The locking protocol in nat.Mapping was racy:
* Mapping.addressMap RLock'd, but then returned a map shared between
caller and Mapping, so the lock didn't do anything.
* Operations inside Service.{verifyExistingMappings,acquireNewMappings}
would lock the map for every update, but that means callers to
Mapping.ExternalAddresses can be looping over the map while the
Service methods are concurrently modifying it. When the Go runtime
detects that happening, it panics.
* Mapping.expires was read and updated without locking.
The Service methods now lock the map once and release the lock only when
done.
Also, subscribers no longer get the added and removed addresses, because
none of them were using the information. This was changed for a previous
attempt to retain the fine-grained locking and not reverted because it
simplifies the code.
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