Bug 1402521

For the problem statement and initial design ideas, see
https://github.com/ClusterLabs/libqb/issues/239

In a nutshell, the top-level goal is to avoid copytruncate option in
logrotate configurations of both corosync and pacemaker (current state)
because, at any rate, any bit missing from logs may be too precious
(e.g. otherwise exactly clarifying the root cause of an issue or putting
an explanation for the observable behavior later in the game, making for
real tough life of anyone trying to inspect past issues) to accept the
connected risk clearly stated in the man page of logrotate (and
logrotate is installed by default so it may definitely kick in).

Currently, most viable implemention would rely on the named pipes:

> libqb will create a named pipe X.pipe for > the target log file X
> which then will be written to per the logrotate > configuration. This
> currently seems the best technical solution:
> 
> - named pipes should be available everywhere we desire
> - custom command in logrotate config would be immediately clear on
>   whether the libqb supporting the signalling mechanism is in use
>   thanks to existing X.pipe file (a la test -p), and thus supporting
>   the sketched fallback scheme for older libqb in use
>   (it could be viceversa: old -> new)

(for the fallback scheme, see below)

> - no sign of "bad polling" (i.e., checking whether inode has changes
>   over and over), just a "good polling" (poll on X.pipe)
> - signal-safe


(signal-safe as opposed to libqb's user having libqb to react on SIGHUP
while handling other signals on its own, with various signal masks,
etc.)

The fallback scheme deals with possible incompatibilities between
libqb's user + its logrotate file vs. libqb version in an actual use:

> One of the possibilities to employ in the updated logrotate files is
> to use firstscript option with custom command to check the libqb
> version (through *.so version info) and if not high enough, to trigger
> logrotate > with an out-of-path original configuration (copytruncate)
> and > subsequently fail to denote the procedure should not continue.