Re: {CREATE INDEX, REINDEX} CONCURRENTLY improvements

> On Mon, Nov 30, 2020 at 04:54:39PM -0300, Alvaro Herrera wrote:
>
> In a previous thread [1], we added smarts so that processes running
> CREATE INDEX CONCURRENTLY would not wait for each other.
>
> One is adding the same to REINDEX CONCURRENTLY.  I've attached patch
> 0002 here which does that.
>
> Why 0002, you ask?  That's because preparatory patch 0001 simplifies the
> ReindexRelationConcurrently somewhat by adding a struct to be used of
> indexes that are going to be processed, instead of just a list of Oids.
> This is a good change in itself because it let us get rid of duplicative
> open/close of the index rels in order to obtain some info that's already
> known at the start.

Thanks! The patch looks pretty good to me, after reading it I only have
a few minor comments/questions:

* ReindexIndexInfo sounds a bit weird for me because of the repeating
  part, although I see there is already a similar ReindexIndexCallbackState
  so probably it's fine.

* This one is mostly for me to understand. There are couple of places
  with a commentary that 'PROC_IN_SAFE_IC is not necessary, because the
  transaction only takes a snapshot to do some catalog manipulation'.
  But for some of them I don't immediately see in the relevant code
  anything related to snapshots. E.g. one in DefineIndex is followed by
  WaitForOlderSnapshots (which seems to only do waiting, not taking a
  snapshot), index_set_state_flags and CacheInvalidateRelcacheByRelid.
  Is taking a snapshot hidden somewhere there inside?