Add SecureCookie.Err for surfacing deferred configuration errors

securecookie.go

@@ -244,6 +244,14 @@ func (s *SecureCookie) SetSerializer(sz Serializer) *SecureCookie {
 	return s
 }
 
+// Err returns the deferred error stored on s during New or BlockFunc. It's
+// the same error every subsequent Encode or Decode call would surface. This
+// lets callers fail fast when configuration is wrong instead of waiting for
+// the first cookie to be processed.
+func (s *SecureCookie) Err() error {
+	return s.err
+}
+
 // Encode encodes a cookie value.
 //
 // It serializes, optionally encrypts, signs with a message authentication code,

securecookie_test.go

@@ -88,6 +88,19 @@ func TestSecureCookieNilKey(t *testing.T) {
 	}
 }
 
+func TestErr(t *testing.T) {
+	if err := New([]byte("hashkey"), nil).Err(); err != nil {
+		t.Fatalf("expected nil Err for a valid hash key, got %v", err)
+	}
+	if err := New(nil, nil).Err(); err != errHashKeyNotSet {
+		t.Fatalf("expected errHashKeyNotSet from Err, got %v", err)
+	}
+	// Invalid block key (wrong size) is surfaced via BlockFunc.
+	if err := New([]byte("hashkey"), []byte{1, 2, 3}).Err(); err == nil {
+		t.Fatal("expected non-nil Err for an invalid block key")
+	}
+}
+
 func TestDecodeInvalid(t *testing.T) {
 	// List of invalid cookies, which must not be accepted, base64-decoded
 	// (they will be encoded before passing to Decode).