Fix potential clock race condition (#814)

packages/clock/synchronization.go

@@ -2,6 +2,7 @@ package clock
 
 import (
 	"errors"
+	"sync"
 	"time"
 
 	"github.com/beevik/ntp"
@@ -13,6 +14,7 @@ var ErrNTPQueryFailed = errors.New("NTP query failed")
 
 // difference between network time and node's local time.
 var offset time.Duration
+var offsetMutex sync.RWMutex
 
 // FetchTimeOffset establishes the difference in local vs network time.
 // This difference is stored in offset so that it can be used to adjust the local clock.
@@ -21,6 +23,8 @@ func FetchTimeOffset(host string) error {
 	if err != nil {
 		return xerrors.Errorf("NTP query error (%v): %w", err, ErrNTPQueryFailed)
 	}
+	offsetMutex.Lock()
+	defer offsetMutex.Unlock()
 	offset = resp.ClockOffset
 
 	return nil
@@ -28,5 +32,7 @@ func FetchTimeOffset(host string) error {
 
 // SyncedTime gets the synchronized time (according to the network) of a node.
 func SyncedTime() time.Time {
+	offsetMutex.RLock()
+	defer offsetMutex.RUnlock()
 	return time.Now().Add(offset)
 }