Re: Re: ALTER SYSTEM SET command to change postgresql.conf parameters (RE: Proposal for Allow postgresql.conf values to be changed via SQL [review])

Andres Freund <andres(at)2ndquadrant(dot)com> writes:
> On 2013-08-05 12:18:25 -0400, Bruce Momjian wrote:
>> I am unclear why we don't need a lock around _each_ GUC, i.e. if two
>> sessions try to modify the same GUC at the same time. And if we need a
>> lock, seems we can have just one and write all the settings to one file
>> --- it is not like we have trouble doing locking, though this is
>> cluster-wide locking.

> If you have two sessions modifying the same variable, one is going to
> win and overwrite the other's setting with or without locking around
> GUCs unless you error out if somebody else holds the lock.

The point of a lock is just to ensure that the end result is one valid
state or the other, and not something corrupt. We would certainly need a
lock if we write to a single file. With file-per-GUC, we could possibly
dispense with a lock if we depend on atomic file rename(); though whether
it's wise to assume that for Windows is unclear. (Note that we ought to
write a temp file and rename it into place anyway, to avoid possibly
corrupting the existing file on out-of-disk-space. The only thing that
needs discussion is whether to add an explicit lock around that.)

regards, tom lane