Related: #3797
Motivation:
There is a race condition where DnsNameResolver.query() can attempt to
increase the reference count of the DNS response which was released
already by other thread.
Modifications:
- Make DnsCacheEntry a top-level class for clear access control
- Use 'synchronized' to avoid the race condition
- Add DnsCacheEntry.retainedResponse() to make sure that the response
is never released while it is retained
- Make retainedResponse() return null when the response has been
released already, so that DnsNameResolver.query() knows that the
cached entry has been released
Result:
The forementioned race condition has been fixed.
c72082ea7f