don't hold global chequebook lock while sending cheque (#986)

pkg/settlement/swap/chequebook/chequebook.go

@@ -73,6 +73,7 @@ type service struct {
 
 	store               storage.StateStorer
 	chequeSigner        ChequeSigner
+	totalIssuedReserved *big.Int
 }
 
 // New creates a new chequebook service for the provided chequebook contract.
@@ -109,6 +110,7 @@ func New(backend transaction.Backend, transactionService transaction.Service, ad
 		erc20Instance:       erc20Instance,
 		store:               store,
 		chequeSigner:        chequeSigner,
+		totalIssuedReserved: big.NewInt(0),
 	}, nil
 }
 
@@ -202,10 +204,7 @@ func lastIssuedChequeKey(beneficiary common.Address) string {
 	return fmt.Sprintf("chequebook_last_issued_cheque_%x", beneficiary)
 }
 
-// Issue issues a new cheque and passes it to sendChequeFunc
-// if sendChequeFunc succeeds the cheque is considered sent and saved
-func (s *service) Issue(ctx context.Context, beneficiary common.Address, amount *big.Int, sendChequeFunc SendChequeFunc) error {
-	// don't allow concurrent issuing of cheques
+func (s *service) reserveTotalIssued(ctx context.Context, amount *big.Int) error {
 	s.lock.Lock()
 	defer s.lock.Unlock()
 
@@ -214,10 +213,29 @@ func (s *service) Issue(ctx context.Context, beneficiary common.Address, amount
 		return err
 	}
 
-	if amount.Cmp(availableBalance) > 0 {
+	if amount.Cmp(big.NewInt(0).Sub(availableBalance, s.totalIssuedReserved)) > 0 {
 		return ErrOutOfFunds
 	}
 
+	s.totalIssuedReserved = s.totalIssuedReserved.Add(s.totalIssuedReserved, amount)
+	return nil
+}
+
+func (s *service) unreserveTotalIssued(amount *big.Int) {
+	s.lock.Lock()
+	defer s.lock.Unlock()
+	s.totalIssuedReserved = s.totalIssuedReserved.Sub(s.totalIssuedReserved, amount)
+}
+
+// Issue issues a new cheque and passes it to sendChequeFunc
+// if sendChequeFunc succeeds the cheque is considered sent and saved
+func (s *service) Issue(ctx context.Context, beneficiary common.Address, amount *big.Int, sendChequeFunc SendChequeFunc) error {
+	err := s.reserveTotalIssued(ctx, amount)
+	if err != nil {
+		return err
+	}
+	defer s.unreserveTotalIssued(amount)
+
 	var cumulativePayout *big.Int
 	lastCheque, err := s.LastCheque(beneficiary)
 	if err != nil {
@@ -262,6 +280,9 @@ func (s *service) Issue(ctx context.Context, beneficiary common.Address, amount
 		return err
 	}
 
+	s.lock.Lock()
+	defer s.lock.Unlock()
+
 	totalIssued, err := s.totalIssued()
 	if err != nil {
 		return err