Fix releasing bug in StringDecoder

This commit fixes an issue in StringDecoder, where, if the buffer did
not contain any delimiters, it was released before it was relayed to
any subscribers.

Closes gh-29119

spring-core/src/main/java/org/springframework/core/codec/StringDecoder.java

@@ -143,13 +143,14 @@ public final class StringDecoder extends AbstractDataBufferDecoder<String> {
 	private Collection<DataBuffer> processDataBuffer(
 			DataBuffer buffer, DataBufferUtils.Matcher matcher, LimitedDataBufferList chunks) {
 
+		boolean release = true;
 		try {
 			List<DataBuffer> result = null;
 			do {
 				int endIndex = matcher.match(buffer);
 				if (endIndex == -1) {
 					chunks.add(buffer);
-					DataBufferUtils.retain(buffer); // retain after add (may raise DataBufferLimitException)
+					release = false;
 					break;
 				}
 				DataBuffer split = buffer.split(endIndex + 1);
@@ -177,7 +178,9 @@ public final class StringDecoder extends AbstractDataBufferDecoder<String> {
 			return (result != null ? result : Collections.emptyList());
 		}
 		finally {
-			DataBufferUtils.release(buffer);
+			if (release) {
+				DataBufferUtils.release(buffer);
+			}
 		}
 	}
 

spring-webflux/src/test/java/org/springframework/web/reactive/function/client/WebClientIntegrationTests.java

@@ -1258,6 +1258,26 @@ class WebClientIntegrationTests {
 				.verify();
 	}
 
+	@ParameterizedWebClientTest
+	void retrieveTextDecodedToFlux(ClientHttpConnector connector) {
+		startServer(connector);
+
+		prepareResponse(response -> response
+				.setHeader("Content-Type", "text/plain")
+				.setBody("Hey now"));
+
+		Flux<String> result = this.webClient.get()
+				.uri("/")
+				.accept(MediaType.TEXT_PLAIN)
+				.retrieve()
+				.bodyToFlux(String.class);
+
+		StepVerifier.create(result)
+				.expectNext("Hey now")
+				.expectComplete()
+				.verify(Duration.ofSeconds(3));
+	}
+
 	private <T> Mono<T> doMalformedChunkedResponseTest(
 			ClientHttpConnector connector, Function<ResponseSpec, Mono<T>> handler) {