sql: show the virtual cluster name via a session var

Release note (cluster virtualization): The name of the virtual cluster
that the SQL client is connected to can now be inspected via the SQL
session var `virtual_cluster_name`.

docs/generated/sql/bnf/stmt_block.bnf

@@ -1886,6 +1886,7 @@ session_var ::=
 	| 'LC_COLLATE'
 	| 'LC_CTYPE'
 	| 'TIME' 'ZONE'
+	| 'VIRTUAL_CLUSTER_NAME'
 
 var_name ::=
 	name

pkg/server/server_sql.go

@@ -98,6 +98,7 @@ import (
 	"github.com/cockroachdb/cockroach/pkg/sql/scheduledlogging"
 	"github.com/cockroachdb/cockroach/pkg/sql/schemachanger/scdeps"
 	"github.com/cockroachdb/cockroach/pkg/sql/schemachanger/scexec"
+	"github.com/cockroachdb/cockroach/pkg/sql/sem/catconstants"
 	"github.com/cockroachdb/cockroach/pkg/sql/sem/eval"
 	"github.com/cockroachdb/cockroach/pkg/sql/sessiondata"
 	"github.com/cockroachdb/cockroach/pkg/sql/sessiondatapb"
@@ -1040,6 +1041,10 @@ func newSQLServer(ctx context.Context, cfg sqlServerArgs) (*SQLServer, error) {
 		AutoConfigProvider:         cfg.AutoConfigProvider,
 	}
 
+	if codec.ForSystemTenant() {
+		execCfg.VirtualClusterName = catconstants.SystemTenantName
+	}
+
 	if sqlSchemaChangerTestingKnobs := cfg.TestingKnobs.SQLSchemaChanger; sqlSchemaChangerTestingKnobs != nil {
 		execCfg.SchemaChangerTestingKnobs = sqlSchemaChangerTestingKnobs.(*sql.SchemaChangerTestingKnobs)
 	} else {
@@ -1438,6 +1443,7 @@ func (s *SQLServer) preStart(
 		// from KV (or elsewhere).
 		if entry, _ := s.tenantConnect.TenantInfo(); entry.Name != "" {
 			s.cfg.idProvider.SetTenantName(entry.Name)
+			s.execCfg.VirtualClusterName = entry.Name
 		}
 		if err := s.startCheckService(ctx, stopper); err != nil {
 			return err

pkg/sql/exec_util.go

@@ -1447,6 +1447,10 @@ type ExecutorConfig struct {
 	// AutoConfigProvider informs the auto config runner job of new
 	// tasks to run.
 	AutoConfigProvider acprovider.Provider
+
+	// VirtualClusterName contains the name of the virtual cluster
+	// (tenant).
+	VirtualClusterName roachpb.TenantName
 }
 
 // UpdateVersionSystemSettingHook provides a callback that allows us

pkg/sql/parser/sql.y

@@ -7486,6 +7486,7 @@ session_var:
 // TIME ZONE is special: it is two tokens, but is really the identifier "TIME ZONE".
 | TIME ZONE { $$ = "timezone" }
 | TIME error // SHOW HELP: SHOW SESSION
+| VIRTUAL_CLUSTER_NAME
 
 session_var_parts:
   '.' IDENT

pkg/sql/tests/BUILD.bazel

@@ -6,10 +6,12 @@ go_library(
         "command_filters.go",
         "data.go",
         "explain_test_util.go",
+        "virtual_cluster_name.go",
     ],
     importpath = "github.com/cockroachdb/cockroach/pkg/sql/tests",
     visibility = ["//visibility:public"],
     deps = [
+        "//pkg/base",
         "//pkg/internal/sqlsmith",
         "//pkg/kv",
         "//pkg/kv/kvpb",
@@ -18,7 +20,10 @@ go_library(
         "//pkg/sql/sem/tree",
         "//pkg/storage",
         "//pkg/testutils/serverutils",
+        "//pkg/testutils/sqlutils",
         "//pkg/testutils/storageutils",
+        "//pkg/util/leaktest",
+        "//pkg/util/log",
         "//pkg/util/syncutil",
         "@com_github_cockroachdb_errors//:errors",
         "@org_golang_x_text//cases",

pkg/sql/tests/virtual_cluster_name.go

@@ -0,0 +1,51 @@
+// Copyright 2023 The Cockroach Authors.
+//
+// Use of this software is governed by the Business Source License
+// included in the file licenses/BSL.txt.
+//
+// As of the Change Date specified in that file, in accordance with
+// the Business Source License, use of this software will be governed
+// by the Apache License, Version 2.0, included in the file
+// licenses/APL.txt.
+
+package tests
+
+import (
+	"context"
+	gosql "database/sql"
+	"testing"
+
+	"github.com/cockroachdb/cockroach/pkg/base"
+	"github.com/cockroachdb/cockroach/pkg/testutils/serverutils"
+	"github.com/cockroachdb/cockroach/pkg/testutils/sqlutils"
+	"github.com/cockroachdb/cockroach/pkg/util/leaktest"
+	"github.com/cockroachdb/cockroach/pkg/util/log"
+)
+
+func TestClusterName(t *testing.T) {
+	defer leaktest.AfterTest(t)()
+	defer log.Scope(t).Close(t)
+
+	ctx := context.Background()
+	s, db, _ := serverutils.StartServer(t, base.TestServerArgs{
+		DefaultTestTenant: base.TestControlsTenantsExplicitly,
+	})
+	defer s.Stopper().Stop(ctx)
+
+	checkName := func(t *testing.T, db *gosql.DB, expected string) {
+		sql := sqlutils.MakeSQLRunner(db)
+		sql.CheckQueryResults(t, `SHOW virtual_cluster_name`, [][]string{{expected}})
+	}
+
+	t.Run("system", func(t *testing.T) {
+		checkName(t, db, "system")
+	})
+
+	t.Run("virtual", func(t *testing.T) {
+		_, db2 := serverutils.StartTenant(t, s, base.TestTenantArgs{
+			TenantID:   serverutils.TestTenantID(),
+			TenantName: "virtual",
+		})
+		checkName(t, db2, "virtual")
+	})
+}

pkg/sql/vars.go

@@ -1624,6 +1624,14 @@ var varGen = map[string]sessionVar{
 		},
 	},
 
+	// CockroachDB extension.
+	`virtual_cluster_name`: {
+		Hidden: true,
+		Get: func(evalCtx *extendedEvalContext, _ *kv.Txn) (string, error) {
+			return string(evalCtx.ExecCfg.VirtualClusterName), nil
+		},
+	},
+
 	// CockroachDB extension.
 	`allow_prepare_as_opt_plan`: {
 		Hidden: true,