NetworkMap text diffs being empty were currently used to short-circuit
calling magicsock's SetNetworkMap (via Engine.SetNetworkMap), but that
went away in c7582dc2 (0.100.0-230)
Prior to c7582dc2 (notably, in 0.100.0-225 and below, down to
0.100.0), a change in only disco key (as when a node restarts) but
without endpoint changes (as would happen for a client not behind a
NAT with random ports) could result in a "netmap diff: (none)" being
printed, as well as Engine.SetNetworkMap being skipped, leading to
broken discovery endpoints.
c7582dc2 fixed the Engine.SetNetworkMap skippage.
This change fixes the "netmap diff: (none)" print so we'll actually see when a peer
restarts with identical endpoints but a new discovery key.
As part of disabling background STUN packets when idle, we want an
emergency override switch to turn it back on, in case it interacts
poorly in the wild. We'll send that via control, but we'll want to
plumb it down to magicsock via NetworkMap.
Updates tailscale/corp#320
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
Brad Fitzpatrick