Re: Patch to implement pg_current_logfile() function

On Tue, 17 Jan 2017 11:22:45 +0900
Michael Paquier <michael(dot)paquier(at)gmail(dot)com> wrote:

> On Tue, Jan 17, 2017 at 2:21 AM, Karl O. Pinc <kop(at)meme(dot)com> wrote:

> >>Also, I would rather see an ERROR returned to the user in case of
> >>bad data in current_logfiles, I did not change that either as
> >>that's the original intention of Gilles.
> >
> > I could be wrong but I seem to recall that Robert recommended
> > against an error message. If there is bad data then something is
> > really wrong, up to some sort of an attack on the back end. Because
> > this sort of thing simply shouldn't happen it's enough in my
> > opinion to guard against buffer overruns etc and just get on with
> > life. If something goes unexpectedly and badly wrong with internal
> > data structures in general there would have to be all kinds of
> > additional code to cover every possible problem and that doesn't
> > seem to make sense.
>
> Hm... OK. At the same time not throwing at least a WARNING is
> confusing, because a NULL result is returned as well even if the input
> method is incorrect and even if the method is correct but it is not
> present in current_logfiles. As the user is thought as a trusted user
> as it has access to this function, I would think that being verbose on
> the error handling, or at least warnings, would make things easier to
> analyze.

These are all valid points.

In the context of reliability it's worth noting that
pg_current_logfile() results are inherently unreliable.
If the OS returns ENFILE or EMFILE when opening the current_logfiles
file (but not previously) the content, and so pg_current_logfile()
results, will be outdated until the next logfile rotation.
You wouldn't expect this to happen, but it might. Which
is similar to the situation where the content of the current_logfiles
is corrupted; very unexpected but possible.

Karl <kop(at)meme(dot)com>
Free Software: "You don't pay back, you pay forward."
-- Robert A. Heinlein