Re: How to shoot yourself in the foot: kill -9 postmaster

* Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> [010306 11:03] wrote:
> Alfred Perlstein <bright(at)wintelcom(dot)net> writes:
> >> Are there any portability problems with relying on shm_nattch to be
> >> available? If not, I like this a lot...
>
> > Well it's available on FreeBSD and Solaris, I'm sure Redhat has
> > some deamon that resets the value to 0 periodically just for kicks
> > so it might not be viable... :)
>
> I notice that our BeOS and QNX emulations of shmctl() don't support
> IPC_STAT, but that could be dealt with, at least to the extent of
> stubbing it out.

Well since we already have spinlocks, I can't see why we can't
keep the refcount and spinlock in a special place in the shm
for all cases?

> This does raise the question of what to do if shmctl(IPC_STAT) fails
> for a reason other than EINVAL. I think the conservative thing to do
> is refuse to start up. On EPERM, for example, it's possible that there
> is a postmaster running in your PGDATA but with a different userid.

Yes, if possible a more meaningfull error message and pointer to
some docco would be nice or even a nice "i don't care, i killed
all the backends, just start darnit" flag, it's really no fun at
all to have to attempt to decypher some cryptic error message at
3am when the database/system is acting up. :)

> > Seriously, there's some dispute on the type that 'shm_nattch' is,
> > under Solaris it's "shmatt_t" (unsigned long afaik), under FreeBSD
> > it's 'short' (i should fix this. :)).
>
> > But since you're really only testing for 0'ness then it shouldn't
> > really be a problem.
>
> We need not copy the value anywhere, so as long as the struct is
> correctly declared in the system header files I don't think it matters
> what the field type is ...

Yup, my point exactly.

--
-Alfred Perlstein - [bright(at)wintelcom(dot)net|alfred(at)freebsd(dot)org]