NIO Promise Leak During Response Body Stream

[jshier]

Steps to reproduce

Attempting to repeatedly write to a Response body stream results in a promise leak.

This route:

app.on(.GET, "chunked", ":count") { request -> Response in
    guard let count = request.parameters["count", as: Int.self], count > 0, count <= 100 else {
        return Response(status: .badRequest)
    }
    let response = Response(body: .init(stream: { writer in
        for i in 0..<count {
            writer.write(.buffer(.init(integer: i)))
        }
    }, count: 0))
    return response
}

Results in a runtime assertion about "leaking promise":

Fatal error: leaking promise created at (file: "/SourcePackages/checkouts/vapor/Sources/Vapor/HTTP/Server/HTTPServerHandler.swift", line: 53): file /SourcePackages/checkouts/vapor/Sources/Vapor/HTTP/Server/HTTPServerHandler.swift, line 53
2020-06-10 17:49:10.639020-0400 firewalk[70185:2526213] Fatal error: leaking promise created at (file: "/SourcePackages/checkouts/vapor/Sources/Vapor/HTTP/Server/HTTPServerHandler.swift", line: 53): file /SourcePackages/checkouts/vapor/Sources/Vapor/HTTP/Server/HTTPServerHandler.swift, line 53

Which is this code:

func serialize(_ response: Response, for request: Request, context: ChannelHandlerContext) {
    switch request.version.major {
    case 2:
        context.write(self.wrapOutboundOut(response), promise: nil)
    default:
        response.headers.add(name: .connection, value: request.isKeepAlive ? "keep-alive" : "close")
        let done = context.write(self.wrapOutboundOut(response)) // Leaked
        if !request.isKeepAlive {
            done.whenComplete { _ in
                context.close(mode: .output, promise: nil)
            }
        }
    }
}

Expected behavior

No leaked promises.

Actual behavior

Runtime assertion.

Environment

• Vapor Framework version: 4.8.0
• Vapor Toolbox version: N/A
• OS version: 10.15.5

[Joannis]

Hello @jshier, coincidentally @Wessi also ran into this issue in #2390 and provides a solution. I think this is a problem that should be primarily clarified in a better error message.

[jshier]

Yes, I filed this bug on his suggestion during a discussion of streaming bodies. He later did his own experiments and hit the same issue.

[tanner0101]

This should be fixed by https://github.com/vapor/vapor/releases/tag/4.11.0, lmk if that works for you.