fix: image optimizer hangs when invalid image is requested (#33719)

When an invalid image is requested, the 'finish' event is never triggered,
which ultimately leads to a 504 Gateway Timeout error.

This is fixed by invoking the 'callback' in `_write()`.

fix #33441



## Bug

- [x] Related issues linked using `fixes #number`
- [x] Integration tests added
- [ ] Errors have helpful link attached, see `contributing.md`

## Feature

- [ ] Implements an existing feature request or RFC. Make sure the feature request has been accepted for implementation before opening a PR.
- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Documentation added
- [ ] Telemetry added. In case of a feature if it's used or not.
- [ ] Errors have helpful link attached, see `contributing.md`

## Documentation / Examples

- [ ] Make sure the linting passes by running `yarn lint`

packages/next/server/image-optimizer.ts

@@ -253,8 +253,17 @@ export async function imageOptimizer(
         mockRes.write = (chunk: Buffer | string) => {
           resBuffers.push(Buffer.isBuffer(chunk) ? chunk : Buffer.from(chunk))
         }
-        mockRes._write = (chunk: Buffer | string) => {
+        mockRes._write = (
+          chunk: Buffer | string,
+          _encoding: string,
+          callback: () => void
+        ) => {
           mockRes.write(chunk)
+          // According to Node.js documentation, the callback MUST be invoked to signal that
+          // the write completed successfully. If this callback is not invoked, the 'finish' event
+          // will not be emitted.
+          // https://nodejs.org/docs/latest-v16.x/api/stream.html#writable_writechunk-encoding-callback
+          callback()
         }
 
         const mockHeaders: Record<string, string | string[]> = {}
@@ -290,7 +299,6 @@ export async function imageOptimizer(
         await handleRequest(mockReq, mockRes, nodeUrl.parse(href, true))
         await isStreamFinished
         res.statusCode = mockRes.statusCode
-
         upstreamBuffer = Buffer.concat(resBuffers)
         upstreamType =
           detectContentType(upstreamBuffer) || mockRes.getHeader('Content-Type')
@@ -328,7 +336,6 @@ export async function imageOptimizer(
         )
         return { finished: true }
       }
-
       if (!upstreamType.startsWith('image/')) {
         res.statusCode = 400
         res.end("The requested resource isn't a valid image.")

test/integration/image-optimizer/test/index.test.js

@@ -779,6 +779,14 @@ function runTests({
     expect(await res.text()).toBe("The requested resource isn't a valid image.")
   })
 
+  it('should error if the image file does not exist', async () => {
+    const query = { url: '/does_not_exist.jpg', w, q: 80 }
+    const opts = { headers: { accept: 'image/webp' } }
+    const res = await fetchViaHTTP(appPort, '/_next/image', query, opts)
+    expect(res.status).toBe(400)
+    expect(await res.text()).toBe("The requested resource isn't a valid image.")
+  })
+
   it('should handle concurrent requests', async () => {
     await fs.remove(imagesDir)
     const query = { url: '/test.png', w, q: 80 }