chain: add rescanMtx to resolve rescan segfault

Add a mutex to synchronize access to the rescan prop of the
neutrino client.  Make calls to Rescan and NotifyReceived
require the rescan mutex lock to execute.  Resolve the segmentation
fault in the TestNeutrinoClientNotifyReceivedRescan unit test.

chain/neutrino.go

@@ -27,8 +27,14 @@ type NeutrinoClient struct {
 	chainParams *chaincfg.Params
 
 	// We currently support only one rescan/notification goroutine per client.
+	// Therefore there can only be one instance of the rescan object and
+	// the rescanMtx synchronizes its access.  Calls to the NotifyReceived
+	// and Rescan methods of the client must hold the rescan mutex lock for
+	// the length of their execution to ensure that all operations that
+	// affect the rescan object are atomic.
 	rescan    rescanner
 	newRescan newRescanFunc
+	rescanMtx sync.Mutex
 
 	enqueueNotification     chan interface{}
 	dequeueNotification     chan interface{}
@@ -46,6 +52,14 @@ type NeutrinoClient struct {
 	finished   bool
 	isRescan   bool
 
+	// The clientMtx synchronizes access to the state variables of the client.
+	//
+	// TODO(mstreet3): Currently the clientMtx synchronizes access to the
+	// rescanQuit and rescanErr channels, which cancel the current rescan
+	// goroutine when closed and is updated each time a new rescan goroutine
+	// is created, respectively.  All state related to the rescan goroutine
+	// should ideally be synchronized by the same lock or via some other
+	// shared mechanism.
 	clientMtx sync.Mutex
 }
 
@@ -351,6 +365,10 @@ func (s *NeutrinoClient) pollCFilter(hash *chainhash.Hash) (*gcs.Filter, error)
 func (s *NeutrinoClient) Rescan(startHash *chainhash.Hash, addrs []btcutil.Address,
 	outPoints map[wire.OutPoint]btcutil.Address) error {
 
+	// Obtain and hold the rescan mutex lock for the duration of the call.
+	s.rescanMtx.Lock()
+	defer s.rescanMtx.Unlock()
+
 	s.clientMtx.Lock()
 	if !s.started {
 		s.clientMtx.Unlock()
@@ -458,6 +476,10 @@ func (s *NeutrinoClient) NotifyBlocks() error {
 
 // NotifyReceived replicates the RPC client's NotifyReceived command.
 func (s *NeutrinoClient) NotifyReceived(addrs []btcutil.Address) error {
+	// Obtain and hold the rescan mutex lock for the duration of the call.
+	s.rescanMtx.Lock()
+	defer s.rescanMtx.Unlock()
+
 	s.clientMtx.Lock()
 
 	// If we have a rescan running, we just need to add the appropriate