fix(sse): Drain stream as recommended

packages/core/router/router-response-controller.ts

@@ -8,6 +8,7 @@ import { isFunction } from '@nestjs/common/utils/shared.utils';
 import { Observable } from 'rxjs';
 import { SseStream, HeaderStream } from '../services';
 import { IncomingMessage, ServerResponse } from 'http';
+import { debounce } from 'rxjs/operators';
 
 export interface CustomHeader {
   name: string;
@@ -93,10 +94,16 @@ export class RouterResponseController {
     const stream = new SseStream(request);
     stream.pipe(response);
 
-    const subscription = observable.subscribe((message: any) => {
-      if (typeof message !== 'object') message = { data: message };
-      stream.writeMessage(message);
-    });
+    const subscription = observable
+      .pipe(
+        debounce((message: any) => {
+          return new Promise(resolve => {
+            if (typeof message !== 'object') message = { data: message };
+            stream.writeMessage(message, resolve);
+          });
+        }),
+      )
+      .subscribe();
 
     request.on('close', () => {
       response.end();

packages/core/services/sse-stream.service.ts

@@ -50,7 +50,7 @@ export class SseStream extends Transform {
   pipe<T extends HeaderStream>(destination: T, options?: { end?: boolean }): T {
     if (destination.writeHead) {
       destination.writeHead(200, {
-        // See https://github.com/dunglas/mercure/blob/master/hub/subscribe.go#L124-L130 
+        // See https://github.com/dunglas/mercure/blob/master/hub/subscribe.go#L124-L130
         'Content-Type': 'text/event-stream',
         Connection: 'keep-alive',
         // Disable cache, even for old browsers and proxies
@@ -74,24 +74,31 @@ export class SseStream extends Transform {
     encoding: string,
     callback: (error?: Error | null, data?: any) => void,
   ) {
-    if (message.type) this.push(`event: ${message.type}\n`);
-    if (message.id) this.push(`id: ${message.id}\n`);
-    if (message.retry) this.push(`retry: ${message.retry}\n`);
-    if (message.data) this.push(toDataString(message.data));
-    this.push('\n');
+    let data = message.type ? `event: ${message.type}\n` : '';
+    data += message.id ? `id: ${message.id}\n` : '';
+    data += message.retry ? `retry: ${message.retry}\n` : '';
+    data += message.data ? toDataString(message.data) : '';
+    data += '\n';
+    this.push(data);
     callback();
   }
 
+  /**
+   * Calls `.write` but handles the drain if needed
+   */
   writeMessage(
     message: MessageEvent,
-    encoding?: string,
-    cb?: (error: Error | null | undefined) => void,
-  ): boolean {
+    cb: (error: Error | null | undefined) => void,
+  ) {
     if (!message.id) {
       this.lastEventId++;
       message.id = this.lastEventId.toString();
     }
 
-    return this.write(message, encoding, cb);
+    if (!this.write(message, 'utf-8', cb)) {
+      this.once('drain', cb);
+    } else {
+      process.nextTick(cb);
+    }
   }
 }

packages/core/test/services/sse-stream.service.spec.ts

@@ -5,6 +5,8 @@ import * as EventSource from 'eventsource';
 import { SseStream, HeaderStream } from '../../services/sse-stream.service';
 import { AddressInfo } from 'net';
 
+const noop = () => {};
+
 const written = (stream: Writable) =>
   new Promise((resolve, reject) =>
     stream.on('error', reject).on('finish', resolve),
@@ -41,12 +43,18 @@ describe('SseStream', () => {
     const sse = new SseStream();
     const sink = new Sink();
     sse.pipe(sink);
-    sse.writeMessage({
-      data: 'hello\nworld',
-    });
-    sse.write({
-      data: 'bonjour\nmonde',
-    });
+    sse.writeMessage(
+      {
+        data: 'hello\nworld',
+      },
+      noop,
+    );
+    sse.writeMessage(
+      {
+        data: 'bonjour\nmonde',
+      },
+      noop,
+    );
     sse.end();
     await written(sink);
     expect(sink.content).to.equal(
@@ -55,6 +63,7 @@ id: 1
 data: hello
 data: world
 
+id: 2
 data: bonjour
 data: monde
 
@@ -66,9 +75,12 @@ data: monde
     const sse = new SseStream();
     const sink = new Sink();
     sse.pipe(sink);
-    sse.writeMessage({
-      data: { hello: 'world' },
-    });
+    sse.writeMessage(
+      {
+        data: { hello: 'world' },
+      },
+      noop,
+    );
     sse.end();
     await written(sink);
     expect(sink.content).to.equal(
@@ -84,12 +96,15 @@ data: {"hello":"world"}
     const sse = new SseStream();
     const sink = new Sink();
     sse.pipe(sink);
-    sse.writeMessage({
-      type: 'tea-time',
-      id: 'the-id',
-      retry: 222,
-      data: 'hello',
-    });
+    sse.writeMessage(
+      {
+        type: 'tea-time',
+        id: 'the-id',
+        retry: 222,
+        data: 'hello',
+      },
+      noop,
+    );
     sse.end();
     await written(sink);
     expect(sink.content).to.equal(
@@ -137,7 +152,7 @@ data: hello
         es.close();
         server.close(callback);
       };
-      es.onopen = () => sse.writeMessage({ data: 'hello' });
+      es.onopen = () => sse.writeMessage({ data: 'hello' }, noop);
       es.onerror = e =>
         callback(new Error(`Error from EventSource: ${JSON.stringify(e)}`));
     });