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The ThreadSanitizer found a data race when updating the stale header. Instead of trying to acquire the write lock and failing occasionally which would skew the statistics, the dns_rdatasetheader_t.attributes field has been promoted to use stdatomics. Updating the attributes in the mark_header_ancient() and mark_header_stale() now uses the cmpxchg to update the attributes forfeiting the need to hold the write lock on the tree. Please note that mark_header_ancient() still needs to hold the lock because .dirty is being updated in the same go.
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