Turbopack: Create junction points instead of symlinks on Windows

packages/next/src/build/utils.ts

@@ -1274,9 +1274,29 @@ export async function copyTracedFiles(
           if (symlink) {
             try {
               await fs.symlink(symlink, fileOutputPath)
-            } catch (e: any) {
-              if (e.code !== 'EEXIST') {
-                throw e
+            } catch (err: any) {
+              // Windows doesn't support creating symlinks without elevated privileges, unless
+              // "Developer Mode" is turned on. If we failed to crate a symlink due to EPERM, try
+              // creating a junction point instead.
+              //
+              // Ideally we'd just preserve the input file type (junction point or symlink), but
+              // there's no API in node.js to differentiate between a junction point and a symlink,
+              // so we just try making a symlink first. Symlinks are preferred because they support
+              // relative paths and non-directory (file) targets.
+              if (
+                process.platform === 'win32' &&
+                err.code === 'EPERM' &&
+                path.isAbsolute(symlink)
+              ) {
+                try {
+                  await fs.symlink(symlink, fileOutputPath, 'junction')
+                } catch (junctionErr: any) {
+                  if (junctionErr.code !== 'EEXIST') {
+                    throw junctionErr
+                  }
+                }
+              } else if (err.code !== 'EEXIST') {
+                throw err
               }
             }
           } else {

turbopack/crates/turbo-tasks-fs/src/attach.rs

@@ -125,12 +125,15 @@ impl FileSystem for AttachedFileSystem {
     }
 
     #[turbo_tasks::function(fs)]
-    async fn write_link(
+    async fn write_symbolic_link_dir(
         self: Vc<Self>,
         path: FileSystemPath,
         target: Vc<LinkContent>,
     ) -> Result<Vc<()>> {
-        Ok(self.get_inner_fs_path(path).await?.write_link(target))
+        Ok(self
+            .get_inner_fs_path(path)
+            .await?
+            .write_symbolic_link_dir(target))
     }
 
     #[turbo_tasks::function]

turbopack/crates/turbo-tasks-fs/src/embed/fs.rs

@@ -75,7 +75,11 @@ impl FileSystem for EmbeddedFileSystem {
     }
 
     #[turbo_tasks::function]
-    fn write_link(&self, _path: FileSystemPath, _target: Vc<LinkContent>) -> Result<Vc<()>> {
+    fn write_symbolic_link_dir(
+        &self,
+        _path: FileSystemPath,
+        _target: Vc<LinkContent>,
+    ) -> Result<Vc<()>> {
         bail!("Writing is not possible to the embedded filesystem")
     }