[undertow-dev] Too many open files: Exception accepting request, closing server channel TCP server (NIO)

[David Lloyd]

On Mon, Mar 2, 2020 at 7:56 AM Stan Rosenberg <stan.rosenberg at acm.org> wrote:
>
> Stuck in CLOSE_WAIT is a symptom of the client-side not properly shutting down [1].

I would partially disagree.  In the article you linked: "It all starts
with a listening application that leaks sockets and forgets to call
close(). This kind of bug does happen in complex applications."  This
seems to be essentially what's happening here: the server isn't
completing the connection (for some reason), stranding the socket in
`CLOSE_WAIT`.

We can't assume that the client is abandoning the connection after
`FIN_WAIT2` (the titular RFC violation); if the server stays in
`CLOSE_WAIT`, then even if the client dutifully stays in `FIN_WAIT2`
forever, the resolving condition still needs to be that the server
shuts down its side of the connection.

This diagram is a useful visual aid, mapping TCP states to the XNIO
API: https://www.lucidchart.com/publicSegments/view/524ec20a-5c40-4fd0-8bde-0a1c0a0046e1/image.png

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- DML