WIP: Failover Slots

Failover Slots

If Logical Decoding is taking place from a master, then if we failover and
create a new master we would like to continue the Logical Decoding from the
new master. To facilitate this, we introduce the concept of “Failover
Slots”, which are slots that generate WAL, so that any downstream standby
can continue to produce logical decoding output from that named plugin.

In the current patch, any slot defined on a master will generate WAL,
leading to a pending-slot being present on all standby nodes. When a
standby is promoted, the slot becomes usable and will have the properties
as of the last fsync on the master.

Failover slots are not accessible until promotion of the standby. Logical
slots from a standby looks like a significant step beyond this and will not
be part of this patch.

Internal design is fairly clear, using a new rmgr. No real problems emerged
so far.

Patch is WIP, posted for comment, so you can see where I'm going.

I'm expecting to have a working version including timeline following for
9.6.

--
Simon Riggs http://www.2ndQuadrant.com/
<http://www.2ndquadrant.com/>
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