sql: add scaffolding for version upgrade hook

This callback will be called after validating a `SET CLUSTER SETTING
version` but before executing it. It will be used in future PRs to
execute arbitrary migrations to allow us to eventually remove code to
support legacy behavior.

This diff was pulled out of the long-running migrations prototype
(cockroachdb#56107). For more details, see the RFC (cockroachdb#48843).

Release note: None

pkg/server/server_sql.go

@@ -465,6 +465,10 @@ func newSQLServer(ctx context.Context, cfg sqlServerArgs) (*sqlServer, error) {
 		RangeDescriptorCache:    cfg.distSender.RangeDescriptorCache(),
 		RoleMemberCache:         &sql.MembershipCache{},
 		TestingKnobs:            sqlExecutorTestingKnobs,
+		VersionUpgradeHook: func(ctx context.Context, to roachpb.Version) error {
+			// TODO(irfansharif): Do something real here.
+			return nil
+		},
 
 		DistSQLPlanner: sql.NewDistSQLPlanner(
 			ctx,

pkg/sql/exec_util.go

@@ -736,6 +736,11 @@ type ExecutorConfig struct {
 	HydratedTables *hydratedtables.Cache
 
 	GCJobNotifier *gcjobnotifier.Notifier
+
+	// VersionUpgradeHook is called after validating a `SET CLUSTER SETTING
+	// version` but before executing it. It can carry out arbitrary migrations
+	// that allow us to eventually remove legacy code.
+	VersionUpgradeHook func(ctx context.Context, to roachpb.Version) error
 }
 
 // Organization returns the value of cluster.organization.

pkg/sql/set_cluster_setting.go

@@ -19,6 +19,7 @@ import (
 
 	"github.com/cockroachdb/cockroach/pkg/clusterversion"
 	"github.com/cockroachdb/cockroach/pkg/kv"
+	"github.com/cockroachdb/cockroach/pkg/roachpb"
 	"github.com/cockroachdb/cockroach/pkg/security"
 	"github.com/cockroachdb/cockroach/pkg/server/telemetry"
 	"github.com/cockroachdb/cockroach/pkg/settings"
@@ -47,6 +48,10 @@ type setClusterSettingNode struct {
 	setting settings.WritableSetting
 	// If value is nil, the setting should be reset.
 	value tree.TypedExpr
+	// versionUpgradeHook is called after validating a `SET CLUSTER SETTING
+	// version` but before executing it. It can carry out arbitrary migrations
+	// that allow us to eventually remove legacy code.
+	versionUpgradeHook func(ctx context.Context, to roachpb.Version) error
 }
 
 func checkPrivilegesForSetting(ctx context.Context, p *planner, name string, action string) error {
@@ -156,7 +161,11 @@ func (p *planner) SetClusterSetting(
 		}
 	}
 
-	return &setClusterSettingNode{name: name, st: st, setting: setting, value: value}, nil
+	csNode := setClusterSettingNode{
+		name: name, st: st, setting: setting, value: value,
+		versionUpgradeHook: p.execCfg.VersionUpgradeHook,
+	}
+	return &csNode, nil
 }
 
 func (n *setClusterSettingNode) startExec(params runParams) error {
@@ -209,7 +218,8 @@ func (n *setClusterSettingNode) startExec(params runParams) error {
 			}
 			reportedValue = tree.AsStringWithFlags(value, tree.FmtBareStrings)
 			var prev tree.Datum
-			if _, ok := n.setting.(*settings.VersionSetting); ok {
+			_, isSetVersion := n.setting.(*settings.VersionSetting)
+			if isSetVersion {
 				datums, err := execCfg.InternalExecutor.QueryRowEx(
 					ctx, "retrieve-prev-setting", txn,
 					sessiondata.InternalExecutorOverride{User: security.RootUserName()},
@@ -232,6 +242,18 @@ func (n *setClusterSettingNode) startExec(params runParams) error {
 			if err != nil {
 				return err
 			}
+
+			if isSetVersion {
+				// toSettingString already validated the input, and checked to
+				// see that we are allowed to transition. Let's call into our
+				// upgrade hook to run migrations, if any.
+				versionStr := string(*value.(*tree.DString))
+				targetVersion := roachpb.MustParseVersion(versionStr)
+				if err := n.versionUpgradeHook(ctx, targetVersion); err != nil {
+					return err
+				}
+			}
+
 			if _, err = execCfg.InternalExecutor.ExecEx(
 				ctx, "update-setting", txn,
 				sessiondata.InternalExecutorOverride{User: security.RootUserName()},