This can be used to load an injection point and prewarm the
backend-level cache before running it, to avoid issues if the point
cannot be loaded due to restrictions in the code path where it would be
run, like a critical section where no memory allocation can happen
(load_external_function() can do allocations when expanding a library
name).
Tests can use a macro called INJECTION_POINT_LOAD() to load an injection
point. The test module injection_points gains some tests, and a SQL
function able to load an injection point.
Based on a request from Andrey Borodin, who has implemented a test for
multixacts requiring this facility.
Reviewed-by: Andrey Borodin
Discussion: https://postgr.es/m/ZkrBE1e2q2wGvsoN@paquier.xyz
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