Fix fingerprint method for null default values (#733)

* Fix default toString for JsonProperties.NULL_VALUE

The normalization of the Schema is inconsistent across restarts due to object's JsonProperties.NULL_VALUE lack of override toString.

This fixes that and allows deterministic creation for fingerprints.

* Assert that fingerprint hash remains the same across executions

Added test case to assess that generated fingerprint is always the same
between multiple executions, to prevent using an invalid stringified
version of Object.

Same Schema string must return same fingerprint, at least as long as
normalize function is not changed. Forced MD5, just in case in future
releases, the default hashing algorithm changes.

schema-registry/common/src/main/java/com/hortonworks/registries/schemaregistry/avro/AvroSchemaProvider.java

@@ -22,6 +22,7 @@
 import com.hortonworks.registries.schemaregistry.SchemaFieldInfo;
 import com.hortonworks.registries.schemaregistry.errors.InvalidSchemaException;
 import com.hortonworks.registries.schemaregistry.errors.SchemaNotFoundException;
+import org.apache.avro.JsonProperties;
 import org.apache.avro.Schema;
 
 import java.io.IOException;
@@ -180,7 +181,11 @@ private static Appendable build(Map<String, String> env,
                     // handle default value
                     Object defaultValue = field.defaultVal();
                     if (defaultValue != null) {
-                        appendable.append(defaultValue.toString());
+                        if (defaultValue == JsonProperties.NULL_VALUE) {
+                            appendable.append("null");
+                        } else {
+                            appendable.append(defaultValue.toString());
+                        }
                     }
 
                     build(env, field.schema(), appendable).append("}");

schema-registry/common/src/test/java/com/hortonworks/registries/schemaregistry/avro/AvroSchemaProviderTest.java

@@ -0,0 +1,44 @@
+package com.hortonworks.registries.schemaregistry.avro;
+
+import com.hortonworks.registries.schemaregistry.SchemaProvider;
+import org.junit.Assert;
+import org.junit.Test;
+
+import java.util.Collections;
+
+import static org.hamcrest.core.IsEqual.equalTo;
+
+public class AvroSchemaProviderTest {
+
+    private String schemaWithNullDefaults = "{\n" +
+            "  \"type\" : \"record\",\n" +
+            "  \"namespace\" : \"com.hortonworks.registries\",\n" +
+            "  \"name\" : \"trucks\",\n" +
+            "  \"fields\" : [\n" +
+            "    { \"name\" : \"driverId\" , \"type\" : [\"null\", \"int\"], \"default\": null }" +
+            "  ]\n" +
+            "}\n";
+
+    @Test
+    public void testGetFingerprintNullTypeDeterminism() throws Exception {
+        AvroSchemaProvider avroSchemaProvider = new AvroSchemaProvider();
+        avroSchemaProvider.init(Collections.singletonMap(SchemaProvider.HASH_FUNCTION_CONFIG, "MD5"));
+
+        String withNullDefaultsFingerprintHex = bytesToHex(avroSchemaProvider.getFingerprint(schemaWithNullDefaults));
+
+        Assert.assertThat(withNullDefaultsFingerprintHex, equalTo("e1e17a3aef8c728c131204bbf49046b2"));
+    }
+
+    private static String bytesToHex(byte[] hash) {
+        StringBuilder hexString = new StringBuilder();
+        for (byte b : hash) {
+            String hex = Integer.toHexString(0xff & b);
+            if (hex.length() == 1) {
+                hexString.append('0');
+            }
+            hexString.append(hex);
+        }
+        return hexString.toString();
+    }
+
+}