Skip EnumCodec registration if the builder has no mappings.

Also, improve PostgresTypes safety if the collection of type names is empty.

[resolves #515]

Signed-off-by: Mark Paluch <mpaluch@vmware.com>

src/main/java/io/r2dbc/postgresql/codec/EnumCodec.java

@@ -212,6 +212,10 @@ public CodecRegistrar build() {
 
             Map<String, Class<? extends Enum<?>>> mapping = new LinkedHashMap<>(this.mapping);
 
+            if (mapping.isEmpty()) {
+                return (connection, allocator, registry) -> Mono.empty();
+            }
+
             return (connection, allocator, registry) -> {
 
                 List<String> missing = new ArrayList<>(mapping.keySet());

src/main/java/io/r2dbc/postgresql/codec/PostgresTypes.java

@@ -25,6 +25,7 @@
 
 import java.util.Objects;
 import java.util.StringJoiner;
+import java.util.concurrent.atomic.AtomicBoolean;
 import java.util.regex.Pattern;
 
 /**
@@ -85,16 +86,22 @@ public Mono<PostgresType> lookupType(String typeName) {
     public Flux<PostgresType> lookupTypes(Iterable<String> typeNames) {
 
         StringJoiner joiner = new StringJoiner(",", "(", ")");
+        AtomicBoolean hasType = new AtomicBoolean();
 
         typeNames.forEach(typeName -> {
 
             if (!TYPENAME.matcher(Assert.requireNonNull(typeName, "typeName must not be null")).matches()) {
                 throw new IllegalArgumentException(String.format("Invalid typename %s", typeName));
             }
 
+            hasType.set(true);
             joiner.add("'" + typeName + "'");
         });
 
+        if (!hasType.get()) {
+            return Flux.empty();
+        }
+
         return this.connection.createStatement(String.format(SELECT_PG_TYPE, "IN", joiner, "")).execute()
             .flatMap(it -> it.map((row, rowMetadata) -> {
 

src/test/java/io/r2dbc/postgresql/codec/EnumCodecIntegrationTests.java

@@ -54,6 +54,24 @@ protected void customize(PostgresqlConnectionConfiguration.Builder builder) {
         builder.codecRegistrar(EnumCodec.builder().withEnum("my_enum_with_codec", MyEnum.class).build());
     }
 
+    @Test
+    void shouldNotRegisterIfEmpty() {
+
+        PostgresqlConnectionConfiguration configuration = PostgresqlConnectionConfiguration.builder()
+            .database(SERVER.getDatabase())
+            .host(SERVER.getHost())
+            .port(SERVER.getPort())
+            .password(SERVER.getPassword())
+            .username(SERVER.getUsername())
+            .codecRegistrar(EnumCodec.builder().build())
+            .build();
+
+        PostgresqlConnectionFactory connectionFactory = new PostgresqlConnectionFactory(configuration);
+        connectionFactory.create().flatMap(PostgresqlConnection::close).as(StepVerifier::create).verifyComplete();
+
+        // we cannot really assert logs so that's up to you.
+    }
+
     @Test
     void shouldReportUnresolvableTypes() {