Re: Replication identifiers, take 4

Hi,

Here's the next version of this patch. I've tried to address the biggest
issue (documentation) and some more. Now that both the more flexible
commit WAL record format and the BKI_FORCE_NOT_NULL thing is in, it
looks much cleaner.

Changes:
* Loads of documentation and comments
* Revamped locking strategy. There's now a LWLock protecting all the
  replication progress array and spinlock for the individual sltos.
* Simpler checkpoint format.
* A new pg_replication_identifier_progress() function returning a
  individual identifier's replication progress; previously there was
  only the view showing all of them.
* Lots of minor cleanup
* Some more tests

I'd greatly appreciate some feedback on the documentation. I'm not
entirely sure into how much detail to go; and where exactly in the docs
to place it. I do wonder if we shouldn't merge this with the logical
decoding section and whether we could also document commit timestamps
somewhere in there.

I've verified that this correctly works on a stanby; replication
progress is replicated correctly.  I think there's two holes though:
Changing the replication progress without replicating anything and
dropping a replication identifier with some replication progress might
not work correctly. That's fairly easily fixed and I intend to do so.

Other than that I'm not aware of outstanding issues.

Comments?

Greetings,

Andres Freund

--
 Andres Freund                       http://www.2ndQuadrant.com/
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