Support empty part in DefaultPartHttpMessageReader

This commit fixes a bug in DefaultPartHttpMessageReader's
MultipartParser, due to which the last token in a part window was not
properly indicated.

See gh-30953
Closes gh-31766

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

@@ -540,7 +540,7 @@ final class MultipartParser extends BaseSubscriber<DataBuffer> {
 					while ((prev = this.queue.pollLast()) != null) {
 						int prevByteCount = prev.readableByteCount();
 						int prevLen = prevByteCount + len;
-						if (prevLen > 0) {
+						if (prevLen >= 0) {
 							// slice body part of previous buffer, and flush it
 							DataBuffer body = prev.split(prevLen + prev.readPosition());
 							DataBufferUtils.release(prev);

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

@@ -301,6 +301,23 @@ class DefaultPartHttpMessageReaderTests  {
 		latch.await();
 	}
 
+	@ParameterizedDefaultPartHttpMessageReaderTest
+	void emptyLastPart(DefaultPartHttpMessageReader reader) throws InterruptedException {
+		MockServerHttpRequest request = createRequest(
+				new ClassPathResource("empty-part.multipart", getClass()), "LiG0chJ0k7YtLt-FzTklYFgz50i88xJCW5jD");
+
+		Flux<Part> result = reader.read(forClass(Part.class), request, emptyMap());
+
+		CountDownLatch latch = new CountDownLatch(2);
+		StepVerifier.create(result)
+				.consumeNextWith(part -> testPart(part, null, "", latch))
+				.consumeNextWith(part -> testPart(part, null, "", latch))
+				.verifyComplete();
+
+		latch.await();
+	}
+
+
 	private void testBrowser(DefaultPartHttpMessageReader reader, Resource resource, String boundary)
 			throws InterruptedException {
 

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

@@ -0,0 +1,13 @@
+--LiG0chJ0k7YtLt-FzTklYFgz50i88xJCW5jD
+Content-Disposition: form-data; name="files"; filename="file17312898095703516893.tmp"
+Content-Type: application/octet-stream
+Content-Length: 0
+
+
+--LiG0chJ0k7YtLt-FzTklYFgz50i88xJCW5jD
+Content-Disposition: form-data; name="files"; filename="file14790463448453253614.tmp"
+Content-Type: application/octet-stream
+Content-Length: 0
+
+
+--LiG0chJ0k7YtLt-FzTklYFgz50i88xJCW5jD--

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

@@ -259,13 +259,19 @@ class MultipartRouterFunctionIntegrationTests extends AbstractRouterFunctionInte
 						assertThat(data).hasSize(2);
 
 						List<PartEvent> fileData = data.get(0);
-						assertThat(fileData).hasSize(1);
+						assertThat(fileData).hasSize(2);
 						assertThat(fileData.get(0)).isInstanceOf(FilePartEvent.class);
 						FilePartEvent filePartEvent = (FilePartEvent) fileData.get(0);
 						assertThat(filePartEvent.name()).isEqualTo("fooPart");
 						assertThat(filePartEvent.filename()).isEqualTo("foo.txt");
 						DataBufferUtils.release(filePartEvent.content());
 
+						assertThat(fileData.get(1)).isInstanceOf(FilePartEvent.class);
+						filePartEvent = (FilePartEvent) fileData.get(1);
+						assertThat(filePartEvent.name()).isEqualTo("fooPart");
+						assertThat(filePartEvent.filename()).isEqualTo("foo.txt");
+						DataBufferUtils.release(filePartEvent.content());
+
 						List<PartEvent> fieldData = data.get(1);
 						assertThat(fieldData).hasSize(1);
 						assertThat(fieldData.get(0)).isInstanceOf(FormPartEvent.class);