The linked code has an issue. It's something like

    switch self.request.bodyStorage.withLockedValue({ $0 }) { // LOCK ACQUSITION
    case .stream(let stream): // ACT UPON value
        return stream.consume(max: max, on: self.request.eventLoop).map { buffer in
            self.request.bodyStorage.withLockedValue({ $0 = .collected(buffer) }) // LOCK RE-ACQUISITON & OVERWRITE

Any code that re-acquires locks and assumes that the state underneath hasn't changed isn't quite right.

If collect() is intended to only be run once, this should be documented. And to catch users doing a bad thing, it should probably be changed to something like

switch self.request.bodyStorage.withLockedValue({ value in let oldValue = value; value = .currentlyStreaming;  return oldValue })

and then you can throw an error if the user attempts to consume the body twice whilst it's streaming.

Most importantly, we should document exactly how collect() is supposed to be used.