This new macro is able to perform a direct lookup from the local cache
of injection points (refreshed each time a point is loaded or run),
without touching the shared memory state of injection points at all.
This works in combination with INJECTION_POINT_LOAD(), and it is better
than INJECTION_POINT() in a critical section due to the fact that it
would avoid all memory allocations should a concurrent detach happen
since a LOAD(), as it retrieves a callback from the backend-private
memory.
The documentation is updated to describe in more details how to use this
new macro with a load. Some tests are added to the module
injection_points based on a new SQL function that acts as a wrapper of
INJECTION_POINT_CACHED().
Based on a suggestion from Heikki Linnakangas.
Author: Heikki Linnakangas, Michael Paquier
Discussion: https://postgr.es/m/58d588d0-e63f-432f-9181-bed29313dece@iki.fi
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