hastd: Use fixed-length protocol names

[des]

Summary

All communication between hastd nodes and internally between hastd and
its worker children passes through the same pair of send / receive
functions. The receive function uses recv(2) with the MSG_WAITALL flag,
which in theory means we should never get a short read. However, when
handing off a socket to a worker child, we also pass a variable-length
string identifying the type of socket we're passing, and reading this
string relies on a short read. This used to work because the arrival of
the descriptor would interrupt the recv(2) call, but this bug was fixed
when the AF_UNIX code was rewritten a while ago and hastd has been
broken ever since.

Fixing the length of the protocol name to four characters including the
terminating null solves the short-read bug by never requiring a short
read (nothing else in hastd requires one).

Reported by: Martin Vidovic <[e-mail]>
MFC after: 3 days
Event: EuroBSDcon DevSummit 2026

[des]

This obsoletes D59465.

[xtronom_gmail.com]

Tested on FreeBSD 15.0-RELEASE-p10, it works fine. I don't see any problems with the code.

[kevans]

I note that there's a second copy of all of this in auditdistd, and we probably want to do similar things.

[des]

In D59521#1366260, @kevans wrote:

I note that there's a second copy of all of this in auditdistd, and we probably want to do similar things.

See D59565

[gjb]

I can confirm this patch fixes hastd in my setup on physical machines running 16-CURRENT from August 24.