bug#78910: tail does not support -r added by POSIX.1-2024

Hi Eric,

Eric Blake <[email protected]> writes:

> Remember, the POSIX synopsis lines lists two distinct invocation
> styles:
>
> tail [-f] [-c number|-n number] [file]
> tail -r [-n number] [file]

Yep, I was going to mention this too. But I was not sure if it should
have a mention in the description to make it more clear.

> which means undefined behavior from the POSIX point of view if -f and
> -r are ever attempted at the same time.  Therefore, whatever GNU
> decides to do on that combo (preferably, diagnose it early with an
> error and exit, since I can't figure out anything else useful it could
> do) will not violate POSIX.

I think it should be standardized to fail when -f and -r are used
together. That seems like the only reasonable behavior to me.

Thanks,
Collin