Determine what type of transaction we're dealing with (regular or stake).
 	// Then, be sure to set the tx tree correctly as it's possible a use submitted
 	// it to the network with TxTreeUnknown.
-	txType := detectTxType(tx)
+	txType := stake.DetermineTxType(tx)
 	if txType == stake.TxTypeRegular {
 		tx.SetTree(dcrutil.TxTreeRegular)
 	} else {
@@ -1317,6 +1298,24 @@ func (mp *txMemPool) maybeAcceptTransaction(tx *dcrutil.Tx, isNew,
 			return nil, err
 		}
 	}
+
+	// Votes that are on too old of blocks are rejected.
+	if txType == stake.TxTypeSSGen {
+		_, voteHeight, err := stake.GetSSGenBlockVotedOn(tx)
+		if err != nil {
+			return nil, err
+		}
+
+		if (int64(voteHeight) < curHeight-maximumVoteAgeDelta) &&
+			!cfg.AllowOldVotes {
+			str := fmt.Sprintf("transaction %v votes on old "+
+				"block height of %v which is before the "+
+				"current cutoff height of %v",
+				tx.Sha(), voteHeight, curHeight-maximumVoteAgeDelta)
+			return nil, txRuleError(wire.RejectNonstandard, str)
+		}
+	}
+
 	// Fetch all of the transactions referenced by the inputs to this
 	// transaction.  This function also attempts to fetch the transaction
 	// itself to be used for detecting a duplicate transaction without
@@ -1693,7 +1692,7 @@ func (mp *txMemPool) PruneStakeTx(requiredStakeDifficulty, height int64) {
 
 func (mp *txMemPool) pruneStakeTx(requiredStakeDifficulty, height int64) {
 	for _, tx := range mp.pool {
-		txType := detectTxType(tx.Tx)
+		txType := stake.DetermineTxType(tx.Tx)
 		if txType == stake.TxTypeSStx &&
 			tx.Height+int64(heightDiffToPruneTicket) < height {
 			mp.removeTransaction(tx.Tx, true)