cli,server: explain in comment why tenant version must be initialized

For the longest time, we had a comment that suggested we weren't sure
about it. We are now.

Release note: None

pkg/cli/mt_start_sql.go

@@ -62,19 +62,16 @@ func runStartSQL(cmd *cobra.Command, args []string) error {
 		if err := serverCfg.InitSQLServer(ctx); err != nil {
 			return err
 		}
-		// This value is injected in order to have something populated during startup.
-		// In the initial 20.2 release of multi-tenant clusters, no version state was
-		// ever populated in the version cluster setting. A value is populated during
-		// the activation of 21.1. See the documentation attached to the TenantCluster
-		// in upgrade/upgradecluster for more details on the tenant upgrade flow.
-		// Note that the value of 21.1 is populated when a tenant cluster is created
-		// during 21.1 in crdb_internal.create_tenant.
-		//
-		// Note that the tenant will read the value in the system.settings table
-		// before accepting SQL connections.
+
+		// We need a value in the version setting prior to the update
+		// coming from the system.settings table. This value must be valid
+		// and compatible with the state of the tenant's keyspace.
 		//
-		// TODO(knz): Check if this special initialization can be removed.
-		// See: https://github.com/cockroachdb/cockroach/issues/90831
+		// Since we don't know at which binary version the tenant
+		// keyspace was initialized, we must be conservative and
+		// assume it was created a long time ago; and that we may
+		// have to run all known migrations since then. So initialize
+		// the version setting to the minimum supported version.
 		st := serverCfg.BaseConfig.Settings
 		return clusterversion.Initialize(
 			ctx, st.Version.BinaryMinSupportedVersion(), &st.SV,

pkg/server/server_controller_new_server.go

@@ -193,12 +193,15 @@ func makeSharedProcessTenantServerConfig(
 ) (baseCfg BaseConfig, sqlCfg SQLConfig, err error) {
 	st := cluster.MakeClusterSettings()
 
-	// This version initialization is copied from cli/mt_start_sql.go.
+	// We need a value in the version setting prior to the update
+	// coming from the system.settings table. This value must be valid
+	// and compatible with the state of the tenant's keyspace.
 	//
-	// TODO(knz): Why is this even useful? The comment refers to v21.1
-	// compatibility, yet if we don't do this, the server panics with
-	// "version not initialized". This might be related to:
-	// https://github.com/cockroachdb/cockroach/issues/84587
+	// Since we don't know at which binary version the tenant
+	// keyspace was initialized, we must be conservative and
+	// assume it was created a long time ago; and that we may
+	// have to run all known migrations since then. So initialize
+	// the version setting to the minimum supported version.
 	if err := clusterversion.Initialize(
 		ctx, st.Version.BinaryMinSupportedVersion(), &st.SV,
 	); err != nil {