Add streaming support for files in RESTEasy Reactive

Description

It would be nice to have a simple way to return file content as a stream in a reactive way in a REST resource method.

The original discussion is here.

Implementation ideas

This discussion took place on Zulip so I’m only conveying the ideas discussed there (by other people).

@geoand suggested to Stephane Epardaud making AsyncFile a known return type and properly handling it under the covers.

@Ladicek chimed in with:

Ladislav Thon: don’t we have a way to do an equivalent of Vert.x’s ReadStream.pipeTo(WriteStream)? Ladislav Thon: I know we don’t support StreamOutput in RESTEasy Reactive, but having a non-blocking variant of that would be really nice IMHO

Also, something to keep in mind is that it would be nice if a solution to this supported both whole files as well as some range from a file. I don’t know enough about the Vert.X FS API to know if this would be handled transparently by calling AsyncFile.setReadPos(long) and AsyncFile.setReadLength(long) before returning it or whether some special handling would be required for this.

[geoand]

The problem you mentione above has nothing to do with Quarkus, it’s a limitation of the var usage. If you use final StreamingOutput streamingOutput then everything works as expected

[Manfred73]

I’m using RestAssured to test my resources. Is it expected behaviour when returning a StreamingOutput, that the body is initially empty? How would you test this with RestAssured?

Real code:

@GET
@Path("/filename/{filename}")
@Produces(MediaType.APPLICATION_OCTET_STREAM)
public Uni<RestResponse<StreamingOutput>> readFileByFilename(@PathParam("filename") String filename) {
   validationService.validate(FileParams.builder().filename(filename).build());
   return resultCreator.create(queryService.getFileByFilename(filename));
}

The test:

@Test
void readByFilename() {
   // GIVEN
   final var testParams = mockSomeThings();

   // WHEN / THEN
   // How to test this StreamingOutput with restassured? Or should an IntegrationTest suffice here?
   given()
      .pathParam("filename", testParams.filename)
      .when()
      .get("/filename/{filename}")
      .then()
      .statusCode(200)
      .headers(
         "Content-Disposition", "attachment;filename=" + testParams.filename,
         "Content-Type", "application/octet-stream")
      .body(emptyString()); // is emptyString expected with StreamingOutput?
}