protect depot with mutex in SignCSR (#185)

depot/signer.go

@@ -3,6 +3,7 @@ package depot
 import (
 	"crypto/rand"
 	"crypto/x509"
+	"sync"
 	"time"
 
 	"github.com/micromdm/scep/v2/cryptoutil"
@@ -12,6 +13,7 @@ import (
 // Signer signs x509 certificates and stores them in a Depot
 type Signer struct {
 	depot            Depot
+	mu               sync.Mutex
 	caPass           string
 	allowRenewalDays int
 	validityDays     int
@@ -61,6 +63,9 @@ func (s *Signer) SignCSR(m *scep.CSRReqMessage) (*x509.Certificate, error) {
 		return nil, err
 	}
 
+	s.mu.Lock()
+	defer s.mu.Unlock()
+
 	serial, err := s.depot.Serial()
 	if err != nil {
 		return nil, err