AVRO-4176: Java parser allows field type to be object with custom type (#3772)

* AVRO-4176: Disallow named type alteration

The test cases for self reference unions contained a bug that seemed to
make the parsing bug necessary.

* AVRO-4176: Add test

Author: opwvhk

lang/java/avro/src/main/java/org/apache/avro/Schema.java

@@ -1842,8 +1842,9 @@ static Schema parse(JsonNode schema, ParseContext context, String currentNameSpa
         return parseMap(schema, context, currentNameSpace);
       } else if ("fixed".equals(type)) { // fixed
         return parseFixed(schema, context, currentNameSpace);
-      } else { // For unions with self reference
-        return context.find(type, currentNameSpace);
+      } else {
+        throw new SchemaParseException("A schema \"type\" MUST be a primitive type or one of"
+            + " \"enum\", \"fixed\", \"record\", \"error\", \"array\" or \"map\".");
       }
     } else if (schema.isArray()) { // union
       return parseUnion(schema, context, currentNameSpace);

lang/java/avro/src/test/java/org/apache/avro/TestSchema.java

@@ -648,6 +648,15 @@ void testParserNullValidate() {
     new Schema.Parser((NameValidator) null).parse("{\"type\":\"record\",\"name\":\"\",\"fields\":[]}"); // Empty name
   }
 
+  @Test
+  void disallowTypeObjectForNamedType() {
+    String withTypeObjectWithNamedType = "{"
+        + "\"namespace\":\"tests\",\"type\":\"record\",\"name\":\"Invalid\",\"fields\":["
+        + "{\"name\":\"good\",\"type\":{\"type\":\"fixed\",\"name\":\"Hash\",\"size\":16}},"
+        + "{\"name\":\"right\",\"type\":{\"type\":\"Hash\"}}]}";
+    assertThrows(SchemaParseException.class, () -> new SchemaParser().parse(withTypeObjectWithNamedType).mainSchema());
+  }
+
   /**
    * Tests when a user tries to write a record with an invalid enum symbol value
    * that the exception returned is more descriptive than just a NPE or an

lang/java/avro/src/test/java/org/apache/avro/TestUnionSelfReference.java

@@ -31,20 +31,15 @@ public class TestUnionSelfReference {
   @SuppressWarnings("unused")
   private static final Logger LOG = LoggerFactory.getLogger(TestUnionSelfReference.class);
 
-  private static final String SIMPLE_BINARY_TREE = "{" + "    \"namespace\": \"tree\"," + "    \"type\": \"record\","
-      + "    \"name\": \"Node\"," + "    \"fields\": [" + "      {" + "        \"name\": \"left\","
-      + "        \"type\": [" + "          \"null\"," + "          {" + "            \"type\": \"Node\"" + "          }"
-      + "        ]," + "        \"default\": null" + "      }," + "      {" + "        \"name\": \"right\","
-      + "        \"type\": [" + "          \"null\"," + "          {" + "            \"type\": \"Node\"" + "          }"
-      + "        ]," + "        \"default\": null" + "      }" + "    ]" + "  }";
+  private static final String SIMPLE_BINARY_TREE = "{"
+      + "\"namespace\":\"tree\",\"type\":\"record\",\"name\":\"Node\",\"fields\":["
+      + "{\"name\":\"left\",\"type\":[\"null\",\"Node\"],\"default\":null},"
+      + "{\"name\":\"right\",\"type\":[\"null\",\"Node\"],\"default\":null}]}";
 
-  private static final String THREE_TYPE_UNION = "{" + "    \"namespace\": \"tree\"," + "    \"type\": \"record\","
-      + "    \"name\": \"Node\"," + "    \"fields\": [" + "      {" + "        \"name\": \"left\","
-      + "        \"type\": [" + "          \"null\"," + "          \"string\"," + "          {"
-      + "            \"type\": \"Node\"" + "          }" + "        ]," + "        \"default\": null" + "      },"
-      + "      {" + "        \"name\": \"right\"," + "        \"type\": [" + "          \"null\","
-      + "          \"string\"," + "          {" + "            \"type\": \"Node\"" + "          }" + "        ],"
-      + "        \"default\": null" + "      }" + "    ]" + "  }";
+  private static final String THREE_TYPE_UNION = "{"
+      + "\"namespace\":\"tree\",\"type\":\"record\",\"name\":\"Node\",\"fields\":["
+      + "{\"name\":\"left\",\"type\":[\"null\",\"string\",\"Node\"],\"default\":null},"
+      + "{\"name\":\"right\",\"type\":[\"null\",\"string\",\"Node\"],\"default\":null}]}";
 
   @Test
   void selfReferenceInUnion() {