userspace-networking incoming TCP doesn't always work right away #2642
Description
I've noticed a few times now, most recently with 1.12.3, that a tailscaled running in userspace-networking mode doesn't always forward incoming TCP connections to localhost reliably for a bit after the process is started.
Note that I can ping it and the first curl fails, but then it works a bit later:
crow5k:~ $ ping foo
PING foo.tailscale.com.beta.tailscale.net (100.127.0.49): 56 data bytes
64 bytes from 100.127.0.49: icmp_seq=0 ttl=64 time=161.106 ms
^C
--- foo.tailscale.com.beta.tailscale.net ping statistics ---
2 packets transmitted, 1 packets received, 50.0% packet loss
round-trip min/avg/max/stddev = 161.106/161.106/161.106/0.000 ms
crow5k:~ $ curl http://foo:8383/debug/varz
curl: (7) Failed to connect to foo port 8383: Connection refused
crow5k:~ $ curl http://foo:8383/debug/varz
[works, output omitted]