rgw: extend SIGHUP handler for realm reconfiguration

[cbodley]

Adds a signal handler to trigger RGWRealmReloader to shut down and recreate RGWRados to reload the realm configuration and restart multisite sync.

[yehudasa]

@cbodley I think SIGUSR2 is used internally in valgrind, so maybe we'd want to hook this with SIGHUP instead? tie it into the generic ceph configuration reloader somehow

[cbodley]

It looks like we could install a different handler for SIGHUP, which calls both RGWRealmReloader::handle_notify() and g_ceph_context->reopen_logs(). I'll give that a try.

[theanalyst]

is it also possible to hook the ceph tell or injectargs into RGW somehow?

[yehudasa]

@theanalyst no, you cannot use ceph tell or injectargs with client ceph apps.

[cbodley]

@yehudasa updated to piggy-pack realm reconfiguration on SIGHUP. the default sighup_handler() only calls g_ceph_context->reopen_logs() though.. was the goal to extend this to reload ceph.conf too? i wasn't able to find any examples of other daemons doing this, and i don't think there's a thread-safe way to do it (the config observers look like a step in this direction, but they haven't been fully adopted)

[yehudasa]

@cbodley eventually we'd probably want to be able to update all config via sighup, but this is fine for now I think.