rangedesciter: support scoped iteration

[irfansharif]

Informs #87503; adds the ability to only paginate through range descriptors in the system that overlap with some given span. We're going to use it in future commits as part of multi-tenant replication reports (#89987) where we'll need to iterate over the set of range descriptors that pertain only to some span of interest (for a given index, partition, tenant, etc.)

Release note: None.

[cockroach-teamcity]

This change is 

[irfansharif]

(Bump.)

[ajwerner]

@ecwall this may or may not interest you

[arulajmani]

@ecwall this may or may not interest you

+1. We might want to use this on the server side, instead of ScanMetaKVs, when we build out the new RPC to enable tenants to run SHOW RANGEs like we were talking about this morning.

Reviewed 16 of 16 files at r1, all commit messages.
Reviewable status: complete! 0 of 0 LGTMs obtained (waiting on @irfansharif and @tbg)

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pkg/util/rangedesciter/rangedesciter.go line 99 at r1 (raw file):

		init()

		// Bound the start key of the meta{1,2} scan as much as possible. If the

Could you instead do:

metaKey := keys.RangeMetaKey(key)
bounds, err := keys.MetaScanBounds(metaKey)
 metaScanStartKey := bounds.Key   
```?

Would this allow you to get rid of the special casing in this comment and the if condition in the for loop below explained by:

					// Keys in meta ranges are encoded using the end keys of
					// range descriptors. It's possible then the first
					// descriptor/key we read has the same (exclusive) end
					// key as our query start. We'll want to skip over this
					// descriptor.

[irfansharif]

bors r+

Reviewable status: complete! 0 of 0 LGTMs obtained (waiting on @arulajmani and @tbg)

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pkg/util/rangedesciter/rangedesciter.go line 99 at r1 (raw file):

Previously, arulajmani (Arul Ajmani) wrote…

Could you instead do:

metaKey := keys.RangeMetaKey(key)
bounds, err := keys.MetaScanBounds(metaKey)
 metaScanStartKey := bounds.Key   
```?

Would this allow you to get rid of the special casing in this comment and the if condition in the for loop below explained by:

					// Keys in meta ranges are encoded using the end keys of
					// range descriptors. It's possible then the first
					// descriptor/key we read has the same (exclusive) end
					// key as our query start. We'll want to skip over this
					// descriptor.

TIL about keys.MetaScanBounds. This stuff always melts my brain, but after staring at this for a while, and reading 1 (thanks for the pointer!), you're right. Done + changed some commentary.

// Bound the start key of the meta{1,2} scan as much as possible. If the
// start key < keys.Meta1KeyMax (we're also interested in the meta1
// range descriptor), start our scan at keys.MetaMin. If not, start it
// at the relevant range meta key -- in meta1 if the start key sits
// within meta2, in meta2 if the start key is an ordinary key.
//
// So what exactly is the "relevant range meta key"? Since keys in meta
// ranges are encoded using the end keys of range descriptors, we're
// looking for the lowest existing range meta key that's strictly
// greater than RangeMetaKey(start key).
rangeMetaKeyForStart := keys.RangeMetaKey(rspan.Key)
metaScanBoundsForStart, err := keys.MetaScanBounds(rangeMetaKeyForStart)
if err != nil {
  return err
}
metaScanStartKey := metaScanBoundsForStart.Key.AsRawKey()

[craig]

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