make local cosigner nonce cache expiration twice the leader nonce cache

signer/cosigner_nonce_cache.go

@@ -14,6 +14,7 @@ const (
 	defaultGetNoncesInterval = 3 * time.Second
 	defaultGetNoncesTimeout  = 4 * time.Second
 	cachePreSize             = 10000
+	nonceCacheExpiration     = 10 * time.Second // half of the local cosigner cache expiration
 )
 
 type CosignerNonceCache struct {
@@ -201,7 +202,7 @@ func (cnc *CosignerNonceCache) LoadN(ctx context.Context, n int) {
 	var wg sync.WaitGroup
 	wg.Add(len(cnc.cosigners))
 
-	expiration := time.Now().Add(nonceExpiration)
+	expiration := time.Now().Add(nonceCacheExpiration)
 
 	for i, p := range cnc.cosigners {
 		i := i

signer/local_cosigner.go

@@ -16,7 +16,9 @@ import (
 
 var _ Cosigner = &LocalCosigner{}
 
-const nonceExpiration = 10 * time.Second
+// double the CosignerNonceCache expiration so that sign requests from the leader
+// never reference nonces which have expired here in the LocalCosigner.
+const nonceExpiration = 20 * time.Second
 
 // LocalCosigner responds to sign requests.
 // It maintains a high watermark to avoid double-signing.