private/signer/v4: Resign requests which are too old

Requests that are delayed to be sent, or are retried so many times that
the original date the request was signed with is past the age a
signature can be, the request needs to be resigned.  This prevents
issues for request being retried many times, and finally succeding but
encounter an expired signature.

Fix aws#486

private/signer/v4/v4.go

@@ -153,6 +153,7 @@ func Sign(req *request.Request) {
 	}
 
 	req.Error = s.sign()
+	req.Time = s.Time
 	req.SignedHeaderVals = s.signedHeaderVals
 }
 
@@ -162,11 +163,12 @@ func (v4 *signer) sign() error {
 	}
 
 	if v4.isRequestSigned() {
-		if !v4.Credentials.IsExpired() {
+		if !v4.Credentials.IsExpired() && time.Now().Before(v4.Time.Add(10*time.Minute)) {
 			// If the request is already signed, and the credentials have not
-			// expired yet ignore the signing request.
+			// expired, and the request is not too old ignore the signing request.
 			return nil
 		}
+		v4.Time = time.Now()
 
 		// The credentials have expired for this request. The current signing
 		// is invalid, and needs to be request because the request will fail.

private/signer/v4/v4_test.go

@@ -267,6 +267,29 @@ func TestPreResignRequestExpiredCreds(t *testing.T) {
 	assert.NotContains(t, signedHeaders, "x-amz-signedHeaders")
 }
 
+func TestResignRequestExpiredRequest(t *testing.T) {
+	creds := credentials.NewStaticCredentials("AKID", "SECRET", "SESSION")
+	svc := awstesting.NewClient(&aws.Config{Credentials: creds})
+	r := svc.NewRequest(
+		&request.Operation{
+			Name:       "BatchGetItem",
+			HTTPMethod: "POST",
+			HTTPPath:   "/",
+		},
+		nil,
+		nil,
+	)
+
+	Sign(r)
+	querySig := r.HTTPRequest.Header.Get("Authorization")
+
+	// Simulate the request occured 15 minutes in the past
+	r.Time = r.Time.Add(-15 * time.Minute)
+
+	Sign(r)
+	assert.NotEqual(t, querySig, r.HTTPRequest.Header.Get("Authorization"))
+}
+
 func BenchmarkPresignRequest(b *testing.B) {
 	signer := buildSigner("dynamodb", "us-east-1", time.Now(), 300*time.Second, "{}")
 	for i := 0; i < b.N; i++ {