add validate next config override function

Author: mathu97

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

@@ -8,7 +8,11 @@ import {
 } from "../utils.js";
 import { join } from "path";
 import { getBuildOptions, runBuild } from "@apphosting/common";
-import { addRouteOverrides, overrideNextConfig } from "../overrides.js";
+import {
+  addRouteOverrides,
+  overrideNextConfig,
+  validateNextConfigOverrides,
+} from "../overrides.js";
 
 const root = process.cwd();
 const opts = getBuildOptions();
@@ -21,19 +25,26 @@ if (!process.env.FRAMEWORK_VERSION) {
   throw new Error("Could not find the nextjs version of the application");
 }
 
-const { distDir, configFileName } = await loadConfig(root, opts.projectDirectory);
-await overrideNextConfig(root, configFileName);
+const originalConfig = await loadConfig(root, opts.projectDirectory);
+
+await overrideNextConfig(root, originalConfig.configFileName);
+await validateNextConfigOverrides(root, opts.projectDirectory);
+
 await runBuild();
 
 const adapterMetadata = getAdapterMetadata();
-const nextBuildDirectory = join(opts.projectDirectory, distDir);
+const nextBuildDirectory = join(opts.projectDirectory, originalConfig.distDir);
 const outputBundleOptions = populateOutputBundleOptions(
   root,
   opts.projectDirectory,
   nextBuildDirectory,
 );
 
-await addRouteOverrides(outputBundleOptions.outputDirectoryAppPath, distDir, adapterMetadata);
+await addRouteOverrides(
+  outputBundleOptions.outputDirectoryAppPath,
+  originalConfig.distDir,
+  adapterMetadata,
+);
 
 await generateBuildOutput(
   root,

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

@@ -314,6 +314,16 @@ describe("next config overrides", () => {
       `),
     );
   });
+
+  it("should not do anything if no next.config.* file exists", async () => {
+    const { overrideNextConfig } = await importOverrides;
+    await overrideNextConfig(tmpDir, "next.config.js");
+
+    // Assert that no next.config* files were created
+    const files = fs.readdirSync(tmpDir);
+    const nextConfigFiles = files.filter((file) => file.startsWith("next.config"));
+    assert.strictEqual(nextConfigFiles.length, 0, "No next.config files should exist");
+  });
 });
 
 // Normalize whitespace for comparison

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

@@ -5,9 +5,11 @@ import {
   loadMiddlewareManifest,
   exists,
   writeFile,
+  loadConfig,
 } from "./utils.js";
 import { join, extname } from "path";
 import { rename as renamePromise } from "fs/promises";
+import { NextConfigComplete } from "next/dist/server/config-shared.js";
 
 /**
  * Overrides the user's Next Config file (next.config.[ts|js|mjs]) to add configs
@@ -98,10 +100,26 @@ function getCustomNextConfig(importStatement: string, fileExtension: string) {
       }
     : fahOptimizedConfig(originalConfig);
 
+  const firebaseAppHostingSymbol = Symbol("__createdByFirebaseAppHosting__");
+  config[firebaseAppHostingSymbol] = true;
+
   ${fileExtension === ".mjs" ? "export default config;" : "module.exports = config;"}
   `;
 }
 
+export async function validateNextConfigOverrides(root: string, projectRoot: string) {
+  try {
+    const overriddenConfig = await loadConfig(root, projectRoot);
+    if (!overriddenConfig.__createdByFirebaseAppHosting__) {
+      throw new Error("Firebase App Hosting overrides are missing");
+    }
+  } catch (error) {
+    throw new Error(
+      `Invalid Next.js config: ${error instanceof Error ? error.message : "Unknown error"}`,
+    );
+  }
+}
+
 /**
  * Modifies the app's route manifest (routes-manifest.json) to add Firebase App Hosting
  * specific overrides (i.e headers).