[HACKERS] Restricting maximum keep segments by repslots

Hello.

Although replication slot is helpful to avoid unwanted WAL
deletion, on the other hand it can cause a disastrous situation
by keeping WAL segments without a limit. Removing the causal
repslot will save this situation but it is not doable if the
standby is active. We should do a rather complex and forcible
steps to relieve the situation especially in an automatic
manner. (As for me, specifically in an HA cluster.)

This patch adds a GUC to put a limit to the number of segments
that replication slots can keep. Hitting the limit during
checkpoint shows a warining and the segments older than the limit
are removed.

> WARNING:  restart LSN of replication slots is ignored by checkpoint
> DETAIL:  Some replication slots lose required WAL segnents to continue.

Another measure would be automatic deletion or inactivation of
the culprit slot but it seems too complex for the problem.


As we have already postponed some patches by the triage for the
last commit fest, this might should be postponed to PG11.


regards,

-- 
Kyotaro Horiguchi
NTT Open Source Software Center