Re: Support for N synchronous standby servers - take 2 - Mailing list pgsql-hackers

Hello, attached is the new version v8.

At Tue, 26 Apr 2016 11:02:25 +0900 (Tokyo Standard Time), Kyotaro HORIGUCHI <horiguchi.kyotaro@lab.ntt.co.jp> wrote in <20160426.110225.35506931.horiguchi.kyotaro@lab.ntt.co.jp>
> At Sat, 23 Apr 2016 10:12:03 -0400, Tom Lane <tgl@sss.pgh.pa.us> wrote in <476.1461420723@sss.pgh.pa.us>
> > Amit Kapila <amit.kapila16@gmail.com> writes:
> > > The main point for this improvement is that the handling for guc s_s_names
> > > is not similar to what we do for other somewhat similar guc's and which
> > > causes in-efficiency in non-hot code path (less used code).
> >
> > This is not about efficiency, this is about correctness.  The proposed
> > v7 patch is flat out not acceptable, not now and not for 9.7 either,
> > because it introduces a GUC assign hook that can easily fail (eg, through
> > out-of-memory for the copy step).  Assign hook functions need to be
> > incapable of failure.

  I do not see any good reason why this one cannot
> > satisfy that requirement, either.  It just needs to make use of the
> > "extra" mechanism to pass back an already-suitably-long-lived result from
> > check_synchronous_standby_names.  See check_timezone_abbreviations/
> > assign_timezone_abbreviations for a model to follow.
>
> I had already seen there before the v7 and had the same feeling
> below in mind but packing in a blob needs to use other than List
> to hold the name list (just should be an array) and it is
> followed by the necessity of many changes in where the list is
> accessed. But the result is hopeless as you mentioned :(
>
> > You are going to
> > need to find a way to package the parse result into a single malloc'd
> > blob, though, because that's as much as guc.c can keep track of for an
> > "extra" value.
>
> Ok, I'll post the v8 with the blob solution sooner.

Hmm. It was way easier than I thought. The attached v8 patch does,

- Changed SyncRepConfigData from a struct using liked list to a
  blob. Since the former struct is useful in parsing, it is still
  used and converted into the latter form in check_s_s_names.

- Make assign_s_s_names not to do nothing other than just
  assigning SyncRepConfig.

- Change SyncRepGetSyncStandbys to read the latter form of
  configuration.

- SyncRepFreeConfig is removed since it is no longer needed.