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There are several methods within the LocalBackend that used an unusual and error-prone lock discipline whereby they require the caller to hold the backend mutex on entry, but release it on the way out. In #11650 we added some support code to make this pattern more visible. Now it is time to eliminate the pattern (at least within this package). This is intended to produce no semantic changes, though I am relying on integration tests and careful inspection to achieve that. To the extent possible I preserved the existing control flow. In a few places, however, I replaced this with an unlock/lock closure. This means we will sometimes reacquire a lock only to release it again one frame up the stack, but these operations are not performance sensitive and the legibility gain seems worthwhile. We can probably also pull some of these out into separate methods, but I did not do that here so as to avoid other variable scope changes that might be hard to see. I would like to do some more cleanup separately. As a follow-up, we could also remove the unlockOnce helper, but I did not do that here either. Updates #11649 Change-Id: I4c92d4536eca629cfcd6187528381c33f4d64e20 Signed-off-by: M. J. Fromberger <fromberger@tailscale.com>