Re: Attaching and using the Postgres shared memory segment

Paul van den Bogaard wrote:
> Since these data structures are for collecting information I was able to 
> create a new function that retrieves its information from these data 
> structures and returns them as a result from a query.
> 
> However this is too intrusive. Since everything is in shared memory it 
> should be possible for an external, yet to be created, process to attach 
> to the shared memory segment (read only mode only) to collect these data 
> structures.

That sounds dangerous. I'm seeing problems with postmaster stop and 
restart if there's an external process attached to the postgres shared 
memory segment.

> I have the address mapping of the shared memory (doing stuff on Solaris 
> only). I can pass this information to InitShmemAccess (ipc/shmem.c).  
> There is however a missing settting for ShmemIndex. This one can be 
> initialsed by calling InitShmemIndex(). The only thing that makes me 
> wonder is its need to use ShmemIndexLock. Although this is just an enum, 
> and  therefore an index in some array created in CreateLWLock 
> (lwlock.c).  I do not see how I can get access to this lock. 

You can't acquire a lightweight lock safely outside a real backend. 
You'd need to have a valid PGPROC entry, at least.

I would suggest forgetting about that idea and sticking to your original 
approach of a function that returns the data as a result from a query. 
Or, create a completely separate shared memory block for your own data, 
using plain Solaris shmem functions.

--   Heikki Linnakangas  EnterpriseDB   http://www.enterprisedb.com