Consistent maxPartSize check in PartEventHttpMessageReader

Closes gh-37028

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

@@ -188,6 +188,9 @@ public class PartEventHttpMessageReader extends LoggingCodecSupport implements H
 			if (this.maxPartSize == -1) {
 				maxSize = this.maxInMemorySize;
 			}
+			else if (this.maxInMemorySize == -1) {
+				maxSize = (int) Math.min(Integer.MAX_VALUE, this.maxPartSize);
+			}
 			else {
 				// maxInMemorySize is an int, so we can safely cast the long result of Math.min
 				maxSize = (int) Math.min(this.maxInMemorySize, this.maxPartSize);

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

@@ -255,6 +255,21 @@ class PartEventHttpMessageReaderTests extends AbstractLeakCheckingTests {
 				.verify();
 	}
 
+	@Test
+	void partSizeTooLargeWithUnlimitedMemorySize() {
+		MockServerHttpRequest request = createRequest("simple.multipart", "simple-boundary");
+
+		PartEventHttpMessageReader reader = new PartEventHttpMessageReader();
+		reader.setMaxPartSize(10);
+		reader.setMaxInMemorySize(-1);
+
+		Flux<PartEvent> result = reader.read(forClass(PartEvent.class), request, emptyMap());
+
+		StepVerifier.create(result)
+				.expectError(DataBufferLimitException.class)
+				.verify();
+	}
+
 	@Test
 	void formPartTooLarge() {
 		MockServerHttpRequest request = createRequest(