cfetcher: correctly update the limit hint

[yuzefovich]

In 41fa8b6 (which was supposed to be a "noop" refactor) we introduced a bug which made it so that we no longer update the remaining limit hint correctly. As a result, the cFetcher might no longer respect the limit hint. What makes things worse is the fact that the KV layer still does everything correctly, so when the cFetcher asks for more rows that exceed the limit, the KV layer does a BatchRequest with 10x of the original limit. This is now fixed by correctly updating the limit hint right before emitting the batch.

Addresses: #88382.

Epic: CRDB-20535

Release note (bug fix): CockroachDB no longer fetches unnecessary rows for queries with LIMITs. The bug was introduced in 22.1.7.

[cockroach-teamcity]

This change is 

[yuzefovich]

Reviewable status: complete! 0 of 0 LGTMs obtained (waiting on @DrewKimball and @mgartner)

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pkg/sql/colfetcher/cfetcher.go line 852 at r1 (raw file):

			cf.shiftState()

			if cf.machine.limitHint > 0 && cf.machine.rowIdx >= cf.machine.limitHint {

The bug was that this condition would never be true if LIMIT exceeds 1024 because rowIdx is at most 1024.

[DrewKimball]

Great find!

Reviewed 3 of 3 files at r1, all commit messages.
Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @mgartner)

[mgartner]

Thanks for the speedy fix!

Reviewed 3 of 3 files at r1, all commit messages.
Reviewable status: complete! 2 of 0 LGTMs obtained (waiting on @DrewKimball and @mgartner)

[yuzefovich]

TFTRs!

bors r+

[craig]

Build succeeded:

• Bazel Essential CI (Cockroach)

[blathers-crl]

Encountered an error creating backports. Some common things that can go wrong:

1. The backport branch might have already existed.
2. There was a merge conflict.
3. The backport branch contained merge commits.

You might need to create your backport manually using the backport tool.

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error creating merge commit from 1d6621c to blathers/backport-release-22.1-88391: POST https://api.github.com/repos/cockroachdb/cockroach/merges: 409 Merge conflict []

you may need to manually resolve merge conflicts with the backport tool.

Backport to branch 22.1.x failed. See errors above.

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