snappy core: "reboot scheduled to update the system"

[Dan Kegel]

Just noticed this message
 "reboot scheduled to update the system"
during a long-running build.  Fortunately I was there and could do shutdown -c.
Presumably this is the gadget snap (raspberry pi 3), and a real
appliance would control this so it doesn't happen during
critical operations.
Where can I read more about how to control this?
15 seconds of googling didn't find much, just one irc log
from three weeks ago.