fix(kernel): fast module loading fails on Windows (EPERM)

In #4181, a faster method to load
modules was introduced: symlinking instead of recursing through the
directory tree, mostly affecting the load times of large modules.

Since Windows Vista, non-Administrator users on Windows aren't allowed
to create symlinks anymore, so this new loading method fails for users
working in corporate Windows environments.

Catch the error and fall back to the slower copying method if that
happens.

Fixes #4208.

packages/@jsii/kernel/src/link.ts

@@ -6,6 +6,7 @@ import {
   statSync,
   symlinkSync,
 } from 'fs';
+import * as os from 'os';
 import { dirname, join } from 'path';
 
 /**
@@ -16,31 +17,50 @@ import { dirname, join } from 'path';
 const PRESERVE_SYMLINKS = process.execArgv.includes('--preserve-symlinks');
 
 /**
- * Creates directories containing hard links if possible, and falls back on
- * copy otherwise.
+ * Link existing to destination directory
  *
- * @param existing    is the original file or directory to link.
- * @param destination is the new file or directory to create.
+ * - If Node has been started with a module resolution strategy that does not
+ *   resolve symlinks (so peerDependencies can be found), use symlinking.
+ *   Symlinking may fail on Windows for non-Admin users.
+ * - If not symlinking the entire directory, crawl the directory tree and
+ *   hardlink all files (if possible), copying them if not.
+ *
+ * @param existingRoot    is the original file or directory to link.
+ * @param destinationRoot is the new file or directory to create.
  */
-export function link(existing: string, destination: string): void {
-  if (PRESERVE_SYMLINKS) {
-    mkdirSync(dirname(destination), { recursive: true });
-    symlinkSync(existing, destination);
-    return;
-  }
+export function link(existingRoot: string, destinationRoot: string): void {
+  mkdirSync(dirname(destinationRoot), { recursive: true });
 
-  const stat = statSync(existing);
-  if (!stat.isDirectory()) {
+  if (PRESERVE_SYMLINKS) {
     try {
-      linkSync(existing, destination);
-    } catch {
-      copyFileSync(existing, destination);
+      symlinkSync(existingRoot, destinationRoot);
+      return;
+    } catch (e: any) {
+      // On Windows, non-Admin users aren't allowed to create symlinks. In that case, fall back to the copying workflow.
+      const winNoSymlink = e.code === 'EPERM' && os.platform() === 'win32';
+
+      if (!winNoSymlink) {
+        throw e;
+      }
     }
-    return;
   }
+  // Fall back to the slow method
+  recurse(existingRoot, destinationRoot);
+
+  function recurse(existing: string, destination: string): void {
+    const stat = statSync(existing);
+    if (!stat.isDirectory()) {
+      try {
+        linkSync(existing, destination);
+      } catch {
+        copyFileSync(existing, destination);
+      }
+      return;
+    }
 
-  mkdirSync(destination, { recursive: true });
-  for (const file of readdirSync(existing)) {
-    link(join(existing, file), join(destination, file));
+    mkdirSync(destination, { recursive: true });
+    for (const file of readdirSync(existing)) {
+      recurse(join(existing, file), join(destination, file));
+    }
   }
 }