What is "Server never wrote a response" ?

[fonsp]

Julia 1.5.3
HTTP.jl 0.9.1

Pluto started printing a new error message since the HTTP 0.9.1 update:

┌ Error: error handling request
│   exception =
│    Server never wrote a response
│    Stacktrace:
│     [1] error(::String) at ./error.jl:33
│     [2] macro expansion at /Users/fons/.julia/packages/HTTP/FV5cL/src/Servers.jl:406 [inlined]
│     [3] (::HTTP.Servers.var"#13#14"{HTTP.Handlers.var"#4#5"{HTTP.Handlers.StreamHandlerFunction{Pluto.var"#213#223"{Pluto.ServerSession,HTTP.Handlers.Router{Symbol("##253")},Base.RefValue{Function}}}},HTTP.ConnectionPool.Transaction{Sockets.TCPSocket},HTTP.Streams.Stream{HTTP.Messages.Request,HTTP.ConnectionPool.Transaction{Sockets.TCPSocket}}})() at ./task.jl:356
└ @ HTTP.Servers ~/.julia/packages/HTTP/FV5cL/src/Servers.jl:413

Why is this an error? Can I disable this message? I also can't tell which request this error is about, so it's difficult for me to fix. I will limit Pluto to HTTP 0.9.0 as a workaround

You can reproduce this error by using the version of Pluto before I set the 0.9.0 limit:

pkg> add Pluto#a01a6e70466292420517533767e9cbc781b3b9ef

[fonsp]

It looks like this is happening for every new WebSocket connection. The error is false: HTTP.jl writes a response to the HTTP upgrade request:

HTTP.jl/src/WebSockets.jl

Line 154 in 39f1139

startwrite(http)

@staticfloat

[fonsp]

@staticfloat helped me fix the problem:

In my case, we check each incoming connection whether it is authenticated. For simple HTTP endpoints, we respond with a 403 error page if it is not.

The problem was the "protocol upgrade" request for establishing a websocket connection. Here we checked for authentication, and if not, we did not write a response to the upgrade request. The solution is tricky, you need to manually write the response:

fonsp/Pluto.jl@163a507#diff-c838b67ba58bb2ea21d4a739b4d8b1930559595218dd045fa2785fb0f4dbbc60R196-R198