Reject free/low-fee transactions with insufficient priority.

By default, have the mempool reject free and low-fee transactions that
have insufficient priority to be mined in the next block.

Addtionally, add a new configuration option, -norelaypriority, to
disable the check.

config.go

@@ -103,6 +103,7 @@ type config struct {
 	DebugLevel         string        `short:"d" long:"debuglevel" description:"Logging level for all subsystems {trace, debug, info, warn, error, critical} -- You may also specify <subsystem>=<level>,<subsystem2>=<level>,... to set the log level for individual subsystems -- Use show to list available subsystems"`
 	Upnp               bool          `long:"upnp" description:"Use UPnP to map our listening port outside of NAT"`
 	FreeTxRelayLimit   float64       `long:"limitfreerelay" description:"Limit relay of transactions with no transaction fee to the given amount in thousands of bytes per minute"`
+	NoRelayPriority    bool          `long:"norelaypriority" description:"Do not require free or low-fee transactions to have high priority for relaying"`
 	Generate           bool          `long:"generate" description:"Generate (mine) bitcoins using the CPU"`
 	MiningAddrs        []string      `long:"miningaddr" description:"Add the specified payment address to the list of addresses to use for generated blocks -- At least one address is required if the generate option is set"`
 	BlockMinSize       uint32        `long:"blockminsize" description:"Mininum block size in bytes to be used when creating a block"`

mempool.go

@@ -1179,6 +1179,24 @@ func (mp *txMemPool) maybeAcceptTransaction(tx *btcutil.Tx, isNew, rateLimit boo
 		return nil, txRuleError(wire.RejectInsufficientFee, str)
 	}
 
+	// Require that free transactions have sufficient priority to be mined
+	// in the next block.
+	if !cfg.NoRelayPriority && txFee < minFee {
+		txD := &TxDesc{
+			Tx:     tx,
+			Added:  time.Now(),
+			Height: curHeight,
+			Fee:    txFee,
+		}
+		currentPriority := txD.CurrentPriority(txStore, nextBlockHeight)
+		if currentPriority <= minHighPriority {
+			str := fmt.Sprintf("transaction %v has insufficient "+
+				"priority (%g <= %g)", txHash,
+				currentPriority, minHighPriority)
+			return nil, txRuleError(wire.RejectInsufficientFee, str)
+		}
+	}
+
 	// Free-to-relay transactions are rate limited here to prevent
 	// penny-flooding with tiny transactions as a form of attack.
 	if rateLimit && txFee < minFee {