kern/56113: Short reads on AF_UNIX/SOCK_STREAM sockets return early in CURRENT

[nia%pkgsrc.org@localhost]

>Number:         56113
>Category:       kern
>Synopsis:       Short reads on AF_UNIX/SOCK_STREAM sockets return early in CURRENT
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    kern-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Sat Apr 17 08:10:00 +0000 2021
>Originator:     nia
>Release:        NetBSD 9.99.81
>Organization:
>Environment:
NetBSD 9.99.81
>Description:
In netbsd-9 and earlier releases, small reads on AF_UNIX SOCK_STREAM
sockets provided a fixed number of requested bytes. It seems at some
point NetBSD -current was changed so that less bytes than requested
could be returned, and poll() will return before the full written
chunk is available.

This means that the JACK Audio Connection Kit (audio/jack) in pkgsrc
no longer works on NetBSD 10, see

https://marc.info/?l=netbsd-current-users&m=161779584825623&w=2

for further analysis.
>How-To-Repeat:
Attempt to use audio/jack on a recent-ish -current system, observe that
no clients can hold a connection and the server gets very confused.
>Fix: