Re: Initializing LWLock Array from Server Code

On Fri, Apr 26, 2019 at 2:58 PM Souvik Bhattacherjee <kivuosb@gmail.com> wrote:
> I have created a shared hash table in partitioned mode inside the postgres server code. In order to guard the partitions, I'm trying to initialize an array of LWLocks. The code that I'm trying to use for that is
>
> void RequestNamedLWLockTranche(const char *tranche_name, int num_lwlocks);
> LWLockPadded *GetNamedLWLockTranche(const char *tranche_name);
>
> I'm not sure where exactly should this code be called from the server code. So I had placed it in
>
> void CreateSharedMemoryAndSemaphores(bool makePrivate, int port);
>
> within ipic.c. However, I'm getting the following error message when starting the database:
>
> FATAL:  requested tranche is not registered
>
> So at this point, I'm a little confused as to where the methods should be called from inside the server code. Any pointers would be appreciated.

RequestNamedLWLockTranche() changes the behavior of LWLockShmemSize()
and CreateLWLocks(), so must be called before either of those.
GetNamedLWLockTranche() must be called after CreateLWLocks().

This machinery works for pg_stat_statements, so see that as an example
of how to make this work from an extension.  If you're hacking the
core server code, then look at the places where the corresponding bits
of pg_stat_statements code get called.  IIRC, _PG_init() gets called
from process_shared_preload_libraries(), so you might look at the
placement of the calls to that function.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company