concurrency: allow shared locking requests into the lock table

[arulajmani]

This patch extends the list of permissible locking strengths to include
SHARED and tests conflict resolution for shared locks with other lock
strengths.

Note that (currently) the lock table doesn't support multiple locks from
different transactions on a single key; as such, 2 transactions cannot
simultaneously hold shared locks on a single key. This restriction will
be lifted in the future.

Release note: None

[cockroach-teamcity]

This change is 

[nvanbenschoten]

That was easy!

Reviewed 3 of 3 files at r4, all commit messages.
Reviewable status: complete! 0 of 0 LGTMs obtained (waiting on @arulajmani)

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pkg/kv/kvserver/concurrency/testdata/lock_table/shared_locks line 36 at r4 (raw file):

# req3 should not actively wait, as it's locking strength is shared, but it
# should be able to acquire a joint claim.

Do joint claims work yet? If so, do you want to add to this test to demonstrate that?

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pkg/kv/kvserver/concurrency/lock_table_test.go line 718 at r4 (raw file):

		case lock.Shared:
			sa = spanset.SpanReadOnly
			ts = hlc.MaxTimestamp

Consider leaving a comment that points to #102264.

[arulajmani]

Reviewable status: complete! 0 of 0 LGTMs obtained (waiting on @nvanbenschoten)

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pkg/kv/kvserver/concurrency/testdata/lock_table/shared_locks line 36 at r4 (raw file):

Previously, nvanbenschoten (Nathan VanBenschoten) wrote…

Do joint claims work yet? If so, do you want to add to this test to demonstrate that?

They don't work yet because of this TODO that we spoke about in a different context last week:

cockroach/pkg/kv/kvserver/concurrency/lock_table.go

Lines 2284 to 2288 in 292c54e

	// TODO(arul): Inactive waiters will need to capture the strength at which
	// they're trying to acquire a lock in their queuedGuard. We can't simply
	// use the guard's curStrength (or curLockMode) -- inactive waiters may have
	// mutated these values as they scan. For now, we can just use the intent
	// lock mode as that's the only lock strength supported by the lock table.

I'll add a test once we've addressed this.

[nvanbenschoten]

Reviewed 34 of 34 files at r5, all commit messages.
Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @arulajmani)

[arulajmani]

TFTR!

bors r=nvanbenschoten

[craig]

Build succeeded:

• Bazel Essential CI (Cockroach)