A number of methods in LocalBackend (with suffixed "LockedOnEntry")
require b.mu be held but unlock it on the way out. That's asymmetric
and atypical and error prone.
This adds a helper method to LocalBackend that locks the mutex and
returns a sync.OnceFunc that unlocks the mutex. Then we pass around
that unlocker func down the chain to make it explicit (and somewhat
type check the passing of ownership) but also let the caller defer
unlock it, in the case of errors/panics that happen before the callee
gets around to calling the unlock.
This revealed a latent bug in LocalBackend.DeleteProfile which double
unlocked the mutex.
Updates #11649
Change-Id: I002f77567973bd77b8906bfa4ec9a2049b89836a
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
5e7c0b025c