Ensure chunks released on cancel in StringDecoder

The current test were not catching the issue because they request 1
via StepVerifier, wait for it, and then cancel. In the case of
StringDecoder it means all chunks are used up to produce that first
String and so the cancel doesn't catch any cached chunks.

Closes gh-30299

Author: rstoyanchev

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

@@ -124,7 +124,7 @@ public Flux<String> decode(Publisher<DataBuffer> input, ResolvableType elementTy
 					chunks.clear();
 					return Mono.just(lastBuffer);
 				}))
-				.doOnTerminate(chunks::releaseAndClear)
+				.doFinally(signalType -> chunks.releaseAndClear())
 				.doOnDiscard(DataBuffer.class, DataBufferUtils::release)
 				.map(buffer -> decode(buffer, elementType, mimeType, hints));
 	}

spring-core/src/test/java/org/springframework/core/codec/StringDecoderTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2020 the original author or authors.
+ * Copyright 2002-2023 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -18,12 +18,15 @@
 
 import java.nio.charset.Charset;
 import java.nio.charset.StandardCharsets;
+import java.time.Duration;
 import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.Collections;
 import java.util.List;
 
 import org.junit.jupiter.api.Test;
+import org.reactivestreams.Subscription;
+import reactor.core.publisher.BaseSubscriber;
 import reactor.core.publisher.Flux;
 import reactor.test.StepVerifier;
 
@@ -75,14 +78,15 @@ public void decode() {
 		String s = String.format("%s\n%s\n%s", u, e, o);
 		Flux<DataBuffer> input = toDataBuffers(s, 1, UTF_8);
 
-		// TODO: temporarily replace testDecodeAll with explicit decode/cancel/empty
-		// see https://github.com/reactor/reactor-core/issues/2041
+		testDecodeAll(input, TYPE, step -> step.expectNext(u, e, o).verifyComplete(), null, null);
+	}
 
-//		testDecode(input, TYPE, step -> step.expectNext(u, e, o).verifyComplete(), null, null);
-//		testDecodeCancel(input, TYPE, null, null);
-//		testDecodeEmpty(TYPE, null, null);
+	@Test // gh-30299
+	public void decodeAndCancelWithPendingChunks() {
+		Flux<DataBuffer> input = toDataBuffers("abc", 1, UTF_8).concatWith(Flux.never());
+		Flux<String> result = this.decoder.decode(input, TYPE, null, null);
 
-		testDecodeAll(input, TYPE, step -> step.expectNext(u, e, o).verifyComplete(), null, null);
+		StepVerifier.create(result).thenAwait(Duration.ofMillis(100)).thenCancel().verify();
 	}
 
 	@Test
@@ -264,4 +268,13 @@ private DataBuffer stringBuffer(String value) {
 		return buffer;
 	}
 
+
+	private static class SingleRequestSubscriber extends BaseSubscriber<String> {
+
+		@Override
+		protected void hookOnSubscribe(Subscription subscription) {
+			subscription.request(1);
+		}
+	}
+
 }