vfs: follow symlinked dirs in recursive mkdir

lib/internal/vfs/providers/memory.js

@@ -695,10 +695,13 @@ class MemoryProvider extends VirtualProvider {
       const segments = this.#splitPath(normalized);
       let current = this[kRoot];
       let currentPath = '/';
+      let resolvedCurrentPath = '/';
       let firstCreated;
 
       for (const segment of segments) {
         currentPath = pathPosix.join(currentPath, segment);
+        const resolvedPath = pathPosix.join(resolvedCurrentPath, segment);
+        this.#ensurePopulated(current, resolvedCurrentPath);
         let entry = current.children.get(segment);
         if (!entry) {
           entry = new MemoryEntry(TYPE_DIR, { mode: options?.mode });
@@ -707,7 +710,23 @@ class MemoryProvider extends VirtualProvider {
           if (firstCreated === undefined) {
             firstCreated = currentPath;
           }
-        } else if (!entry.isDirectory()) {
+          resolvedCurrentPath = resolvedPath;
+        } else if (entry.isSymbolicLink()) {
+          const targetPath = this.#resolveSymlinkTarget(resolvedPath, entry.target);
+          const result = this.#lookupEntry(targetPath, true, 0);
+          if (result.eloop) {
+            throw createELOOP('mkdir', path);
+          }
+          if (!result.entry) {
+            throw createENOENT('mkdir', path);
+          }
+          entry = result.entry;
+          resolvedCurrentPath = result.resolvedPath;
+        } else {
+          resolvedCurrentPath = resolvedPath;
+        }
+
+        if (!entry.isDirectory()) {
           throw createENOTDIR('mkdir', path);
         }
         current = entry;

test/parallel/test-vfs-mkdir.js

@@ -4,7 +4,7 @@
 // mkdirSync / rmdirSync behaviour: return value, recursive option, mode
 // option, error cases.
 
-require('../common');
+const common = require('../common');
 const assert = require('assert');
 const vfs = require('node:vfs');
 
@@ -17,6 +17,51 @@ const vfs = require('node:vfs');
   assert.strictEqual(result, '/a/b');
 }
 
+// Recursive mkdir follows symlinked intermediate directories, but returns the
+// path of the first created directory as requested by the caller.
+{
+  const myVfs = vfs.create();
+  myVfs.mkdirSync('/target');
+  myVfs.symlinkSync('/target', '/link');
+
+  const result = myVfs.mkdirSync('/link/subdir/deep', { recursive: true });
+
+  assert.strictEqual(result, '/link/subdir');
+  assert.strictEqual(myVfs.existsSync('/target/subdir/deep'), true);
+  assert.strictEqual(myVfs.existsSync('/link/subdir/deep'), true);
+}
+
+// Recursive mkdir also resolves relative symlink targets from the symlink's
+// resolved parent directory.
+{
+  const myVfs = vfs.create();
+  myVfs.mkdirSync('/parent/target', { recursive: true });
+  myVfs.symlinkSync('target', '/parent/link');
+
+  myVfs.mkdirSync('/parent/link/subdir', { recursive: true });
+
+  assert.strictEqual(myVfs.existsSync('/parent/target/subdir'), true);
+}
+
+// Recursive mkdir through symlinks keeps native error behavior for bad
+// intermediate targets.
+{
+  const myVfs = vfs.create();
+  myVfs.symlinkSync('/missing', '/dangling');
+  assert.throws(
+    () => myVfs.mkdirSync('/dangling/subdir', { recursive: true }),
+    { code: 'ENOENT' });
+}
+
+{
+  const myVfs = vfs.create();
+  myVfs.writeFileSync('/file', 'x');
+  myVfs.symlinkSync('/file', '/link');
+  assert.throws(
+    () => myVfs.mkdirSync('/link/subdir', { recursive: true }),
+    { code: 'ENOTDIR' });
+}
+
 // mkdirSync with explicit mode (non-recursive)
 {
   const myVfs = vfs.create();
@@ -47,3 +92,16 @@ const vfs = require('node:vfs');
   myVfs.writeFileSync('/d/x', '');
   assert.throws(() => myVfs.rmdirSync('/d'), { code: 'ENOTEMPTY' });
 }
+
+// promises.mkdir uses the same recursive symlink handling.
+(async () => {
+  const myVfs = vfs.create();
+  myVfs.mkdirSync('/target');
+  myVfs.symlinkSync('/target', '/link');
+
+  const result = await myVfs.promises.mkdir('/link/subdir/deep',
+                                            { recursive: true });
+
+  assert.strictEqual(result, '/link/subdir');
+  assert.strictEqual(myVfs.existsSync('/target/subdir/deep'), true);
+})().then(common.mustCall());