Restore Next config after build.

packages/@apphosting/adapter-nextjs/e2e/config-override.spec.ts

@@ -27,9 +27,40 @@ const compiledFilesPath = posix.join(
   ".next",
 );
 
+const standalonePath = posix.join(process.cwd(), "e2e", "runs", runId, ".next", "standalone");
+
+const appPath = posix.join(process.cwd(), "e2e", "runs", runId);
+
 const requiredServerFilePath = posix.join(compiledFilesPath, "required-server-files.json");
 
 describe("next.config override", () => {
+  it("Should not overwrite original next config", async function () {
+    if (
+      scenario.includes("with-empty-config") ||
+      scenario.includes("with-images-unoptimized-false") ||
+      scenario.includes("with-custom-image-loader")
+    ) {
+      // eslint-disable-next-line @typescript-eslint/no-invalid-this
+      this.skip();
+    }
+    const files = await fsExtra.readdir(appPath);
+    const configRegex = /^next\.config\..*$/g;
+    const configOriginalRegex = /^next\.config\.(?!original).*$/g;
+    const configFiles = files.filter((file) => file.match(configRegex));
+    assert.strictEqual(configFiles.length, 1);
+    assert.ok(
+      configFiles[0].match(configOriginalRegex),
+      "temporary original config not properly removed",
+    );
+
+    const standaloneFiles = await fsExtra.readdir(standalonePath);
+    const standaloneConfigFiles = standaloneFiles.filter((file) => file.match(configRegex));
+    assert.strictEqual(standaloneConfigFiles.length, 2);
+    assert.ok(
+      standaloneConfigFiles.some((file) => file.match(configOriginalRegex)),
+      "no original config found in standalone",
+    );
+  });
   it("should have images optimization disabled", async function () {
     if (
       scenario.includes("with-empty-config") ||

packages/@apphosting/adapter-nextjs/src/bin/build.ts

@@ -9,7 +9,12 @@ import {
 } from "../utils.js";
 import { join } from "path";
 import { getBuildOptions, runBuild } from "@apphosting/common";
-import { addRouteOverrides, overrideNextConfig, validateNextConfigOverride } from "../overrides.js";
+import {
+  addRouteOverrides,
+  overrideNextConfig,
+  restoreNextConfig,
+  validateNextConfigOverride,
+} from "../overrides.js";
 
 const root = process.cwd();
 const opts = getBuildOptions();
@@ -19,48 +24,55 @@ process.env.NEXT_PRIVATE_STANDALONE = "true";
 // Opt-out sending telemetry to Vercel
 process.env.NEXT_TELEMETRY_DISABLED = "1";
 
-const originalConfig = await loadConfig(root, opts.projectDirectory);
+const nextConfig = await loadConfig(root, opts.projectDirectory);
 
 /**
  * Override user's Next Config to optimize the app for Firebase App Hosting
  * and validate that the override resulted in a valid config that Next.js can
  * load.
  *
+ * We restore the user's Next Config at the end of the build, after the config file has been
+ * copied over to the output directory, so that the user's original code is not modified.
+ *
  * If the app does not have a next.config.[js|mjs|ts] file in the first place,
  * then can skip config override.
  *
  * Note: loadConfig always returns a fileName (default: next.config.js) even if
  * one does not exist in the app's root: https://github.com/vercel/next.js/blob/23681508ca34b66a6ef55965c5eac57de20eb67f/packages/next/src/server/config.ts#L1115
  */
-const originalConfigPath = join(root, originalConfig.configFileName);
-if (await exists(originalConfigPath)) {
-  await overrideNextConfig(root, originalConfig.configFileName);
-  await validateNextConfigOverride(root, opts.projectDirectory, originalConfig.configFileName);
+const nextConfigPath = join(root, nextConfig.configFileName);
+if (await exists(nextConfigPath)) {
+  await overrideNextConfig(root, nextConfig.configFileName);
+  await validateNextConfigOverride(root, opts.projectDirectory, nextConfig.configFileName);
 }
 
-await runBuild();
+try {
+  await runBuild();
 
-const adapterMetadata = getAdapterMetadata();
-const nextBuildDirectory = join(opts.projectDirectory, originalConfig.distDir);
-const outputBundleOptions = populateOutputBundleOptions(
-  root,
-  opts.projectDirectory,
-  nextBuildDirectory,
-);
+  const adapterMetadata = getAdapterMetadata();
+  const nextBuildDirectory = join(opts.projectDirectory, nextConfig.distDir);
+  const outputBundleOptions = populateOutputBundleOptions(
+    root,
+    opts.projectDirectory,
+    nextBuildDirectory,
+  );
 
-await addRouteOverrides(
-  outputBundleOptions.outputDirectoryAppPath,
-  originalConfig.distDir,
-  adapterMetadata,
-);
+  await addRouteOverrides(
+    outputBundleOptions.outputDirectoryAppPath,
+    nextConfig.distDir,
+    adapterMetadata,
+  );
 
-const nextjsVersion = process.env.FRAMEWORK_VERSION || "unspecified";
-await generateBuildOutput(
-  root,
-  opts.projectDirectory,
-  outputBundleOptions,
-  nextBuildDirectory,
-  nextjsVersion,
-  adapterMetadata,
-);
-await validateOutputDirectory(outputBundleOptions, nextBuildDirectory);
+  const nextjsVersion = process.env.FRAMEWORK_VERSION || "unspecified";
+  await generateBuildOutput(
+    root,
+    opts.projectDirectory,
+    outputBundleOptions,
+    nextBuildDirectory,
+    nextjsVersion,
+    adapterMetadata,
+  );
+  await validateOutputDirectory(outputBundleOptions, nextBuildDirectory);
+} finally {
+  await restoreNextConfig(root, nextConfig.configFileName);
+}

packages/@apphosting/adapter-nextjs/src/overrides.spec.ts

@@ -378,6 +378,67 @@ describe("validateNextConfigOverride", () => {
   });
 });
 
+describe("next config restore", () => {
+  let tmpDir: string;
+  const nextConfigOriginalBody = `
+    // @ts-check
+
+    /** @type {import('next').NextConfig} */
+    const nextConfig = {
+      /* config options here */
+    }
+
+    module.exports = nextConfig
+    `;
+  const nextConfigBody = `
+  // This file was automatically generated by Firebase App Hosting adapter
+  const fahOptimizedConfig = (config) => ({
+    ...config,
+    images: {
+      ...(config.images || {}),
+      ...(config.images?.unoptimized === undefined && config.images?.loader === undefined
+          ? { unoptimized: true }
+          : {}),
+    },
+  });
+
+  const config = typeof originalConfig === 'function'
+    ? async (...args) => {
+        const resolvedConfig = await originalConfig(...args);
+        return fahOptimizedConfig(resolvedConfig);
+      }
+    : fahOptimizedConfig(originalConfig);
+  `;
+
+  beforeEach(() => {
+    tmpDir = fs.mkdtempSync(path.join(os.tmpdir(), "test-overrides"));
+  });
+
+  it("handle no original config file found", async () => {
+    const { restoreNextConfig } = await importOverrides;
+    fs.writeFileSync(path.join(tmpDir, "next.config.mjs"), nextConfigBody);
+    await restoreNextConfig(tmpDir, "next.config.mjs");
+
+    const restoredConfig = fs.readFileSync(path.join(tmpDir, "next.config.mjs"), "utf-8");
+    assert.equal(restoredConfig, nextConfigBody);
+  });
+
+  it("handle no config file found", async () => {
+    const { restoreNextConfig } = await importOverrides;
+    assert.doesNotReject(restoreNextConfig(tmpDir, "next.config.mjs"));
+  });
+
+  it("original config file restored", async () => {
+    const { restoreNextConfig } = await importOverrides;
+    fs.writeFileSync(path.join(tmpDir, "next.config.mjs"), nextConfigBody);
+    fs.writeFileSync(path.join(tmpDir, "next.config.original.mjs"), nextConfigOriginalBody);
+    await restoreNextConfig(tmpDir, "next.config.mjs");
+
+    const restoredConfig = fs.readFileSync(path.join(tmpDir, "next.config.mjs"), "utf-8");
+    assert.equal(restoredConfig, nextConfigOriginalBody);
+  });
+});
+
 // Normalize whitespace for comparison
 function normalizeWhitespace(str: string) {
   return str.replace(/\s+/g, " ").trim();

packages/@apphosting/adapter-nextjs/src/overrides.ts

@@ -142,6 +142,29 @@ export async function validateNextConfigOverride(
   }
 }
 
+/**
+ * Restores the user's original Next Config file (next.config.original.[ts|js|mjs])
+ * to leave user code the way we found it.
+ */
+export async function restoreNextConfig(projectRoot: string, nextConfigFileName: string) {
+  // Determine the file extension
+  const fileExtension = extname(nextConfigFileName);
+  const originalConfigPath = join(projectRoot, `next.config.original${fileExtension}`);
+
+  if (!(await exists(originalConfigPath))) {
+    // No backup file found, nothing to restore.
+    return;
+  }
+  console.log(`Restoring original next config in project root`);
+
+  const configPath = join(projectRoot, nextConfigFileName);
+  try {
+    await renamePromise(originalConfigPath, configPath);
+  } catch (error) {
+    console.error(`Error restoring Next config: ${error}`);
+  }
+}
+
 /**
  * Modifies the app's route manifest (routes-manifest.json) to add Firebase App Hosting
  * specific overrides (i.e headers).