Corrupt multipart should not hang SynchronossPartHttpMessageReader

This commit notifies the Synchronoss listener that the buffer stream
has ended.

Closes gh-23768

(cherry picked from commit bf36f49)

Author: poutsma

spring-web/src/test/java/org/springframework/http/codec/multipart/SynchronossPartHttpMessageReaderTests.java

@@ -18,6 +18,7 @@
 
 import java.io.File;
 import java.io.IOException;
+import java.nio.channels.ReadableByteChannel;
 import java.time.Duration;
 import java.util.Map;
 import java.util.function.Consumer;
@@ -44,6 +45,7 @@
 import org.springframework.util.MultiValueMap;
 
 import static java.util.Collections.emptyMap;
+import static java.util.Collections.singletonMap;
 import static org.hamcrest.core.StringStartsWith.startsWith;
 import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertFalse;
@@ -149,6 +151,24 @@ public void readPartsWithoutDemand() {
 		subscriber.cancel();
 	}
 
+	@Test
+	public void gh23768() throws IOException {
+		ReadableByteChannel channel = new ClassPathResource("invalid.multipart", getClass()).readableChannel();
+		Flux<DataBuffer> body = DataBufferUtils.readByteChannel(() -> channel, this.bufferFactory, 1024);
+
+		MediaType contentType = new MediaType("multipart", "form-data",
+				singletonMap("boundary", "NbjrKgjbsaMLdnMxMfDpD6myWomYc0qNX0w"));
+		ServerHttpRequest request = MockServerHttpRequest.post("/")
+				.contentType(contentType)
+				.body(body);
+
+		Mono<MultiValueMap<String, Part>> parts = this.reader.readMono(PARTS_ELEMENT_TYPE, request, emptyMap());
+
+		StepVerifier.create(parts)
+				.assertNext(result -> assertThat(result).isEmpty())
+				.verifyComplete();
+	}
+
 	@Test
 	public void readTooManyParts() {
 		testMultipartExceptions(reader -> reader.setMaxParts(1), ex -> {

spring-web/src/test/resources/org/springframework/http/codec/multipart/invalid.multipart

@@ -0,0 +1,5 @@
+--NbjrKgjbsaMLdnMxMfDpD6myWomYc0qNX0w
+Content-Disposition: form-data; name="part-00-name"
+
+post-payload-text-23456789ABCDEF:post-payload-0001-3456789ABCDEF:post-payload-0002-3456789ABCDEF:post-payload-0003-3456789ABCDEF
+--NbjrKgjbsaMLdnMxMfDpD6myWomYc