add validation for the app directory prompt

src/init/features/apphosting.ts

@@ -31,7 +31,7 @@
  */
 export async function doSetup(setup: Setup, config: Config, options: Options): Promise<void> {
   // Use dynamicImport to bypass Node ESM linkage cycles during mocha test loading.
   const { dynamicImport } = eval("require")("../../dynamicImport");
 
   const projectId = setup.projectId as string;
 
@@ -91,8 +91,8 @@
       utils.logWarning(`Firebase web app not set`);
     }
 
     const prompts = await dynamicImport("./apphosting/prompts");
     const runtime = await prompts.resolveRuntime(projectId, location, options.nonInteractive);
 
     const createBackendSpinner = ora("Creating your new backend...").start();
     const backend = await createBackend(
@@ -103,7 +103,7 @@
       /* repository= */ undefined,
       webApp?.id,
       /* rootDir= */ "/",
       runtime,
     );
 
     createBackendSpinner.succeed(`Successfully created backend!\n\t${backend.name}\n`);
@@ -113,13 +113,20 @@
   backendConfig.rootDir = await input({
     default: "/",
     message: "Specify your app's root directory relative to your firebase.json directory",
+    validate: (input: string) => {
+      const absPath = path.join(config.projectDir, input);
+      if (!existsSync(absPath)) {
+        return `Directory ${absPath} does not exist. Please enter a valid directory.`;
+      }
+      return true;
+    },
   });
 
   upsertAppHostingConfig(backendConfig, config);
   config.writeProjectFile("firebase.json", config.src);
 
   utils.logBullet("Writing default settings to " + clc.bold("apphosting.yaml") + "...");
-  const absRootDir = path.join(process.cwd(), backendConfig.rootDir);
+  const absRootDir = path.join(config.projectDir, backendConfig.rootDir);
   if (!existsSync(absRootDir)) {
     throw new FirebaseError(
       `Failed to write apphosting.yaml file because app root directory ${absRootDir} does not exist. Please try again with a valid directory.`,