middlewares: limit process.env to inferred usage (vercel#33186)

Production middlewares will only expose env vars that are statically analyzable, as mentioned here: https://nextjs.org/docs/api-reference/next/server#how-do-i-access-environment-variables

This creates some incompatibility with `next dev` and `next start`, where all `process.env` data is shared and can lead to unexpected behavior in runtime.

This PR fixes it by limiting the data in `process.env` with the inferred env vars from the code usage. I believe the test speaks for itself 🕺 

<!--
## Bug

- [ ] Related issues linked using `fixes #number`
- [ ] 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/build/webpack/plugins/middleware-plugin.ts

@@ -7,8 +7,8 @@ import {
   MIDDLEWARE_BUILD_MANIFEST,
   MIDDLEWARE_REACT_LOADABLE_MANIFEST,
   MIDDLEWARE_RUNTIME_WEBPACK,
+  MIDDLEWARE_SSR_RUNTIME_WEBPACK,
 } from '../../../shared/lib/constants'
-import { MIDDLEWARE_ROUTE } from '../../../lib/constants'
 import { nonNullable } from '../../../lib/non-nullable'
 
 const PLUGIN_NAME = 'MiddlewarePlugin'
@@ -141,7 +141,10 @@ export default class MiddlewarePlugin {
           envPerRoute.clear()
 
           for (const [name, info] of compilation.entries) {
-            if (name.match(MIDDLEWARE_ROUTE)) {
+            if (
+              info.options.runtime === MIDDLEWARE_SSR_RUNTIME_WEBPACK ||
+              info.options.runtime === MIDDLEWARE_RUNTIME_WEBPACK
+            ) {
               const middlewareEntries = new Set<webpack5.Module>()
               const env = new Set<string>()
 

packages/next/server/base-server.ts

@@ -725,6 +725,7 @@ export default abstract class Server {
         result = await run({
           name: middlewareInfo.name,
           paths: middlewareInfo.paths,
+          env: middlewareInfo.env,
           request: {
             headers: params.request.headers,
             method: params.request.method || 'GET',

packages/next/server/require.ts

@@ -85,7 +85,7 @@ export function getMiddlewareInfo(params: {
   distDir: string
   page: string
   serverless: boolean
-}): { name: string; paths: string[] } {
+}): { name: string; paths: string[]; env: string[] } {
   const serverBuildPath = join(
     params.distDir,
     params.serverless && !params.dev ? SERVERLESS_DIRECTORY : SERVER_DIRECTORY
@@ -112,5 +112,6 @@ export function getMiddlewareInfo(params: {
   return {
     name: pageInfo.name,
     paths: pageInfo.files.map((file) => join(params.distDir, file)),
+    env: pageInfo.env ?? [],
   }
 }

packages/next/server/web/sandbox/context.ts

@@ -56,6 +56,7 @@ export function getModuleContext(options: {
   module: string
   onWarning: (warn: Error) => void
   useCache: boolean
+  env: string[]
 }) {
   let moduleCache = options.useCache
     ? caches.get(options.module)
@@ -97,12 +98,13 @@ export function getModuleContext(options: {
 function createModuleContext(options: {
   onWarning: (warn: Error) => void
   module: string
+  env: string[]
 }) {
   const requireCache = new Map([
     [require.resolve('next/dist/compiled/cookie'), { exports: cookie }],
   ])
 
-  const context = createContext()
+  const context = createContext(options)
 
   requireDependencies({
     requireCache: requireCache,
@@ -171,7 +173,10 @@ function createModuleContext(options: {
  * Create a base context with all required globals for the runtime that
  * won't depend on any externally provided dependency.
  */
-function createContext() {
+function createContext(options: {
+  /** Environment variables to be provided to the context */
+  env: string[]
+}) {
   const context: { [key: string]: unknown } = {
     _ENTRIES: {},
     atob: polyfills.atob,
@@ -196,7 +201,9 @@ function createContext() {
     crypto: new polyfills.Crypto(),
     File,
     FormData,
-    process: { env: { ...process.env } },
+    process: {
+      env: buildEnvironmentVariablesFrom(options.env),
+    },
     ReadableStream: polyfills.ReadableStream,
     setInterval,
     setTimeout,
@@ -245,3 +252,10 @@ function createContext() {
         : undefined,
   })
 }
+
+function buildEnvironmentVariablesFrom(
+  keys: string[]
+): Record<string, string | undefined> {
+  const pairs = keys.map((key) => [key, process.env[key]])
+  return Object.fromEntries(pairs)
+}

packages/next/server/web/sandbox/sandbox.ts

@@ -3,6 +3,7 @@ import { getModuleContext } from './context'
 
 export async function run(params: {
   name: string
+  env: string[]
   onWarning: (warn: Error) => void
   paths: string[]
   request: RequestData
@@ -12,6 +13,7 @@ export async function run(params: {
     module: params.name,
     onWarning: params.onWarning,
     useCache: params.useCache !== false,
+    env: params.env,
   })
 
   for (const paramPath of params.paths) {

test/production/middleware-environment-variables-in-node-server-reflect-the-usage-inference/index.test.ts

@@ -0,0 +1,50 @@
+import { createNext } from 'e2e-utils'
+import { NextInstance } from 'test/lib/next-modes/base'
+import { renderViaHTTP } from 'next-test-utils'
+
+describe('middleware environment variables in node server reflect the usage inference', () => {
+  let next: NextInstance
+
+  beforeAll(() => {
+    process.env.CAN_BE_INFERRED = 'can-be-inferred'
+    process.env.X_CUSTOM_HEADER = 'x-custom-header'
+    process.env.IGNORED_ENV_VAR = 'ignored-env-var'
+  })
+
+  beforeAll(async () => {
+    next = await createNext({
+      files: {
+        'pages/_middleware.js': `
+          export default function middleware() {
+            return new Response(JSON.stringify({
+              canBeInferred: process.env.CAN_BE_INFERRED,
+              rest: process.env
+            }), {
+              headers: {
+                'Content-Type': 'application/json',
+                'X-Custom-Header': process.env.X_CUSTOM_HEADER,
+              }
+            })
+          }
+        `,
+      },
+      dependencies: {},
+    })
+  })
+  afterAll(() => next.destroy())
+
+  it('limits process.env to only contain env vars that are inferred from usage', async () => {
+    const html = await renderViaHTTP(next.url, '/test')
+    let parsed: any
+    expect(() => {
+      parsed = JSON.parse(html)
+    }).not.toThrow()
+    expect(parsed).toEqual({
+      canBeInferred: 'can-be-inferred',
+      rest: {
+        CAN_BE_INFERRED: 'can-be-inferred',
+        X_CUSTOM_HEADER: 'x-custom-header',
+      },
+    })
+  })
+})