presets(vercel): hard link custom function dirs instead of copying

Author: RihanArfan

src/presets/_utils/fs.ts

@@ -1,5 +1,5 @@
 import fsp from "node:fs/promises";
-import { relative, dirname } from "pathe";
+import { relative, dirname, join } from "pathe";
 import consola from "consola";
 import { colors } from "consola/utils";
 
@@ -23,3 +23,45 @@ export async function isDirectory(path: string) {
     return false;
   }
 }
+
+/**
+ * Recursively recreate `src` at `dest` using hard links for files.
+ *
+ * Hard links share inodes with the source, so deployment targets that
+ * read files into per-function bundles (e.g. Vercel Lambda packaging)
+ * see them as regular files while local disk usage stays flat.
+ *
+ * Directories cannot be hard-linked, so the tree is recreated and each
+ * file is linked individually. Symlinks are preserved as symlinks.
+ * Falls back to `copyFile` when crossing filesystems (`EXDEV`) or when
+ * the platform refuses the link operation (`EPERM`).
+ *
+ * Entries listed in `skip` are ignored at the top level only.
+ */
+export async function hardLinkDir(src: string, dest: string, options: { skip?: Set<string> } = {}) {
+  await fsp.mkdir(dest, { recursive: true });
+  const entries = await fsp.readdir(src, { withFileTypes: true });
+  for (const entry of entries) {
+    if (options.skip?.has(entry.name)) {
+      continue;
+    }
+    const srcPath = join(src, entry.name);
+    const destPath = join(dest, entry.name);
+    if (entry.isDirectory()) {
+      await hardLinkDir(srcPath, destPath);
+    } else if (entry.isSymbolicLink()) {
+      const target = await fsp.readlink(srcPath);
+      await fsp.symlink(target, destPath);
+    } else {
+      try {
+        await fsp.link(srcPath, destPath);
+      } catch (err: any) {
+        if (err?.code === "EXDEV" || err?.code === "EPERM") {
+          await fsp.copyFile(srcPath, destPath);
+        } else {
+          throw err;
+        }
+      }
+    }
+  }
+}

src/presets/vercel/utils.ts

@@ -1,6 +1,6 @@
 import fsp from "node:fs/promises";
 import { defu } from "defu";
-import { writeFile } from "../_utils/fs.ts";
+import { hardLinkDir, writeFile } from "../_utils/fs.ts";
 import type { Nitro, NitroRouteRules } from "nitro/types";
 import { dirname, relative, resolve } from "pathe";
 import { Router } from "../../routing.ts";
@@ -636,11 +636,9 @@ async function createFunctionDirWithCustomConfig(
   overrides: VercelServerlessFunctionConfig,
   functionPath: string
 ) {
-  // Copy the entire server directory instead of symlinking individual
-  // entries. Vercel's build container preserves symlinks in the Lambda
-  // zip, but symlinks pointing outside the .func directory break at
-  // runtime because the target path doesn't exist on Lambda.
-  await fsp.cp(serverDir, funcDir, { recursive: true });
+  await hardLinkDir(serverDir, funcDir, {
+    skip: new Set([".vc-config.json"]),
+  });
   const mergedConfig = defu(overrides, baseFunctionConfig);
   for (const [key, value] of Object.entries(overrides)) {
     if (Array.isArray(value)) {