We don't need no public dir (#5142)

* We don't need no public dir

* Add docs

* Add changelog

CHANGELOG.md

@@ -0,0 +1 @@
+Fix a bug where next.js applications would fail to deploy if they did not have a public dir (#5142)

src/frameworks/next/index.ts

@@ -51,13 +51,19 @@ function getNextVersion(cwd: string) {
   return findDependency("next", { cwd, depth: 0, omitDev: false })?.version;
 }
 
+/**
+ * Returns whether this codebase is a Next.js backend.
+ */
 export async function discover(dir: string) {
   if (!(await pathExists(join(dir, "package.json")))) return;
   if (!(await pathExists("next.config.js")) && !getNextVersion(dir)) return;
   // TODO don't hardcode public dir
   return { mayWantBackend: true, publicDirectory: join(dir, "public") };
 }
 
+/**
+ * Build a next.js application.
+ */
 export async function build(dir: string): Promise<BuildResult> {
   const { default: nextBuild } = relativeRequire(dir, "next/dist/build");
 
@@ -133,6 +139,9 @@ export async function build(dir: string): Promise<BuildResult> {
   return { wantsBackend, headers, redirects, rewrites };
 }
 
+/**
+ * Utility method used during project initialization.
+ */
 export async function init(setup: any) {
   const language = await promptOnce({
     type: "list",
@@ -148,6 +157,9 @@ export async function init(setup: any) {
   );
 }
 
+/**
+ * Create a directory for SSG content.
+ */
 export async function ɵcodegenPublicDirectory(sourceDir: string, destDir: string) {
   const { distDir } = await getConfig(sourceDir);
   const exportDetailPath = join(sourceDir, distDir, "export-detail.json");
@@ -157,8 +169,11 @@ export async function ɵcodegenPublicDirectory(sourceDir: string, destDir: strin
   if (exportDetailJson?.success) {
     copy(exportDetailJson.outDirectory, destDir);
   } else {
+    const publicPath = join(sourceDir, "public");
     await mkdir(join(destDir, "_next", "static"), { recursive: true });
-    await copy(join(sourceDir, "public"), destDir);
+    if (await pathExists(publicPath)) {
+      await copy(publicPath, destDir);
+    }
     await copy(join(sourceDir, distDir, "static"), join(destDir, "_next", "static"));
 
     const serverPagesDir = join(sourceDir, distDir, "server", "pages");
@@ -202,6 +217,9 @@ export async function ɵcodegenPublicDirectory(sourceDir: string, destDir: strin
   }
 }
 
+/**
+ * Create a directory for SSR content.
+ */
 export async function ɵcodegenFunctionsDirectory(sourceDir: string, destDir: string) {
   const { distDir } = await getConfig(sourceDir);
   const packageJsonBuffer = await readFile(join(sourceDir, "package.json"));
@@ -227,13 +245,18 @@ export async function ɵcodegenFunctionsDirectory(sourceDir: string, destDir: st
       platform: "node",
     });
   }
-  await mkdir(join(destDir, "public"));
+  if (await pathExists(join(sourceDir, "public"))) {
+    await mkdir(join(destDir, "public"));
+    await copy(join(sourceDir, "public"), join(destDir, "public"));
+  }
   await mkdirp(join(destDir, distDir));
-  await copy(join(sourceDir, "public"), join(destDir, "public"));
   await copy(join(sourceDir, distDir), join(destDir, distDir));
   return { packageJson, frameworksEntry: "next.js" };
 }
 
+/**
+ * Create a dev server.
+ */
 export async function getDevModeHandle(dir: string) {
   const { default: next } = relativeRequire(dir, "next");
   const nextApp = next({