[arch-general] User actions following system package upgrades?

[Paul Gideon Dann]

I've looked everywhere, but I can't find any reference to what the accepted 
wisdom is:

I know that due to the way the kernel is upgraded, it is best to save kernel 
upgrade until the system can be rebooted, since the running kernel will be 
unable to load modules because the modules directory (/lib/modules/<version>) 
changes when a new kernel package is installed.

For this reason, I have the "linux" package in my IgnorePkg line in 
pacman.conf, and upgrade it explicitly before a planned reboot.

However, for what other packages is this true?  I also have "udev" in 
IgnorePkg, but is this necessary?  Is it safe to upgrade udev without 
rebooting?  I suspect that certain rules may disappear or change, and the 
running udev may not be happy about that.

I'm guessing it's also best to shut down X and restart DBus after a DBus 
upgrade too, although in practice a reboot is usually simpler, and that it 
would also be sensible to reboot after a "glibc" upgrade too, although that's 
rare.  Any other thoughts?  Are these sorts of considerations documented 
anywhere?

This issue of upgrading system packages while the system is running feels like 
a potentially dangerous situation to me.  Are we lucky that it doesn't cause 
too much trouble, or are in-place upgrades specifically handled gracefully by 
core system components such as udev?  (I don't see anything special in udev's 
install script.)  I suspect that most of us shrug it off and reboot only when 
we see breakage, but I hope you agree that's not a very safe way to work.

Paul