I had neither written code to test it nor was speculating - I was reading the kernel source, which is pretty clear (if you know in general how file polling is implemented in the Linux kernel).
However, because of what you have observed I wrote up a quick test, which confirms to my satisfaction that epoll_wait() respects SO_RCVLOWAT on TCP sockets: https://gist.github.com/keaston/d2473b8b996a34a5860b0744684f...
It seems likely that you must have been dealing with an old "enterprise" kernel in 2015 that hadn't had this fix backported to it?
Wow. When I saw your code, I thought to myself "yeah, but does it work when the socket is for a client?" Turns out, it does.
My only guess is that I was botching something else when I was digging into this a few years ago.
I can't tell you how much I appreciate you putting that together. This seriously changes a lot of what I work on. Thank you.
The client code I wrote: https://gist.github.com/benwills/95ee844853d3b18588fb3df7d56...
Unless I'm missing something (and I could very well be missing something), I'm not seeing how that is any different than anything I tried...and I went through every socket option I could find, even if it seemed irrelevant.
Have you written code that actually performs this way? Or are you speculating? I really do wish I could do this with epoll, but I (and several others) just never were able to figure it out. And kqueue simply performed at least an order of magnitude better of any variation I could hack together with epoll.