The genuine-failure cases fed a fabricated error straight to
logIfaceLookupFailure, so they exercised only its own branching. A
restoration failure never reaches that helper, and no test reached the
enumeration classification in netInterface at all: discarding the
enumeration error, or downgrading an actual restoration failure to
debug, both kept the suite green.
The host boundaries of the reset path are now variables, in the style
the package already uses: the interface enumeration, the interface
lookup, the DNS setters and the NetworkManager restore. The tests stub
them, so a run changes no host DNS or NetworkManager state, and drive
the real netInterface and resetDNSForRunningIface to assert that a
failed enumeration keeps its cause and is not reported as a missing
interface, and that a failure to restore the saved static config or to
reset to DHCP on an interface that does exist stays at error level.
Both surviving mutations, and three more, now fail.
A successful macOS upgrade printed "could not get interface
error=interface not found iface=en5" before reporting success, because
the DNS cleanup logged every interface lookup failure at error level.
An interface that is gone, an unplugged adapter or a torn down tether,
has no DNS settings left to restore, so the skip is now a debug
diagnostic naming the interface and the work it skipped.
Only that condition is quiet. netInterface returns a sentinel error for
a missing interface and no longer reports a failed enumeration as one,
so any other lookup failure and any restoration failure on an interface
that does exist stay at error level.
Cuong Manh Le