Return null elements for absent keys using reactive MGET.

We now use correctly null to indicate absent keys when using reactive MGET. Previously, we used an empty byte buffer that could be incorrectly translated to an empty string when using the string codec. An empty byte buffer can also be returned if the value length is zero leading to a state that doesn't allow distinguishing between absence and empty value.

Closes #2402

Author: mp911de

src/main/java/org/springframework/data/redis/connection/lettuce/LettuceReactiveStringCommands.java

@@ -17,11 +17,13 @@
 
 import io.lettuce.core.BitFieldArgs;
 import io.lettuce.core.GetExArgs;
+import io.lettuce.core.KeyValue;
 import io.lettuce.core.SetArgs;
 import reactor.core.publisher.Flux;
 import reactor.core.publisher.Mono;
 
 import java.nio.ByteBuffer;
+import java.util.ArrayList;
 import java.util.List;
 import java.util.stream.Collectors;
 
@@ -47,8 +49,6 @@
  */
 class LettuceReactiveStringCommands implements ReactiveStringCommands {
 
-	private static final ByteBuffer EMPTY_BYTE_BUFFER = ByteBuffer.wrap(new byte[0]);
-
 	private final LettuceReactiveRedisConnection connection;
 
 	/**
@@ -70,8 +70,9 @@ public Flux<MultiValueResponse<List<ByteBuffer>, ByteBuffer>> mGet(Publisher<Lis
 
 			Assert.notNull(keys, "Keys must not be null");
 
-			return cmd.mget(keys.toArray(new ByteBuffer[0])).map((value) -> value.getValueOrElse(EMPTY_BYTE_BUFFER))
-					.collectList().map((values) -> new MultiValueResponse<>(keys, values));
+			return cmd.mget(keys.toArray(new ByteBuffer[0])).collectList().map((value) -> {
+				return value.stream().map(keyValue -> keyValue.getValueOrElse(null)).collect(Collectors.toList());
+			}).map((values) -> new MultiValueResponse<>(keys, values));
 		}));
 	}
 
@@ -295,9 +296,9 @@ public Flux<NumericResponse<BitCountCommand, Long>> bitCount(Publisher<BitCountC
 
 			Range<Long> range = command.getRange();
 
-			return (!Range.unbounded().equals(range) ? cmd.bitcount(command.getKey(),
-					LettuceConverters.getLowerBoundIndex(range), //
-					LettuceConverters.getUpperBoundIndex(range)) //
+			return (!Range.unbounded().equals(range)
+					? cmd.bitcount(command.getKey(), LettuceConverters.getLowerBoundIndex(range), //
+							LettuceConverters.getUpperBoundIndex(range)) //
 					: cmd.bitcount(command.getKey())).map(responseValue -> new NumericResponse<>(command, responseValue));
 		}));
 	}

src/test/java/org/springframework/data/redis/connection/lettuce/LettuceReactiveStringCommandsIntegrationTests.java

@@ -202,7 +202,7 @@ void mGetShouldRetrieveNullValueCorrectly() {
 		Mono<List<ByteBuffer>> result = connection.stringCommands()
 				.mGet(Arrays.asList(KEY_1_BBUFFER, KEY_2_BBUFFER, KEY_3_BBUFFER));
 
-		assertThat(result.block()).containsExactly(VALUE_1_BBUFFER, ByteBuffer.allocate(0), VALUE_3_BBUFFER);
+		assertThat(result.block()).containsExactly(VALUE_1_BBUFFER, null, VALUE_3_BBUFFER);
 	}
 
 	@ParameterizedRedisTest // DATAREDIS-525

src/test/java/org/springframework/data/redis/core/DefaultReactiveValueOperationsIntegrationTests.java

@@ -306,13 +306,6 @@ void multiGet() {
 		V value2 = valueFactory.instance();
 		V absentValue = null;
 
-		if (serializer instanceof StringRedisSerializer) {
-			absentValue = (V) "";
-		}
-		if (value1 instanceof ByteBuffer) {
-			absentValue = (V) ByteBuffer.wrap(new byte[0]);
-		}
-
 		Map<K, V> map = new LinkedHashMap<>();
 		map.put(key1, value1);
 		map.put(key2, value2);