fix(bigquerystorage): pick the base64 alphabet up front instead of by fallback

Falling back through a caught IllegalArgumentException put an exception on the
happy path for every record when a producer uses the URL-safe alphabet. A value
containing - or _ is never valid standard base64, so the alphabet can be chosen
before decoding. The original exception is now kept as the cause.

Author: laughingman7743

java-bigquerystorage/google-cloud-bigquerystorage/src/main/java/com/google/cloud/bigquery/storage/v1/JsonToProtoMessage.java

@@ -1122,23 +1122,24 @@ private static void throwWrongFieldType(
   /**
    * Decodes a base64 string into bytes, the encoding protobuf's canonical JSON mapping defines for
    * the {@code bytes} type. Standard and URL-safe alphabets are both accepted, with or without
-   * padding, as that mapping requires; the two alphabets are mutually exclusive on {@code +/}
-   * versus {@code -_}, so the fallback is what makes both work.
+   * padding, as that mapping requires. The two differ only in {@code +/} versus {@code -_}, so the
+   * alphabet is picked by looking for the URL-safe characters rather than by decoding twice, which
+   * would make an exception part of the happy path for every URL-safe record.
    *
-   * @throws IllegalArgumentException if the value decodes under neither alphabet
+   * @throws IllegalArgumentException if the value is not valid base64
    */
   private static byte[] parseBase64(String currentScope, String value) {
     try {
-      return Base64.getDecoder().decode(value);
-    } catch (IllegalArgumentException standardAlphabetFailed) {
-      try {
+      if (value.indexOf('-') >= 0 || value.indexOf('_') >= 0) {
         return Base64.getUrlDecoder().decode(value);
-      } catch (IllegalArgumentException urlSafeAlphabetFailed) {
-        throw new IllegalArgumentException(
-            "Error: "
-                + currentScope
-                + " could not be converted to byte[]: not a valid base64 string.");
       }
+      return Base64.getDecoder().decode(value);
+    } catch (IllegalArgumentException e) {
+      throw new IllegalArgumentException(
+          "Error: "
+              + currentScope
+              + " could not be converted to byte[]: not a valid base64 string.",
+          e);
     }
   }