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

@@ -1120,25 +1120,23 @@ 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.
+   * Decodes a base64 string, the encoding protobuf's canonical JSON mapping defines for {@code
+   * bytes}. Both the standard and URL-safe alphabets are accepted, with or without padding.
    *
-   * @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);
     }
   }
 

java-bigquerystorage/google-cloud-bigquerystorage/src/test/java/com/google/cloud/bigquery/storage/v1/JsonToProtoMessageTest.java

@@ -1123,8 +1123,6 @@ void testBytesFromBase64() throws Exception {
     TableSchema tableSchema = bytesTableSchema("test_field_type", TableFieldSchema.Mode.NULLABLE);
     BytesType expectedProto =
         BytesType.newBuilder().setTestFieldType(ByteString.copyFromUtf8("hi")).build();
-    // Base64 for "hi". Padding is optional in protobuf's canonical JSON mapping, and "hi" encodes
-    // to characters the standard and URL-safe alphabets share.
     for (String encoded : ImmutableList.of("aGk=", "aGk")) {
       JSONObject json = new JSONObject();
       json.put("test_field_type", encoded);
@@ -1140,8 +1138,7 @@ void testBytesFromBase64EitherAlphabet() throws Exception {
     TableSchema tableSchema = bytesTableSchema("test_field_type", TableFieldSchema.Mode.NULLABLE);
     BytesType expectedProto =
         BytesType.newBuilder().setTestFieldType(ByteString.copyFrom(new byte[] {-5, -1})).build();
-    // 0xFB 0xFF is "+/8" in the standard alphabet and "-_8" in the URL-safe one — the only place
-    // the two differ. Protobuf's mapping accepts both, with or without padding.
+    // 0xFB 0xFF is "+/8" in the standard alphabet and "-_8" in the URL-safe one.
     for (String encoded : ImmutableList.of("+/8=", "-_8=", "+/8", "-_8")) {
       JSONObject json = new JSONObject();
       json.put("test_field_type", encoded);
@@ -1185,8 +1182,7 @@ void testBytesFromInvalidBase64() throws Exception {
 
   @Test
   void testBytesFromBase64NeedsTableSchema() throws Exception {
-    // Without a TableSchema the converter cannot tell BYTES from NUMERIC and BIGNUMERIC, which
-    // share the same proto type, so a string stays an error rather than being decoded.
+    // Without a TableSchema, BYTES is indistinguishable from NUMERIC and BIGNUMERIC.
     JSONObject json = new JSONObject();
     json.put("test_field_type", "aGk=");
     IllegalArgumentException e =