storage: avoid merge deadlock when concurrent split wins

[benesch]

@bdarnell, in a stroke of luck, the existing conditional put API supports deleting the existing value when you pass nil.

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When merging two adjacent ranges P and Q, we need to be careful to
delete Q's range descriptor with a conditional put, or we can deadlock
with a concurrent split. See the comments and test case within for
details. This commit fixes the last issue that is blocking merges from
being turned on by default.

Release note: None

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This change is 

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[bdarnell]

That's a surprisingly simple test for such a complex issue.

Reviewed 3 of 3 files at r1.
Reviewable status: complete! 1 of 0 LGTMs obtained

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pkg/storage/client_merge_test.go, line 1133 at r1 (raw file):

			if bt := req.GetBeginTransaction(); bt != nil &&
				bt.Key.Equal(keys.RangeDescriptorKey(roachpb.RKeyMin)) &&
				atomic.AddInt64(&launchSplit, -1) == 0 {

Nit: Why use a negative value here?

[tschottdorf]

good job.

Reviewed 3 of 3 files at r1.
Reviewable status: complete! 1 of 0 LGTMs obtained (and 1 stale)

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pkg/storage/client_merge_test.go, line 1133 at r1 (raw file):

Previously, bdarnell (Ben Darnell) wrote…

Nit: Why use a negative value here?

I'm confused by the comment, Ben. He could use a CAS, is that what you mean? Or subtract 1 below and add +1 here?

[bdarnell]

Reviewable status: complete! 2 of 0 LGTMs obtained

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pkg/storage/client_merge_test.go, line 1133 at r1 (raw file):

Previously, tschottdorf (Tobias Schottdorf) wrote…

I'm confused by the comment, Ben. He could use a CAS, is that what you mean? Or subtract 1 below and add +1 here?

I missed the Store below and was just confused. Nevermind (although I think a CAS might capture the intent a little more cleanly).

[benesch]

Ack, switched to a CAS. Thanks for the reviews!

bors r=tschottdorf,bdarnell

[benesch]

Ack, switched to a CAS. Thanks for the reviews!

bors r=tschottdorf,bdarnell

[craig]

Build succeeded

• GitHub CI (Cockroach)

[craig]

Build succeeded

• GitHub CI (Cockroach)