Backport fixes for discarding data buffers

Closes gh-26232

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

@@ -128,7 +128,8 @@ public class EncoderHttpMessageWriter<T> implements HttpMessageWriter<T> {
 						message.getHeaders().setContentLength(buffer.readableByteCount());
 						return message.writeWith(Mono.just(buffer)
 								.doOnDiscard(PooledDataBuffer.class, DataBufferUtils::release));
-					});
+					})
+					.doOnDiscard(PooledDataBuffer.class, DataBufferUtils::release);
 		}
 
 		if (isStreamingMediaType(contentType)) {

spring-web/src/main/java/org/springframework/http/server/reactive/AbstractServerHttpResponse.java

@@ -203,10 +203,28 @@ public abstract class AbstractServerHttpResponse implements ServerHttpResponse {
 		// We must resolve value first however, for a chance to handle potential error.
 		if (body instanceof Mono) {
 			return ((Mono<? extends DataBuffer>) body)
-					.flatMap(buffer -> doCommit(() ->
+					.flatMap(buffer -> {
-							writeWithInternal(Mono.fromCallable(() -> buffer)
+						AtomicReference<Boolean> subscribed = new AtomicReference<>(false);
-									.doOnDiscard(PooledDataBuffer.class, DataBufferUtils::release))))
+						return doCommit(
-					.doOnError(t -> getHeaders().clearContentHeaders());
+								() -> {
+									try {
+										return writeWithInternal(Mono.fromCallable(() -> buffer)
+												.doOnSubscribe(s -> subscribed.set(true))
+												.doOnDiscard(PooledDataBuffer.class, DataBufferUtils::release));
+									}
+									catch (Throwable ex) {
+										return Mono.error(ex);
+									}
+								})
+								.doOnError(ex -> DataBufferUtils.release(buffer))
+								.doOnCancel(() -> {
+									if (!subscribed.get()) {
+										DataBufferUtils.release(buffer);
+									}
+								});
+					})
+					.doOnError(t -> getHeaders().clearContentHeaders())
+					.doOnDiscard(PooledDataBuffer.class, DataBufferUtils::release);
 		}
 		else {
 			return new ChannelSendOperator<>(body, inner -> doCommit(() -> writeWithInternal(inner)))

spring-web/src/test/java/org/springframework/http/server/reactive/ServerHttpResponseTests.java

@@ -19,7 +19,9 @@ package org.springframework.http.server.reactive;
 import java.nio.ByteBuffer;
 import java.nio.charset.StandardCharsets;
 import java.util.ArrayList;
+import java.util.Collections;
 import java.util.List;
+import java.util.Map;
 import java.util.function.Consumer;
 import java.util.function.Supplier;
 
@@ -27,14 +29,22 @@ import org.junit.jupiter.api.Test;
 import org.reactivestreams.Publisher;
 import reactor.core.publisher.Flux;
 import reactor.core.publisher.Mono;
+import reactor.netty.channel.AbortedException;
 import reactor.test.StepVerifier;
 
+import org.springframework.core.ResolvableType;
 import org.springframework.core.io.buffer.DataBuffer;
 import org.springframework.core.io.buffer.DefaultDataBuffer;
 import org.springframework.core.io.buffer.DefaultDataBufferFactory;
+import org.springframework.core.testfixture.io.buffer.LeakAwareDataBufferFactory;
 import org.springframework.http.HttpHeaders;
 import org.springframework.http.MediaType;
 import org.springframework.http.ResponseCookie;
+import org.springframework.http.codec.EncoderHttpMessageWriter;
+import org.springframework.http.codec.HttpMessageWriter;
+import org.springframework.http.codec.json.Jackson2JsonEncoder;
+import org.springframework.web.testfixture.http.server.reactive.MockServerHttpRequest;
+import org.springframework.web.testfixture.http.server.reactive.MockServerHttpResponse;
 
 import static org.assertj.core.api.Assertions.assertThat;
 
@@ -176,6 +186,24 @@ public class ServerHttpResponseTests {
 		});
 	}
 
+	@Test // gh-26232
+	void monoResponseShouldNotLeakIfCancelled() {
+		LeakAwareDataBufferFactory bufferFactory = new LeakAwareDataBufferFactory();
+		MockServerHttpRequest request = MockServerHttpRequest.get("/").build();
+		MockServerHttpResponse response = new MockServerHttpResponse(bufferFactory);
+		response.setWriteHandler(flux -> {
+			throw AbortedException.beforeSend();
+		});
+
+		HttpMessageWriter<Object> messageWriter = new EncoderHttpMessageWriter<>(new Jackson2JsonEncoder());
+		Mono<Void> result = messageWriter.write(Mono.just(Collections.singletonMap("foo", "bar")),
+				ResolvableType.forClass(Mono.class), ResolvableType.forClass(Map.class), null,
+				request, response, Collections.emptyMap());
+
+		StepVerifier.create(result).expectError(AbortedException.class).verify();
+
+		bufferFactory.checkForLeaks();
+	}
 
 	private DefaultDataBuffer wrap(String a) {
 		return new DefaultDataBufferFactory().wrap(ByteBuffer.wrap(a.getBytes(StandardCharsets.UTF_8)));