Don't require return value backup

Fixes #91703

All custom editors should implement backup or throw an exception if they cannot for some reason.

src/vs/vscode.proposed.d.ts

@@ -1273,7 +1273,7 @@ declare module 'vscode' {
 		 * in an operation that takes time to complete, your extension may decide to finish the ongoing backup rather
 		 * than cancelling it to ensure that VS Code has some valid backup.
 		 */
-		backup(cancellation: CancellationToken): Thenable<boolean>;
+		backup(cancellation: CancellationToken): Thenable<void>;
 	}
 
 	/**

src/vs/workbench/api/common/extHost.protocol.ts

@@ -620,7 +620,7 @@ export interface ExtHostWebviewsShape {
 	$onSave(resource: UriComponents, viewType: string): Promise<void>;
 	$onSaveAs(resource: UriComponents, viewType: string, targetResource: UriComponents): Promise<void>;
 
-	$backup(resource: UriComponents, viewType: string, cancellation: CancellationToken): Promise<boolean>;
+	$backup(resource: UriComponents, viewType: string, cancellation: CancellationToken): Promise<void>;
 }
 
 export interface MainThreadUrlsShape extends IDisposable {

src/vs/workbench/api/common/extHostWebview.ts

@@ -727,7 +727,7 @@ export class ExtHostWebviews implements ExtHostWebviewsShape {
 		return document._saveAs(URI.revive(targetResource));
 	}
 
-	async $backup(resourceComponents: UriComponents, viewType: string, cancellation: CancellationToken): Promise<boolean> {
+	async $backup(resourceComponents: UriComponents, viewType: string, cancellation: CancellationToken): Promise<void> {
 		const document = this.getDocument(viewType, resourceComponents);
 		return document._backup(cancellation);
 	}

src/vs/workbench/contrib/customEditor/common/customEditor.ts

@@ -74,7 +74,7 @@ export interface ICustomEditorModel extends IWorkingCopy {
 	readonly onWillSave: Event<CustomEditorSaveEvent>;
 	readonly onWillSaveAs: Event<CustomEditorSaveAsEvent>;
 
-	onBackup(f: () => CancelablePromise<boolean>): void;
+	onBackup(f: () => CancelablePromise<void>): void;
 
 	setDirty(dirty: boolean): void;
 	undo(): void;

src/vs/workbench/contrib/customEditor/common/customEditorModel.ts

@@ -29,7 +29,7 @@ namespace HotExitState {
 		readonly type = Type.Pending;
 
 		constructor(
-			public readonly operation: CancelablePromise<boolean>,
+			public readonly operation: CancelablePromise<void>,
 		) { }
 	}
 
@@ -90,9 +90,9 @@ export class CustomEditorModel extends Disposable implements ICustomEditorModel
 	private readonly _onWillSaveAs = this._register(new Emitter<CustomEditorSaveAsEvent>());
 	public readonly onWillSaveAs = this._onWillSaveAs.event;
 
-	private _onBackup: undefined | (() => CancelablePromise<boolean>);
+	private _onBackup: undefined | (() => CancelablePromise<void>);
 
-	public onBackup(f: () => CancelablePromise<boolean>) {
+	public onBackup(f: () => CancelablePromise<void>) {
 		if (this._onBackup) {
 			throw new Error('Backup already implemented');
 		}
@@ -182,7 +182,11 @@ export class CustomEditorModel extends Disposable implements ICustomEditorModel
 		this._hotExitState = pendingState;
 
 		try {
-			this._hotExitState = await pendingState.operation ? HotExitState.Allowed : HotExitState.NotAllowed;
+			await pendingState.operation;
+			// Make sure state has not changed in the meantime
+			if (this._hotExitState === pendingState) {
+				this._hotExitState = HotExitState.Allowed;
+			}
 		} catch (e) {
 			// Make sure state has not changed in the meantime
 			if (this._hotExitState === pendingState) {