Copy stable/10@r272459 to releng/10.1 as part of
the 10.1-RELEASE process.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Add a comment to the example that pidfile_* functions can handle pfh being NULL.

MFC after: 3 days

Note that calling pidfile_write(3) multiple times is fine.

MFC after: 3 days

Remove unnecessary includes from these libutil man pages.

Requested by bde (as was the previous commit).

pidfile_open() no longer uses fcntl() to set the close-on-exec flag.

Style fixes courtesy of pjd.

Document that pidfile_open() now sets the FD_CLOEXEC flag on the open
file descriptor, and that errors can be returned as a result of the
fcntl(2) system call.

Add pidfile_fileno() to obtain the file descriptor for an open
pidfile.

In pidfile_open(), if the pidfile is locked, but empty (PID is not stored yet)
and the caller requested other process' PID by passing non-NULL pidptr
argument, we will wait at most 100ms for the PID to show up in the file and if
it won't, we will store -1 in *pidptr.

From now on, pidfile_open() function never sets errno to EAGAIN on failure.

In collaboration with: des
MFC after: 1 week

Add missing .Pp

pidfile(3) uses flopen(3) - don't make any assumptions about how the
latter is implemented.

Minor mdoc cleanup: Every sentence should start on its own line.

When pidfile is already locked and has zero length, do not return
success and zero pid from pidfile_read(). Return EAGAIN instead. Sleep
up to three times for 5 ms while waiting for pidfile to be written.

mount(8) does the kill(mountpid, SIGHUP). If mountd pidfile is truncated,
that would result in the SIGHUP delivered to the mount' process group
instead of the mountd.

Found and analyzed by: Peter Holm
Tested by: Peter Holm, kris
Reviewed by: pjd
MFC after: 1 week

Use `intmax_t' instead of plain `int' for pid_t casts.

Useful tips from: ru, bde
Approved by: pjd
MFC after: 3 days

- Add a note that passing NULL to pidfile_write(), pidfile_remove() and
pidfile_close() functions is safe. This possibility is used in example code.
- Cast pid_t to int.

Requested by: yar

Fix markup, grammar and spelling.

Pidfiles should be created with permission preventing users from opening
them for reading. When user can open file for reading, he can also
flock(2) it, which can lead to confusions.

Pointed out by: green

Add a family of functions for reliable pidfiles handling.

Idea from: jmg
Discussed on: arch@