Re: Add a hint to the "WAL summaries are required" errors

Hi Sai,

I tried the patch on master (v1 applies cleanly, and 007 still passes
3/3) and reproduced the error on several paths, including the
wal_level=minimal case via 007. The hint shows up in all of them.

> Would a shorter "Take a new full backup." be preferable?

I think there is a case for it. On the minimal path the hint still
fires, but its two causes do not really fit: summarize_wal is off there
only because minimal requires it -- the actual reason is that the
summarizer reaches that WAL and skips it on purpose (fast_forward) --
and nothing was removed. And even in the main case the first cause is
more a symptom than the condition: with the WAL still on disk, enabling
summarize_wal after the full backup does not error, since the summarizer
backfills from the oldest WAL present; it only fails once that WAL has
been recycled before summarization caught up.

So the remedy-only form you suggested reads correctly in all of them.
If you would rather keep naming the minimal case, there is existing
wording to mirror at xlog.c:6037:

Use a backup taken after setting "wal_level" to higher than "minimal".

Happy to share the repro script if it helps.

Regards,
Manu