s6-linux-utils: the s6-logwatch program

The `s6-logwatch` program

s6-logwatch watches the current file of a logdir, printing it in real time.

Interface

     s6-logwatch [ -m buflen ] logdir

s6-logwatch prints logdir/current and watches the file.
logdir must be managed by a s6-log instance.
When new logs are appended to the current file, s6-logwatch prints them in real-time to stdout.
When a rotation happens, s6-logwatch notices, and keeps watching the new current file.
s6-logwatch runs forever until killed.

-m buflen : accumulate at most buflen bytes into the stdout buffer before flushing it. By default, buflen is 4000.

s6-logwatch is not entirely reliable because there is an unavoidable race condition when a rotation occurs; it's a hack for humans to keep reading logs across rotations, not a tool to be used in safe programming. When the race condition is triggered, s6-logwatch will be unable to understand what state logdir is in and will exit 101 with an error message.
Specific support in the logger program would be needed to avoid this race condition; it would significantly bloat the logger program, so it has not been deemed useful.