Re: Feature: POSIX Shared memory support

Tom Lane wrote:
> Chris Marcellino <maps(at)levelview(dot)com> writes:
>> To this end, I have "ported" the svsv_shmem.c layer to use the POSIX
>> calls (which are some ways more robust w.r.t reducing collision by
>> using strings as shared memory id's, instead of ints).
>
> This has been suggested before, and rejected before, on the grounds that
> the POSIX API provides no way to detect whether anyone else is attached
> to the segment. Not being able to tell that is a tremendous robustness
> hit for us. We are not going to risk destroying someone's database
> (or in the alternative, failing to restart after most crashes, which
> it looks like your patch would do) in order to make installation
> fractionally easier.
>
> I read through your patch in the hopes that you had a solution for this,
> but all I find is a copied-and-pasted comment
>
>> /*
>> * We detect whether a shared memory segment is in use by seeing whether
>> * it (a) exists and (b) has any processes are attached to it.
>> */
>
> followed by code that does no such thing.

Just an idea, but would it be possible to have a small SysV area as an
"advisory lock" (using the existing semantics) to protect the POSIX segment.

Best Regards
Michael Paesold