testutils: add DeploymentMode method to ApplicationLayerInterface

Certain tests need to determine the deployment mode of the test
server—whether it's single-tenant, shared-process, or external-process.
While this can be queried via SQL, a dedicated helper method improves
usability.

This commit adds `DeploymentMode` to `ApplicationLayerInterface` to provide
a simpler way to retrieve this information.

Informs: #138912
Epic: CRDB-38970
Release note: None

Author: shubhamdhama

pkg/server/testserver.go

@@ -910,6 +910,9 @@ type testTenant struct {
 	// pgPreServer handles SQL connections prior to routing them to a
 	// specific tenant.
 	pgPreServer *pgwire.PreServeConnHandler
+	// deploymentMode specifies the tenant's deployment mode.
+	// Allowed values: ExternalProcess or SharedProcess.
+	deploymentMode serverutils.DeploymentMode
 }
 
 var _ serverutils.ApplicationLayerInterface = &testTenant{}
@@ -1258,6 +1261,11 @@ func (t *testTenant) SettingsWatcher() interface{} {
 	return t.sql.settingsWatcher
 }
 
+// DeploymentMode is part of the serverutils.ApplicationLayerInterface.
+func (t *testTenant) DeploymentMode() serverutils.DeploymentMode {
+	return t.deploymentMode
+}
+
 // WaitForTenantCapabilities is part of the serverutils.TenantControlInterface.
 func (ts *testServer) WaitForTenantCapabilities(
 	ctx context.Context,
@@ -1415,6 +1423,7 @@ func (ts *testServer) StartSharedProcessTenant(
 		pgL:            sqlServerWrapper.loopbackPgL,
 		httpTestServer: hts,
 		drain:          sqlServerWrapper.drainServer,
+		deploymentMode: serverutils.SharedProcess,
 	}
 
 	sqlDB, err := ts.SQLConnE(serverutils.DBName("cluster:" + string(args.TenantName) + "/" + args.UseDatabase))
@@ -1791,6 +1800,7 @@ func (ts *testServer) StartTenant(
 		http:           sw.http,
 		drain:          sw.drainServer,
 		pgL:            sw.loopbackPgL,
+		deploymentMode: serverutils.ExternalProcess,
 	}, err
 }
 
@@ -1920,6 +1930,11 @@ func (ts *testServer) MustGetSQLNetworkCounter(name string) int64 {
 	return mustGetSQLCounterForRegistry(reg, name)
 }
 
+// DeploymentMode is part of the serverutils.ApplicationLayerInterface.
+func (ts *testServer) DeploymentMode() serverutils.DeploymentMode {
+	return serverutils.SingleTenant
+}
+
 // Locality is part of the serverutils.ApplicationLayerInterface.
 func (ts *testServer) Locality() roachpb.Locality {
 	return ts.cfg.Locality

pkg/testutils/serverutils/api.go

@@ -130,6 +130,17 @@ type TestServerController interface {
 	RunInitialSQL(ctx context.Context, startSingleNode bool, adminUser, adminPassword string) error
 }
 
+// DeploymentMode defines the mode of the underlying test server or tenant,
+// which can be single-tenant (system-only), shared-process, or
+// external-process.
+type DeploymentMode uint8
+
+const (
+	SingleTenant DeploymentMode = iota
+	SharedProcess
+	ExternalProcess
+)
+
 // ApplicationLayerInterface defines accessors to the application
 // layer of a test server. Tests written against this interface are
 // effectively agnostic to whether they use a virtual cluster or not.
@@ -451,6 +462,11 @@ type ApplicationLayerInterface interface {
 
 	// DistSQLPlanningNodeID returns the NodeID to use by the DistSQL span resolver.
 	DistSQLPlanningNodeID() roachpb.NodeID
+
+	// DeploymentMode returns the deployment mode of the underlying server or
+	// tenant, which can be single-tenant (system-only), shared-process, or
+	// external-process.
+	DeploymentMode() DeploymentMode
 }
 
 // TenantControlInterface defines the API of a test server that can