fix(router): honor SSR backpressure via drain event for compression compatibility

.changeset/compressed-streams-drain.md

@@ -0,0 +1,5 @@
+---
+'@qwik.dev/router': patch
+---
+
+fix(router): Node SSR no longer hangs when using `compression` (or other middleware that wraps `res.write` / `res.end`).

packages/qwik-router/src/middleware/node/http.ts

@@ -97,12 +97,21 @@ function createNodeResponseSink(res: ServerResponse) {
 
     return new Promise<void>((resolve, reject) => {
       let settled = false;
+      let onDrain: (() => void) | undefined;
+
+      const cleanupDrain = () => {
+        if (onDrain) {
+          res.off('drain', onDrain);
+          onDrain = undefined;
+        }
+      };
 
       const finish = () => {
         if (settled) {
           return;
         }
         settled = true;
+        cleanupDrain();
 
         // Let Node process the completed write before SSR keeps emitting more chunks.
         setImmediate(resolve);
@@ -119,6 +128,7 @@ function createNodeResponseSink(res: ServerResponse) {
         }
         settled = true;
         closed = true;
+        cleanupDrain();
         reject(error);
       };
 
@@ -130,8 +140,9 @@ function createNodeResponseSink(res: ServerResponse) {
         finish();
       });
 
+      let ok: boolean;
       try {
-        res.write(chunk, (error) => {
+        ok = res.write(chunk, (error) => {
           if (error) {
             if (isClientAbortWriteError(error)) {
               finishClosed();
@@ -148,6 +159,28 @@ function createNodeResponseSink(res: ServerResponse) {
           return;
         }
         fail(error);
+        return;
+      }
+
+      // Some response wrappers (notably `compression`) replace res.write
+      // with a 2-arg signature and silently drop the per-write callback,
+      // which would otherwise hang the SSR walker forever. Resolve via
+      // Node's standard backpressure protocol instead:
+      //   - res.write() returned true  → no backpressure, resolve now.
+      //   - res.write() returned false → wait for 'drain'.
+      // The per-write callback (if it ever fires) still surfaces errors,
+      // and becomes a no-op for resolution once we've already settled.
+      if (settled) {
+        return;
+      }
+      if (ok) {
+        finish();
+      } else {
+        onDrain = () => {
+          onDrain = undefined;
+          finish();
+        };
+        res.once('drain', onDrain);
       }
     });
   };
@@ -189,10 +222,15 @@ function createNodeResponseSink(res: ServerResponse) {
         finish();
       });
 
+      // Resolve on 'finish' (data fully flushed). We do NOT pass a callback
+      // to res.end because some response wrappers replace it with a 2-arg
+      // signature `(chunk, encoding)` (notably `compression@1.x`) and would
+      // treat the callback as a chunk, then throw inside Buffer.from(...) —
+      // leaving the underlying stream un-ended and the socket hanging.
+      res.once('finish', finish);
+
       try {
-        res.end(() => {
-          finish();
-        });
+        res.end();
       } catch (error) {
         if (isClientAbortWriteError(error)) {
           finish();

packages/qwik-router/src/middleware/node/http.unit.ts

@@ -55,26 +55,57 @@ import { fromNodeHttp, normalizeUrl } from './http';
 });
 
 describe('fromNodeHttp()', () => {
-  test('should resolve writes from the node write callback without waiting for drain', async () => {
+  test('should resolve writes immediately when res.write returns true (no backpressure)', async () => {
     const req = new EventEmitter() as IncomingMessage & EventEmitter;
     req.method = 'GET';
     req.url = '/';
     req.headers = { host: 'localhost' };
     (req as any).socket = {};
 
-    let writeCallback: ((error?: Error | null) => void) | undefined;
     const res = new EventEmitter() as ServerResponse & EventEmitter;
     Object.defineProperty(res, 'closed', { value: false, configurable: true });
     Object.defineProperty(res, 'destroyed', { value: false, configurable: true });
     res.setHeader = vi.fn();
-    res.write = vi.fn((_chunk: Uint8Array, cb?: (error?: Error | null) => void) => {
-      writeCallback = cb;
-      // Returning false simulates backpressure. This adapter does not attach
-      // drain listeners because ServerResponse.write() provides a per-write callback.
-      return false;
+    res.write = vi.fn(() => true) as any;
+    // Mimic vanilla Node: 'finish' fires once res.end() has flushed.
+    res.end = vi.fn(() => {
+      queueMicrotask(() => res.emit('finish'));
+      return res;
     }) as any;
-    res.end = vi.fn((cb?: () => void) => {
-      cb?.();
+
+    const requestEv = await fromNodeHttp(new URL('http://localhost/'), req, res, 'server');
+    const writableStream = requestEv.getWritableStream(
+      200,
+      new Headers([['Content-Type', 'text/html; charset=utf-8']]),
+      { headers: () => [] } as any,
+      () => {},
+      undefined as any
+    );
+    const writer = writableStream.getWriter();
+
+    await writer.write(new Uint8Array([1, 2, 3]));
+    // No drain listener should be attached when there's no backpressure.
+    expect(res.listenerCount('drain')).toBe(0);
+
+    await writer.close();
+    expect(res.write).toHaveBeenCalledTimes(1);
+    expect(res.end).toHaveBeenCalledTimes(1);
+  });
+
+  test('should resolve writes via the drain event when res.write returns false', async () => {
+    const req = new EventEmitter() as IncomingMessage & EventEmitter;
+    req.method = 'GET';
+    req.url = '/';
+    req.headers = { host: 'localhost' };
+    (req as any).socket = {};
+
+    const res = new EventEmitter() as ServerResponse & EventEmitter;
+    Object.defineProperty(res, 'closed', { value: false, configurable: true });
+    Object.defineProperty(res, 'destroyed', { value: false, configurable: true });
+    res.setHeader = vi.fn();
+    res.write = vi.fn(() => false) as any; // simulate backpressure
+    res.end = vi.fn(() => {
+      queueMicrotask(() => res.emit('finish'));
       return res;
     }) as any;
 
@@ -90,17 +121,52 @@ describe('fromNodeHttp()', () => {
 
     const writePromise = writer.write(new Uint8Array([1, 2, 3]));
     await Promise.resolve();
-    // No drain listener should be attached in this path; Node calls the write
-    // callback once this chunk has flushed, even when write() returned false.
-    expect(res.listenerCount('drain')).toBe(0);
-    expect(writeCallback).toBeDefined();
+    // A drain listener should be attached because res.write() returned false.
+    expect(res.listenerCount('drain')).toBe(1);
 
-    writeCallback?.(null);
+    res.emit('drain');
     await writePromise;
+    expect(res.listenerCount('drain')).toBe(0);
+
     await writer.close();
+  });
 
-    expect(res.write).toHaveBeenCalledTimes(1);
-    expect(res.end).toHaveBeenCalledTimes(1);
+  test('should not hang when wrapper middleware drops the per-write callback (e.g. compression)', async () => {
+    // Reproduces the regression from https://github.com/QwikDev/qwik/pull/8557:
+    // `compression` middleware wraps res.write with a 2-arg signature
+    // (chunk, encoding) and never invokes the optional callback.
+    const req = new EventEmitter() as IncomingMessage & EventEmitter;
+    req.method = 'GET';
+    req.url = '/';
+    req.headers = { host: 'localhost' };
+    (req as any).socket = {};
+
+    const res = new EventEmitter() as ServerResponse & EventEmitter;
+    Object.defineProperty(res, 'closed', { value: false, configurable: true });
+    Object.defineProperty(res, 'destroyed', { value: false, configurable: true });
+    res.setHeader = vi.fn();
+    // Wrapper that mimics `compression`: callback is dropped.
+    res.write = vi.fn((_chunk: Uint8Array, _encoding?: any) => false) as any;
+    res.end = vi.fn(() => res) as any; // also drops callback
+
+    const requestEv = await fromNodeHttp(new URL('http://localhost/'), req, res, 'server');
+    const writableStream = requestEv.getWritableStream(
+      200,
+      new Headers([['Content-Type', 'text/html; charset=utf-8']]),
+      { headers: () => [] } as any,
+      () => {},
+      undefined as any
+    );
+    const writer = writableStream.getWriter();
+
+    const writePromise = writer.write(new Uint8Array([1, 2, 3]));
+    await Promise.resolve();
+    res.emit('drain');
+    await writePromise; // would hang forever before the fix
+
+    const closePromise = writer.close();
+    res.emit('finish'); // close() must resolve even though res.end's cb is dropped
+    await closePromise;
   });
 
   test('should resolve the response when the writable stream is created', async () => {
@@ -118,8 +184,8 @@ describe('fromNodeHttp()', () => {
       cb?.(null);
       return true;
     }) as any;
-    res.end = vi.fn((cb?: () => void) => {
-      cb?.();
+    res.end = vi.fn(() => {
+      queueMicrotask(() => res.emit('finish'));
       return res;
     }) as any;