Sighup for Reloading Configuration

SIGHUP for reloading configuration

SIGHUP as a notification about terminal closing event doesn't make sense for a daemon, because deamons are detached from their terminal. So the system will never send this signal to them.
Then it is common practice for daemons to use it for another meaning, typically reloading the daemon's configuration.
This is not a rule, just kind of a convention. That's why it's not documented in the manpage.

See the wikipedia entry for SIGHUP
and from there, a longer description with implementation example

How to know if a SIGHUP signal has been handled?

The inotify-tools C library may solve your problem. Its installation gives you access to new commands: inotifywait and inotifywatch.

Let's say you have this /tmp/foo.bar file. You can start watching for any read access on this file with the following command:

inotifywait --event access /tmp/foo.bar

Then, do a cat /tmp/foo.bar and the program will return.

As I said, it's a C library and I guess there are other language implementations of it. So feel free to not use this Bash example and write you own program using this library.

Logstash configuration reload

Hot Reload is not yet supported. There is a Issue.

The logstash configuration is loaded at startup, so, if you kill your process (to restart it later), there is no reason why it does not work.

Upstart - how to send reload signal to script started as different user?

I ended up running my script as root and then dropping privileges to desired user.
To run script as root I've uncommented exec python app.py from Upstart config above.
To drop privileges I use slightly modified code from this answer:

def drop_privileges(uid_name, gid_name):
    '''If running as root, this function will try to change current uid and gid
    to given values.
    May raise OSError in case of error.
    :param uid_name: Name of the user we need to be running as.
    :param gid_name: Name of the group we need to be running as.
    '''

    starting_uid = os.getuid()
    starting_gid = os.getgid()

    log.info('drop_privileges: started as %s/%s' %
        (pwd.getpwuid(starting_uid).pw_name,
        grp.getgrgid(starting_gid).gr_name))

    if starting_uid == 0:
        # If we started as root, drop privs and become the specified user/group
        log.info("drop_privileges: trying to drop provileges to '%s'" % uid_name)

        # Get the uid/gid from the name
        running_uid = pwd.getpwnam(uid_name).pw_uid
        running_gid = grp.getgrnam(gid_name).gr_gid

        # Try setting the new uid/gid
        # These calls may result in exception, let it propagate back
        # Exception thrown is: OSError (e.errno == 1 means 'Operation not
        # permitted')
        os.setgid(running_gid)
        os.setuid(running_uid)

        final_uid = os.getuid()
        final_gid = os.getgid()
        log.info('drop_privileges: running as %s/%s' %
            (pwd.getpwuid(final_uid).pw_name,
             grp.getgrgid(final_gid).gr_name))

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