AVRO-4135: [Java] JSON decoding unqualified union types

Support unqualified type references for unions when decoding them from
json.
AVRO-2287 makes it unclear if type reference for a JSON encoded union
needs to be qualified or not.
Today all encoders use the fully qualified types except the C JSON
encoder which uses the unqualified type.

In this patch we make the java JSON Decoder more lenient and let it
fallback to unqualified types names when no qualified type name
matches.  Which matches the behavior currently implemented in the
Javascript Json decoder.

This patch is an alternative for apache#3373 where it is proposed
to update the C library instead.

Author: steven-aerts

lang/java/avro/src/main/java/org/apache/avro/io/parsing/Symbol.java

@@ -475,6 +475,16 @@ public int findLabel(String label) {
             return i;
           }
         }
+        // AVRO-4135 We are a bit more flexible here since we have to deal with the C
+        // serializer being less strict, so
+        // we fall back to looking up unqualified names.
+        for (int i = 0; i < labels.length; i++) {
+          String candidate = labels[i];
+          if (candidate != null && candidate.length() > label.length()
+              && candidate.charAt(candidate.length() - label.length() - 1) == '.' && candidate.endsWith(label)) {
+            return i;
+          }
+        }
       }
       return -1;
     }

lang/java/avro/src/test/java/org/apache/avro/io/TestJsonDecoder.java

@@ -92,4 +92,16 @@ void testIntWithError() throws IOException {
     JsonDecoder decoder = DecoderFactory.get().jsonDecoder(schema, record);
     Assertions.assertThrows(AvroTypeException.class, () -> reader.read(null, decoder));
   }
+
+  @Test
+  void testUnionTypeQualification() throws Exception {
+    final Schema schema = SchemaBuilder.unionOf().nullType().and()
+        .type(SchemaBuilder.record("r").namespace("space").fields().endRecord()).endUnion();
+    GenericDatumReader<GenericRecord> reader = new GenericDatumReader<>(schema, schema);
+    JsonDecoder decoder = DecoderFactory.get().jsonDecoder(schema, "{\"space.r\": {}}");
+    assertEquals("space.r", reader.read(null, decoder).getSchema().getFullName());
+    // AVRO-4135: C json encoder uses unqualified types.
+    decoder = DecoderFactory.get().jsonDecoder(schema, "{\"r\": {}}");
+    assertEquals("space.r", reader.read(null, decoder).getSchema().getFullName());
+  }
 }