TestSocket_UNIXSocket: stop testing empty packets

[casperisfine]

Followup: #6407
[Bug #19012]

OpenBSD and Solaris behave differently here.

Linux does deliver the empty packet, which is questionable as it's undistinguishable from a closed connection.

It seems that OpenBSD and Solaris simply drop it.

We could test the platform before doing the assertion, but it would likely be fragile, and the entire web recommend to not ever send an empty packet, so the value of this assertion is low.

[ioquatix]

Linux does deliver the empty packet, which is questionable as it's undistinguishable from a closed connection.

What's the actual semantics? readmsg returns 0?

[casperisfine]

readmsg returns 0?

Yhe issue is with recvmsg, but essentially yes https://linux.die.net/man/2/recvmsg

Return Value
These calls return the number of bytes received, or -1 if an error occurred. The return value will be 0 when the peer has performed an orderly shutdown.

So if you received a 0B packet, it returns 0. If the peer did shutdown the connection, it also returns 0.

There is no way to differentiate the two cases as far as I'm aware. Most discussions on this topic I was able to see basically concluded with: "Don't send empty packets".

[ioquatix]

Yhe issue is with recvmsg

Thanks for the correction, sorry I was half asleep :)

basically concluded with: "Don't send empty packets".

Makes sense. I wonder if we can get around that by using something like wait_readable... a closed connection will return different from a connection that's still open IIRC.