search: always pass a session to file search providers

FileSearchProviderOptions.session is declared as an object, but
searches without a cache key got undefined, so providers that key a
WeakMap on the session threw.

Give those searches their own session and cancel it once the search
is done, like clearCache does for cached sessions.

src/vs/workbench/api/test/node/extHostSearch.test.ts

@@ -293,6 +293,20 @@ suite('ExtHostSearch', () => {
 			disposables.forEach(d => d?.dispose());
 		});
 
+		test('session is defined when there is no cache key', async () => {
+			let session: object | undefined;
+			disposables.add(extHostSearch.registerFileSearchProvider('file', {
+				provideFileSearchResults(pattern: string, options: vscode.FileSearchProviderOptions, token: vscode.CancellationToken): Promise<URI[]> {
+					session = options.session;
+					return Promise.resolve([]);
+				}
+			}));
+			await rpcProtocol.sync();
+
+			await runFileSearch(getSimpleQuery());
+			assert.strictEqual(typeof session, 'object');
+		});
+
 		test('provider returns null', async () => {
 			await registerTestFileSearchProvider({
 				provideFileSearchResults(query: vscode.FileSearchQuery, options: vscode.FileSearchOptions, token: vscode.CancellationToken): Promise<URI[]> {

src/vs/workbench/services/search/common/fileSearchManager.ts

@@ -347,7 +347,11 @@ export class FileSearchManager {
 			onBatch(batch.map(m => this.rawMatchToSearchItem(m)));
 		};
 
-		return this.doSearch(engine, FileSearchManager.BATCH_SIZE, onInternalResult, token).then(
+		return this.doSearch(engine, FileSearchManager.BATCH_SIZE, onInternalResult, token).finally(() => {
+			if (!config.cacheKey) {
+				sessionTokenSource.cancel();
+			}
+		}).then(
 			result => {
 				return {
 					limitHit: result.limitHit,
@@ -369,16 +373,16 @@ export class FileSearchManager {
 		this.sessions.delete(cacheKey);
 	}
 
-	private getSessionTokenSource(cacheKey: string | undefined): SessionLifecycle | undefined {
+	private getSessionTokenSource(cacheKey: string | undefined): SessionLifecycle {
 		if (!cacheKey) {
-			return undefined;
+			return new SessionLifecycle();
 		}
 
 		if (!this.sessions.has(cacheKey)) {
 			this.sessions.set(cacheKey, new SessionLifecycle());
 		}
 
-		return this.sessions.get(cacheKey);
+		return this.sessions.get(cacheKey)!;
 	}
 
 	private rawMatchToSearchItem(match: IInternalFileMatch): IFileMatch {