pgsql: injection_points: Add initialization of shmem state when loading

injection_points: Add initialization of shmem state when loading module

This commits adds callbacks to initialize the shared memory state of the
module when loaded with shared_preload_libraries. This is necessary to
be able to update the test introduced in 768a9fd5535f to use the macros
INJECTION_POINT_{LOAD,CACHED}() rather than a SQL function in the module
injection_points forcing a load, as this test runs a callback in a
critical section where no memory allocation should happen.

Initializing the shared memory state of the module while loading
provides a strict control on the timing of its allocation. If the
module is not loaded at startup, it will use a GetNamedDSMSegment()
instead to initialize its shmem state on-the-fly.

Per discussion with Álvaro Herrera.

Author: Michael Paquier
Discussion: https://postgr.es/m/ZsUnJUlSOBNAzwW1@paquier.xyz

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/b2b023aa3706ec6b3978708545301f7436205c6d

Modified Files
--------------
.../modules/injection_points/injection_points.c | 62 ++++++++++++++++++++--
1 file changed, 59 insertions(+), 3 deletions(-)