Assertion 'Response body stream writer deinitialized before .end or .error was sent.' hit in Vapor when using body.collect in Middleware & streaming

[weissi]

I've got a Vapor app which has a small Middleware which just attempts to print the first bits of the incoming body and then just continue the request chain. This appears to work fine until I use a route (/echo) which uses body streaming.

When I then post to that route, Vapor hits this assertion

assert(self.isComplete, "Response body stream writer deinitialized before .end or .error was sent.")

var env = try Environment.detect()
try LoggingSystem.bootstrap(from: &env)

let app = Application(env)
defer { app.shutdown() }

struct MyMiddleware: Middleware {
    func respond(to request: Request, chainingTo next: Responder) -> EventLoopFuture<Response> {
        request.body.collect(max: 1).whenSuccess { print("first body bits", $0.map { String(buffer: $0) } ?? "n/a") }
        return next.respond(to: request)
    }
}

app.middleware.use(MyMiddleware(), at: .beginning)

app.on(.POST, "echo", body: .stream)  { request -> Response in
    let r = Response(body: .init(stream: { writer in
        request.body.drain { body in
            switch body {
            case .buffer(let buffer):
                return writer.write(.buffer(buffer))
            case .error(let error):
                return writer.write(.error(error))
            case .end:
                return writer.write(.end)
            }
        }
    }))
    r.headers.add(name: "content-type", value: "application/octet-stream")
    return r
}

try app.run()

To trigger the crash, you can post anything that's like 2000 bytes or more usually. For example

curl -v -d "$(python -c 'print 100000*"x"')" http://localhost:8080/echo

When the middleware isn't used, this appears to work ok.

---

Vapor: 4.68.0

[mkll]

You can't use request.body.collect() while streaming. Your route handler is correct, your middleware is not.

[weissi]

@mkll Hmm, but the middleware doesn't really know anything about the request, right? Basically this would mean that any middleware that looks into the body at all is incompatible with all routes that stream, correct?

Because body.drain from a middleware also doesn't work.

If that's all working as designed, then at least a better error message would be good, no?

[Joannis]

Interesting use case. I think this should be supported nonetheless.

[weissi]

Thanks @0xTim, this might be related to #2742