ServerSentEventHttpMessageReader internal refactoring

Eliminate use of .block() which Reactor now flags as illegal on
schedulers where that's not expected.

spring-web/src/main/java/org/springframework/http/codec/ServerSentEventHttpMessageReader.java

@@ -24,6 +24,7 @@ import java.util.Collections;
 import java.util.List;
 import java.util.Map;
 import java.util.function.IntPredicate;
+import java.util.stream.Collectors;
 
 import reactor.core.publisher.Flux;
 import reactor.core.publisher.Mono;
@@ -40,8 +41,6 @@ import org.springframework.http.MediaType;
 import org.springframework.http.ReactiveHttpInputMessage;
 import org.springframework.lang.Nullable;
 
-import static java.util.stream.Collectors.joining;
-
 /**
  * Reader that supports a stream of {@link ServerSentEvent}s and also plain
  * {@link Object}s which is the same as an {@link ServerSentEvent} with data only.
@@ -120,9 +119,10 @@ public class ServerSentEventHttpMessageReader implements HttpMessageReader<Objec
 				})
 				.bufferUntil(line -> line.equals("\n"))
 				.concatMap(rawLines -> {
-					String[] lines = rawLines.stream().collect(joining()).split("\\r?\\n");
-					ServerSentEvent<Object> event = buildEvent(lines, valueType, hints);
-					return (shouldWrap ? Mono.just(event) : Mono.justOrEmpty(event.data()));
+					String[] lines = rawLines.stream().collect(Collectors.joining()).split("\\r?\\n");
+					return buildEvent(lines, valueType, hints)
+							.filter(event -> shouldWrap || event.data() != null)
+							.map(event -> shouldWrap ? event : event.data());
 				})
 				.cast(Object.class);
 	}
@@ -144,12 +144,12 @@ public class ServerSentEventHttpMessageReader implements HttpMessageReader<Objec
 		return Flux.fromIterable(results);
 	}
 
-	private ServerSentEvent<Object> buildEvent(String[] lines, ResolvableType valueType,
+	private Mono<ServerSentEvent<Object>> buildEvent(String[] lines, ResolvableType valueType,
 			Map<String, Object> hints) {
 
 		ServerSentEvent.Builder<Object> sseBuilder = ServerSentEvent.builder();
-		StringBuilder mutableData = new StringBuilder();
-		StringBuilder mutableComment = new StringBuilder();
+		StringBuilder data = null;
+		StringBuilder comment = null;
 
 		for (String line : lines) {
 			if (line.startsWith("id:")) {
@@ -159,42 +159,43 @@ public class ServerSentEventHttpMessageReader implements HttpMessageReader<Objec
 				sseBuilder.event(line.substring(6));
 			}
 			else if (line.startsWith("data:")) {
-				mutableData.append(line.substring(5)).append("\n");
+				data = (data != null ? data : new StringBuilder());
+				data.append(line.substring(5)).append("\n");
 			}
 			else if (line.startsWith("retry:")) {
 				sseBuilder.retry(Duration.ofMillis(Long.valueOf(line.substring(6))));
 			}
 			else if (line.startsWith(":")) {
-				mutableComment.append(line.substring(1)).append("\n");
+				comment = (comment != null ? comment : new StringBuilder());
+				comment.append(line.substring(1)).append("\n");
 			}
 		}
-		if (mutableData.length() > 0) {
-			String data = mutableData.toString();
-			sseBuilder.data(decodeData(data, valueType, hints));
+		if (comment != null) {
+			sseBuilder.comment(comment.toString().substring(0, comment.length() - 1));
+		}
+		if (data != null) {
+			return decodeData(data.toString(), valueType, hints).map(decodedData -> {
+				sseBuilder.data(decodedData);
+				return sseBuilder.build();
+			});
 		}
-		if (mutableComment.length() > 0) {
-			String comment = mutableComment.toString();
-			sseBuilder.comment(comment.substring(0, comment.length() - 1));
+		else {
+			return Mono.just(sseBuilder.build());
 		}
-		return sseBuilder.build();
 	}
 
-	@Nullable
-	private Object decodeData(String data, ResolvableType dataType, Map<String, Object> hints) {
+	private Mono<?> decodeData(String data, ResolvableType dataType, Map<String, Object> hints) {
 		if (String.class == dataType.resolve()) {
-			return data.substring(0, data.length() - 1);
+			return Mono.just(data.substring(0, data.length() - 1));
 		}
 
 		if (this.decoder == null) {
-			return Flux.error(new CodecException("No SSE decoder configured and the data is not String."));
+			return Mono.error(new CodecException("No SSE decoder configured and the data is not String."));
 		}
 
 		byte[] bytes = data.getBytes(StandardCharsets.UTF_8);
 		Mono<DataBuffer> input = Mono.just(bufferFactory.wrap(bytes));
-
-		return this.decoder
-				.decodeToMono(input, dataType, MediaType.TEXT_EVENT_STREAM, hints)
-				.block(Duration.ZERO);
+		return this.decoder.decodeToMono(input, dataType, MediaType.TEXT_EVENT_STREAM, hints);
 	}
 
 	@Override