migrations: populate initial version setting

[tbg]

Add a bootstrap version which is populated when a cluster running >1.0 is
bootstrapped. Use it to populate the settings table via a sql migration. At the
same time, prevent SET CLUSTER SETTING version = 'x' from working until that
migration has run.

This solves one remaining headache for version migrations by giving it
authoritative information on the currently running cluster version during
upgrades.

Fixes #17389.

[cockroach-teamcity]

This change is 

[bdarnell]

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Reviewed 8 of 8 files at r1, 8 of 8 files at r2.
Review status: all files reviewed at latest revision, 1 unresolved discussion, all commit checks successful.

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pkg/migration/sqlmigrations/migrations.go, line 416 at r2 (raw file):

func populateVersionSetting(ctx context.Context, r runner) error {
	var v roachpb.Version
	if err := r.db.Txn(ctx, func(ctx context.Context, txn *client.Txn) error {

Can this be r.db.Get instead of using an explicit Txn?

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Comments from Reviewable

[tbg]

Review status: all files reviewed at latest revision, 1 unresolved discussion, all commit checks successful.

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pkg/migration/sqlmigrations/migrations.go, line 416 at r2 (raw file):

Previously, bdarnell (Ben Darnell) wrote…

Can this be r.db.Get instead of using an explicit Txn?

Yes, but I'd have to plumb more stuff onto runner, and so I decided against it. I can add a comment and update the runner, whichever you prefer.

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Comments from Reviewable

[bdarnell]

Review status: all files reviewed at latest revision, 1 unresolved discussion, all commit checks successful.

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pkg/migration/sqlmigrations/migrations.go, line 416 at r2 (raw file):

Previously, tschottdorf (Tobias Schottdorf) wrote…

Yes, but I'd have to plumb more stuff onto runner, and so I decided against it. I can add a comment and update the runner, whichever you prefer.

Leaving it is fine; I didn't realize this wasn't a full client.DB

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Comments from Reviewable

[tbg]

TFTR, Ben!