tests: Avoid sleep when causing a write error

[jilles]

Summary

Cause a write error using a fifo and wait, rather than needing a sleep
to wait for something to terminate. This is faster and avoids a
potential test failure on a heavily loaded system.

Using /dev/full would be simpler, but it is not portable enough (it
tends not to be available in jails, even). Starting programs with stdout
not open or only open for read/execute/search may have unexpected side
effects.

[ngie]

What implements /dev/full these days?

$ uname -a
FreeBSD universe16a.freebsd.org 16.0-CURRENT FreeBSD 16.0-CURRENT #0 main-n285776-10e342c1ec78: Sat May  9 09:21:00 UTC 2026     [e-mail]:/usr/obj/usr/src/sys/CLUSTER16 amd64
$ ls /dev/full
ls: /dev/full: No such file or directory

% uname -a
FreeBSD gilot.local 14.4-RELEASE-p3 FreeBSD 14.4-RELEASE-p3 GENERIC amd64
% ls -l /dev/full
crw-rw-rw-  1 root wheel 0x13 May 21 11:19 /dev/full

require.files needs to be used to make sure /dev/full exists before use -- otherwise all of these tests will fail.

[des]

Please don't, I need these tests to remain portable.

[jilles]

In D57213#1313293, @des wrote:

Please don't, I need these tests to remain portable.

OK, then I can build something using fifos that is portable and still does not have a sleep. The complexity will be similar to the existing code.

[jilles]

Use fifo and wait instead of /dev/full.

[ngie]

LGTM -- the only thing I might consider is some commentary linking the patterns together or adding the pattern to a common script.

[jilles]

In D57213#1315545, @ngie wrote:

LGTM -- the only thing I might consider is some commentary linking the patterns together

I think the existing descriptions like "Error writing to stdout" should be clear enough.

or adding the pattern to a common script.

Such a common script would make it harder to copy test cases around to other systems, and due to the limitations of the shell language the whole thing would become more complicated (ideally I would like to return an open file from a function, but shell doesn't support that).