Calling .hasNext() or .next() on MultiPart Iterator after the first call to .next() causes NullPointerException

[brain-dev-null]

When iterating over the parts of a multipart request using the io.helidon.http.media.multipart.MultiPart iterator, calliing .hasNext() of .next() after the first call to .next() causes a NullPointerException to be thrown.

Environment Details

• Helidon Version: 4.0.5
• Helidon SE
• JDK version: 21
• OS: Linux x86
• Docker version (if applicable): /

---

Problem Description

Current behavior: Iterating over parts of a multipart requests fails when calling .hasNext() or .next() on io.helidon.http.media.multipart.MultiPart after .next() was called once.

Expected behavior: Calling .hasNext() should return true as long as unread parts remain and a subsequent call to .next() should return the next io.helidon.http.media.multipart.ReadablePart instance from the request. If no more parts remain, .hasNext() should return false.

Theis can be reproduced by running the Helidon SE MultiPart Example or the minimal example shown below.

When I did this, the following stacktrace was printed:

io.helidon.http.RequestException: Cannot invoke "io.helidon.common.buffers.BufferData.available()" because "this.nextBuffer" is null
	at io.helidon.http.RequestException$Builder.build(RequestException.java:139)
	at io.helidon.webserver.http.ErrorHandlers.unhandledError(ErrorHandlers.java:202)
	at io.helidon.webserver.http.ErrorHandlers.lambda$handleError$1(ErrorHandlers.java:182)
	at java.base/java.util.Optional.ifPresentOrElse(Optional.java:198)
	at io.helidon.webserver.http.ErrorHandlers.handleError(ErrorHandlers.java:181)
	at io.helidon.webserver.http.ErrorHandlers.runWithErrorHandling(ErrorHandlers.java:118)
	at io.helidon.webserver.http.Filters.filter(Filters.java:77)
	at io.helidon.webserver.http.HttpRouting.route(HttpRouting.java:109)
	at io.helidon.webserver.http1.Http1Connection.route(Http1Connection.java:396)
	at io.helidon.webserver.http1.Http1Connection.handle(Http1Connection.java:169)
	at io.helidon.webserver.ConnectionHandler.run(ConnectionHandler.java:155)
	at io.helidon.common.task.InterruptableTask.call(InterruptableTask.java:47)
	at io.helidon.webserver.ThreadPerTaskExecutor$ThreadBoundFuture.run(ThreadPerTaskExecutor.java:239)
	at java.base/java.lang.VirtualThread.run(VirtualThread.java:311)
Caused by: java.lang.NullPointerException: Cannot invoke "io.helidon.common.buffers.BufferData.available()" because "this.nextBuffer" is null
	at io.helidon.http.media.multipart.ReadablePartNoLength$PartInputStream.finish(ReadablePartNoLength.java:198)
	at io.helidon.http.media.multipart.ReadablePartNoLength.finish(ReadablePartNoLength.java:77)
	at io.helidon.http.media.multipart.MultiPartImpl.hasNext(MultiPartImpl.java:70)
	at io.helidon.examples.media.multipart.FileService.upload(FileService.java:136)
	at io.helidon.webserver.http.HttpRouting$RoutingExecutor.doRoute(HttpRouting.java:668)
	at io.helidon.webserver.http.HttpRouting$RoutingExecutor.call(HttpRouting.java:627)
	at io.helidon.webserver.http.HttpRouting$RoutingExecutor.call(HttpRouting.java:605)
	at io.helidon.webserver.http.ErrorHandlers.runWithErrorHandling(ErrorHandlers.java:75)
	... 8 more

Steps to reproduce

1. Create a new Project using this code snippet and run it

   import io.helidon.http.media.multipart.MultiPart;
   import io.helidon.webserver.WebServer;
   
   public class Main {
       public static void main(String[] args) {
           WebServer.builder()
                   .port(8080)
                   .routing(routing -> routing.post("/", (serverRequest, serverResponse) -> {
                       var parts = serverRequest.content().as(MultiPart.class);
                       while (parts.hasNext()) {
                           var part = parts.next();
                           System.out.println(part.name());
                       }
                       serverResponse.status(200).send();
                   })).build().start();
           while (true) {}
       }
   }

2. Send a POST multipart request with more than 2 parts, e.g. using:

   curl --request POST 'http://localhost:8080' --form 'field_1=a' --form 'field_2=b'

The expected behavior of this procedure would be that the following output is printed:

field_1
field_2

The actual behavior is that only the first line including field_1 is printed, then the NullPointerException is thrown.

[tomas-langer]

Reproduced - requires streaming (no Content-Length defined) multipart parts, which we did not test.
Added test to make sure both approaches work.
Fixed an additional problem, where calling consume would fail with an exception.