tail: --pid option does not work when input is a FIFO

[Bernhard Voelker]

Hi *,

I noticed that 'tail --pid' does not work when the file to follow is a FIFO.
Shouldn't --pid behave the same regardless whether it's a pipe or not?

# Good case.
$ rm f; touch f  # tail to follow regular file.
$ sleep 5 & timeout 10 tail -f --pid=$! f; echo $?
[3] 1030528
[3]+  Done                    sleep 5
0

# Bad case.
$ rm f; mkfifo f  # tail to follow a FIFO file.
$ sleep 5 & timeout 10 tail -f --pid=$! f; echo $?
[3] 1030559
[3]+  Done                    sleep 5
124

Looking at strace output, one can see that kill() is only invoked once in the 
FIFO case.

FWIW: it doesn't make a difference whether one uses -f or -F, whether the
monitored file is written to by another process or stays empty, or whether one
tries with the  --sleep-interval option.

Have a nice day,
Berny