Improve UI that prevents user from starting debug session multiple times

fixes #39707

src/vs/workbench/parts/debug/browser/debugActions.ts

@@ -156,7 +156,6 @@ export class StartAction extends AbstractDebugAction {
 
 	public static isEnabled(debugService: IDebugService, contextService: IWorkspaceContextService, configName: string) {
 		const sessions = debugService.getModel().getSessions();
-		const launch = debugService.getConfigurationManager().selectedConfiguration.launch;
 
 		if (debugService.state === State.Initializing) {
 			return false;
@@ -164,13 +163,6 @@ export class StartAction extends AbstractDebugAction {
 		if (contextService && contextService.getWorkbenchState() === WorkbenchState.EMPTY && sessions.length > 0) {
 			return false;
 		}
-		if (sessions.some(p => p.getName(false) === configName && (!launch || !launch.workspace || !p.raw.root || p.raw.root.uri.toString() === launch.workspace.uri.toString()))) {
-			return false;
-		}
-		const compound = launch && launch.getCompound(configName);
-		if (compound && compound.configurations && sessions.some(p => compound.configurations.indexOf(p.getName(false)) !== -1)) {
-			return false;
-		}
 
 		return true;
 	}

src/vs/workbench/parts/debug/electron-browser/debugService.ts

@@ -721,6 +721,15 @@ export class DebugService implements debug.IDebugService {
 				if (typeof configOrName === 'string' && launch) {
 					config = launch.getConfiguration(configOrName);
 					compound = launch.getCompound(configOrName);
+
+					const sessions = this.model.getSessions();
+					const alreadyRunningMessage = nls.localize('configurationAlreadyRunning', "There is already a debug configuration \"{0}\" running.", configOrName);
+					if (sessions.some(p => p.getName(false) === configOrName && (!launch || !launch.workspace || !p.raw.root || p.raw.root.uri.toString() === launch.workspace.uri.toString()))) {
+						return TPromise.wrapError(new Error(alreadyRunningMessage));
+					}
+					if (compound && compound.configurations && sessions.some(p => compound.configurations.indexOf(p.getName(false)) !== -1)) {
+						return TPromise.wrapError(new Error(alreadyRunningMessage));
+					}
 				} else if (typeof configOrName !== 'string') {
 					config = configOrName;
 				}