When a client waits for a prefetch- or RPZ-triggered recursion to
complete, its netmgr handle is attached to client->prefetchhandle and a
reference to the resolver fetch is stored in client->query.prefetch.
Both of these features use the same fields mentioned above. This makes
the code fragile and hard to follow as its logically distinct parts
become intertwined for no obvious reason.
Furthermore, storing pointers related to a specific recursion process in
two different structures makes their purpose harder to grasp than it has
to be.
To alleviate the problem, extend ns_query_t with an array of structures
containing recursion-related pointers. Each feature able to initiate
recursion is supposed to use its own slot in that array, allowing
logically unrelated code paths to be untangled. Prefetch and RPZ will
be the first users of that array.
Define helper macros for accessing specific recursion-related pointers
in order to improve code readability.
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