Might want to read the section on Lingering Close from here:

https://httpd.apache.org/docs/2.4/misc/perf-tuning.html

> Send off the data and close the socket. If there's data still pending to be read, this will cause a RST, I think.

Um, yes? That's how TCP has been universally implemented for more than 30 years. See [0], 2.17 for discussion.

[0] https://www.rfc-editor.org/rfc/rfc2525#page-50

Part 2 shows this comment from the Linux TCP code:

    /* As outlined in RFC 2525, section 2.17, we send a RST here because
     * data was lost. To witness the awful effects of the old behavior of
     * always doing a FIN, run an older 2.1.x kernel or 2.0.x, start a bulk
     * GET in an FTP client, suspend the process, wait for the client to
     * advertise a zero window, then kill -9 the FTP client, wheee...
     * Note: timeout is always zero in such a case.
     */

Ok, so the RST is explained and well justified by the literature. But what are the “awful effects” of sending FIN instead? Can someone explain?