If Close is called after ReceiveIPvX, then ReceiveIPvX will block on an
invalid or potentially reused fd.
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
The sticky socket code stays in the device package for now,
as it reaches deeply into the peer list.
This is the first step in an effort to split some code out of
the very busy device package.
Signed-off-by: David Crawshaw <crawshaw@tailscale.com>
