Protect the relay fee field with a mutex.

This prevents races when setting a new relay fee through the legacy
RPC server (settxfee).

Fixes #379.

rpc/legacyrpc/methods.go

@@ -512,7 +512,7 @@ func GetInfo(icmd interface{}, w *wallet.Wallet, chainClient *chain.RPCClient) (
 	// to using the manager version.
 	info.WalletVersion = int32(waddrmgr.LatestMgrVersion)
 	info.Balance = bal.ToBTC()
-	info.PaytxFee = w.RelayFee.ToBTC()
+	info.PaytxFee = w.RelayFee().ToBTC()
 	// We don't set the following since they don't make much sense in the
 	// wallet architecture:
 	//  - unlocked_until
@@ -1572,11 +1572,11 @@ func SetTxFee(icmd interface{}, w *wallet.Wallet) (interface{}, error) {
 		return nil, ErrNeedPositiveAmount
 	}
 
-	incr, err := btcutil.NewAmount(cmd.Amount)
+	relayFee, err := btcutil.NewAmount(cmd.Amount)
 	if err != nil {
 		return nil, err
 	}
-	w.RelayFee = incr
+	w.SetRelayFee(relayFee)
 
 	// A boolean true result is returned upon success.
 	return true, nil

wallet/createtx.go

@@ -134,7 +134,7 @@ func (w *Wallet) txToOutputs(outputs []*wire.TxOut, account uint32, minconf int3
 		}
 		return txscript.PayToAddrScript(changeAddr)
 	}
-	tx, err := txauthor.NewUnsignedTransaction(outputs, w.RelayFee,
+	tx, err := txauthor.NewUnsignedTransaction(outputs, w.RelayFee(),
 		inputSource, changeSource)
 	if err != nil {
 		return nil, err

wallet/wallet.go

@@ -75,8 +75,8 @@ type Wallet struct {
 	chainClientSyncMtx sync.Mutex
 
 	lockedOutpoints map[wire.OutPoint]struct{}
-	RelayFee        btcutil.Amount
-	DisallowFree    bool
+	relayFee        btcutil.Amount
+	relayFeeMu      sync.Mutex
 
 	// Channels for rescan processing.  Requests are added and merged with
 	// any waiting requests, before being sent to another goroutine to
@@ -357,6 +357,23 @@ func (w *Wallet) ChainClient() *chain.RPCClient {
 	return chainClient
 }
 
+// RelayFee returns the current minimum relay fee (per kB of serialized
+// transaction) used when constructing transactions.
+func (w *Wallet) RelayFee() btcutil.Amount {
+	w.relayFeeMu.Lock()
+	relayFee := w.relayFee
+	w.relayFeeMu.Unlock()
+	return relayFee
+}
+
+// SetRelayFee sets a new minimum relay fee (per kB of serialized
+// transaction) used when constructing transactions.
+func (w *Wallet) SetRelayFee(relayFee btcutil.Amount) {
+	w.relayFeeMu.Lock()
+	w.relayFee = relayFee
+	w.relayFeeMu.Unlock()
+}
+
 // quitChan atomically reads the quit channel.
 func (w *Wallet) quitChan() <-chan struct{} {
 	w.quitMu.Lock()
@@ -2009,7 +2026,7 @@ func (w *Wallet) SendOutputs(outputs []*wire.TxOut, account uint32,
 	}
 
 	for _, output := range outputs {
-		err = txrules.CheckOutput(output, w.RelayFee)
+		err = txrules.CheckOutput(output, w.RelayFee())
 		if err != nil {
 			return nil, err
 		}
@@ -2230,7 +2247,7 @@ func Open(pubPass []byte, params *chaincfg.Params, db walletdb.DB, waddrmgrNS, w
 		Manager:             addrMgr,
 		TxStore:             txMgr,
 		lockedOutpoints:     map[wire.OutPoint]struct{}{},
-		RelayFee:            txrules.DefaultRelayFeePerKb,
+		relayFee:            txrules.DefaultRelayFeePerKb,
 		rescanAddJob:        make(chan *RescanJob),
 		rescanBatch:         make(chan *rescanBatch),
 		rescanNotifications: make(chan interface{}),