Warn when response body is larger than 5mb (vercel#26831)

This PR adds a warning when api responses exceed 5mb since this will end up failing once deployed. In a future version this scenario will throw an error.

## Bug

- [x] Integration tests added

## Documentation / Examples

- [x] Make sure the linting passes

errors/api-routes-body-size-limit.md

@@ -0,0 +1,13 @@
+# API Routes Body Size Limited to 5mb
+
+#### Why This Error Occurred
+
+API Routes are meant to respond quickly and are not intended to support responding with large amounts of data. The maximum size of responses is 5 MB.
+
+#### Possible Ways to Fix It
+
+Limit your API Route responses to less than 5 MB. If you need to support sending large files to the client, you should consider using a dedicated media host for those assets. See link below for suggestions.
+
+### Useful Links
+
+[Tips to avoid the 5 MB limit](https://vercel.com/support/articles/how-to-bypass-vercel-5mb-body-size-limit-serverless-functions)

errors/manifest.json

@@ -13,6 +13,10 @@
           "title": "amp-export-validation",
           "path": "/errors/amp-export-validation.md"
         },
+        {
+          "title": "api-routes-body-size-limit",
+          "path": "/errors/api-routes-body-size-limit.md"
+        },
         {
           "title": "api-routes-static-export",
           "path": "/errors/api-routes-static-export.md"

packages/next/server/api-utils.ts

@@ -63,6 +63,26 @@ export async function apiResolver(
       )
     }
 
+    let contentLength = 0
+    const writeData = apiRes.write
+    const endResponse = apiRes.end
+    apiRes.write = (...args: any[2]) => {
+      contentLength += Buffer.byteLength(args[0])
+      return writeData.apply(apiRes, args)
+    }
+    apiRes.end = (...args: any[2]) => {
+      if (args.length && typeof args[0] !== 'function') {
+        contentLength += Buffer.byteLength(args[0])
+      }
+
+      if (contentLength >= 5 * 1024 * 1024) {
+        console.warn(
+          `API response for ${req.url} exceeds 5MB. This will cause the request to fail in a future version. https://nextjs.org/docs/messages/api-routes-body-size-limit`
+        )
+      }
+
+      endResponse.apply(apiRes, args)
+    }
     apiRes.status = (statusCode) => sendStatusCode(apiRes, statusCode)
     apiRes.send = (data) => sendData(apiReq, apiRes, data)
     apiRes.json = (data) => sendJson(apiRes, data)

test/integration/api-support/pages/api/large-chunked-response.js

@@ -0,0 +1,6 @@
+export default (req, res) => {
+  for (let i = 0; i <= 5 * 1024 * 1024; i++) {
+    res.write('.')
+  }
+  res.end()
+}

test/integration/api-support/pages/api/large-response.js

@@ -0,0 +1,4 @@
+export default (req, res) => {
+  let body = '.'.repeat(5 * 1024 * 1024)
+  res.send(body)
+}

test/integration/api-support/test/index.test.js

@@ -398,6 +398,20 @@ function runTests(dev = false) {
     expect(data).toBe('hi')
   })
 
+  it('should warn if response body is larger than 5MB', async () => {
+    let res = await fetchViaHTTP(appPort, '/api/large-response')
+    expect(res.ok).toBeTruthy()
+    expect(stderr).toContain(
+      'API response for /api/large-response exceeds 5MB. This will cause the request to fail in a future version.'
+    )
+
+    res = await fetchViaHTTP(appPort, '/api/large-chunked-response')
+    expect(res.ok).toBeTruthy()
+    expect(stderr).toContain(
+      'API response for /api/large-chunked-response exceeds 5MB. This will cause the request to fail in a future version.'
+    )
+  })
+
   if (dev) {
     it('should compile only server code in development', async () => {
       await fetchViaHTTP(appPort, '/api/users')