On Wed, Mar 23, 2022 at 09:03:18PM +0800, Julien Rouhaud wrote: > On Wed, Mar 23, 2022 at 08:32:39AM -0400, Robert Haas wrote: >> Well, the conclusion upthread was that extensions might change the >> values of those GUCs from _PG_init(). If that's a real thing, then >> what you're asking for here is impossible, because the final value is >> indeterminate until all such extensions have finished twiddling those >> the GUCs. On the other hand, it's definitely intended that extensions >> should RequestAddinShmemSpace() from _PG_init(), and wanting to size >> that memory based on MaxBackends is totally reasonable. Do we need to >> add another function, alongside _PG_init(), that gets called after >> MaxBackends is determined and before it's too late to >> RequestAddinShmemSpace()? > > Yes, I don't see how we can support such extensions without an additional > hook, > probably called right after InitializeMaxBackends() since at least > InitializeShmemGUCs() will need to know about those extra memory needs.
Another possibility could be to add a hook that is called _before_ _PG_init() where libraries are permitted to adjust GUCs. After the library is loaded, we first call this _PG_change_GUCs() function, then we initialize MaxBackends, and then we finally call _PG_init(). This way, extensions would have access to MaxBackends within _PG_init(), and if an extension really needed to alter GUCs, іt could define this new function. > I'm not sure how to prevent third party code from messing with the gucs in it, > but a clear indication in the hook comment should probably be enough. It's > not > like it's hard for third-party code author to break something anyway. ERROR-ing in SetConfigOption() might be another way to dissuade folks from messing with GUCs. This is obviously not a perfect solution. -- Nathan Bossart Amazon Web Services: https://aws.amazon.com
