Getting the function definition from oid in before firing the function

Dear Postgres Team,

For my thesis about secure access control for database systems I need to
implement a new decision algorithm for deciding if commands can be
executed. For that algorithm to work I need to keep a stack that holds
all the commands that are currently running, like a stacktrace.

Each stack entry should contain the following:
- Session user
- User that runs the command (differs if we have security definer on
triggers)
- Command
- Oid of trigger (if it is one)

Now my problem consists of two steps:
- First I need to decide if the algorithm supports the trigger type
(only of LANGUAGE PLPGSQL).
- Second I need to get the SQL command that the function will execute.

What I tried was the following:
In the file trigger.c I have found the location where the function is
called. There I have access to the FunctionCallInfoData struct.
I have seen on
http://www.postgresql.org/docs/9.4/static/functions-info.html that there
is a function which should return the functions definition as a Datum
from the function oid. Now this is a bit weird since in the source code
the function expects a pointer to a FunctionCallInfoData struct, which
is no problem since I have access to both there.
But when I pass the the pointer to the FunctionCallInfoData I always get
an error stating that the cache lookup failed. I really don't know why,
since when I investigate the struct fcinfo in gdb it has valid entries
and also the oid of the function is accessible.

I would be glad for some help here.

Also another problem that I have at the moment is the following:
My advisor wanted to have a uniform interface for access control, thus I
have refactored the code of postgres and moved the relevant access
control parts to a new module (src/backend/access_control). Now I have
put the header files in src/include/access_control. What I don't quite
get is how to get make a Makefile that delivers the object files as
postgres needs them to link them to the main binary. Is it enough to
make a Makefile that just creates the object files and leaves them in
the correct folder for the root Makefile to pick them up or do I need to
modify the root Makefile too?

Thank you very much for your help in advance, for both problems I have
tried many variations and researched quite much, but did not find a
solution.

Kind regards,

Mohammed Ajil

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Mohammed Ajil
Bsc CS D-INFK
ajilm(at)student(dot)ethz(dot)ch