fix: make sure register handler when ipc emitter add listener

     When the last listener is disposed, we will remove the handler from the map.
     However, we will not add the handler back to the map when re-subscribing to the emitter,
     so the event will not be emitted anymore. It is better register handler in onWillAddFirstListener and
     delete it in onDidRemoveLastListener.

src/vs/base/parts/ipc/common/ipc.ts

@@ -680,6 +680,8 @@ export class ChannelClient implements IChannelClient, IDisposable {
 
 		const emitter = new Emitter<any>({
 			onWillAddFirstListener: () => {
+				const handler: IHandler = (res: IRawResponse) => emitter.fire((res as IRawEventFireResponse).data);
+				this.handlers.set(id, handler);
 				const doRequest = () => {
 					this.activeRequests.add(emitter);
 					this.sendRequest(request);
@@ -706,9 +708,6 @@ export class ChannelClient implements IChannelClient, IDisposable {
 			}
 		});
 
-		const handler: IHandler = (res: IRawResponse) => emitter.fire((res as IRawEventFireResponse).data);
-		this.handlers.set(id, handler);
-
 		return emitter.event;
 	}
 

src/vs/base/parts/ipc/test/common/ipc.test.ts

@@ -320,6 +320,27 @@ suite('Base IPC', function () {
 			assert.deepStrictEqual(messages, ['hello', 'world']);
 		});
 
+		test('listen to events (resubscribe)', async function () {
+			const onPong = ipcService.onPong;
+			const messages: string[] = [];
+
+			const disposable1 = onPong(msg => messages.push(msg));
+			await timeout(0);
+			assert.deepStrictEqual(messages, []);
+			service.ping('hello');
+			await timeout(0);
+			assert.deepStrictEqual(messages, ['hello']);
+			disposable1.dispose();
+
+			const disposable2 = onPong(msg => (messages as string[]).push(msg));
+			await timeout(0);
+			assert.deepStrictEqual(messages, ['hello']);
+			service.ping('world');
+			await timeout(0);
+			assert.deepStrictEqual(messages, ['hello', 'world']);
+			disposable2.dispose();
+		});
+
 		test('buffers in arrays', async function () {
 			const r = await ipcService.buffersLength([VSBuffer.alloc(2), VSBuffer.alloc(3)]);
 			return assert.strictEqual(r, 5);
@@ -443,6 +464,27 @@ suite('Base IPC', function () {
 			assert.deepStrictEqual(messages, ['hello', 'world']);
 		});
 
+		test('listen to events (resubscribe)', async function () {
+			const onPong = ipcService.onPong;
+			const messages: string[] = [];
+
+			const disposable1 = onPong(msg => messages.push(msg));
+			await timeout(0);
+			assert.deepStrictEqual(messages, []);
+			service.ping('hello');
+			await timeout(0);
+			assert.deepStrictEqual(messages, ['hello']);
+			disposable1.dispose();
+
+			const disposable2 = onPong(msg => (messages as string[]).push(msg));
+			await timeout(0);
+			assert.deepStrictEqual(messages, ['hello']);
+			service.ping('world');
+			await timeout(0);
+			assert.deepStrictEqual(messages, ['hello', 'world']);
+			disposable2.dispose();
+		});
+
 		test('marshalling uri', async function () {
 			const uri = URI.file('foobar');
 			const r = await ipcService.marshall(uri);