vfs: fix rename over non-empty directory

The memory provider only rejected renames whose destination had a
different type than the source, so renaming a directory onto another
directory silently dropped the destination and everything under it.
An existing destination directory has to be empty; rename(2) reports
ENOTEMPTY otherwise, and RealFSProvider already does so because it
delegates to fs.renameSync().

Also, decrement nlink on a file that is replaced by a rename, and make
a rename whose two names resolve to the same entry a no-op, which
covers renaming one hard link onto another.

Signed-off-by: Christian Aurich <christian.aurichzm@gmail.com>

Author: christianaurichzm

lib/internal/vfs/providers/memory.js

@@ -840,6 +840,10 @@ class MemoryProvider extends VirtualProvider {
 
     // Check if destination exists
     const existingDest = newParent.children.get(newName);
+    if (existingDest === entry) {
+      // Both names resolve to the same entry: rename does nothing
+      return;
+    }
     if (existingDest) {
       // Cannot overwrite a directory with a non-directory
       if (existingDest.isDirectory() && !entry.isDirectory()) {
@@ -849,6 +853,14 @@ class MemoryProvider extends VirtualProvider {
       if (!existingDest.isDirectory() && entry.isDirectory()) {
         throw createENOTDIR('rename', newPath);
       }
+      if (existingDest.isDirectory()) {
+        // Cannot overwrite a non-empty directory
+        if (existingDest.children.size > 0) {
+          throw createENOTEMPTY('rename', newPath);
+        }
+      } else {
+        existingDest.nlink--;
+      }
     }
 
     // Remove from old location (after destination validation)

test/parallel/test-vfs-rename.js

@@ -58,3 +58,67 @@ const vfs = require('node:vfs');
   assert.strictEqual(myVfs.existsSync('/a/b/c'), false);
   assert.strictEqual(myVfs.readFileSync('/a/file.txt', 'utf8'), 'data');
 }
+
+// Renaming a directory onto a non-empty directory throws ENOTEMPTY
+{
+  const myVfs = vfs.create();
+  myVfs.mkdirSync('/src');
+  myVfs.mkdirSync('/dst');
+  myVfs.writeFileSync('/dst/keep.txt', 'keep');
+
+  assert.throws(() => myVfs.renameSync('/src', '/dst'), { code: 'ENOTEMPTY' });
+  assert.strictEqual(myVfs.readFileSync('/dst/keep.txt', 'utf8'), 'keep');
+  assert.strictEqual(myVfs.existsSync('/src'), true);
+}
+
+// Renaming a directory onto an empty directory succeeds
+{
+  const myVfs = vfs.create();
+  myVfs.mkdirSync('/src');
+  myVfs.writeFileSync('/src/a.txt', 'a');
+  myVfs.mkdirSync('/dst');
+
+  myVfs.renameSync('/src', '/dst');
+  assert.strictEqual(myVfs.existsSync('/src'), false);
+  assert.strictEqual(myVfs.readFileSync('/dst/a.txt', 'utf8'), 'a');
+}
+
+// Overwriting a file drops one of its links
+{
+  const myVfs = vfs.create();
+  myVfs.writeFileSync('/a.txt', 'a');
+  myVfs.writeFileSync('/b.txt', 'b');
+  myVfs.linkSync('/b.txt', '/b-link.txt');
+  assert.strictEqual(myVfs.statSync('/b-link.txt').nlink, 2);
+
+  myVfs.renameSync('/a.txt', '/b.txt');
+  assert.strictEqual(myVfs.statSync('/b-link.txt').nlink, 1);
+  assert.strictEqual(myVfs.readFileSync('/b-link.txt', 'utf8'), 'b');
+}
+
+// Renaming a path onto itself is a no-op
+{
+  const myVfs = vfs.create();
+  myVfs.writeFileSync('/a.txt', 'a');
+  myVfs.mkdirSync('/d');
+  myVfs.writeFileSync('/d/keep.txt', 'keep');
+
+  myVfs.renameSync('/a.txt', '/a.txt');
+  assert.strictEqual(myVfs.readFileSync('/a.txt', 'utf8'), 'a');
+  assert.strictEqual(myVfs.statSync('/a.txt').nlink, 1);
+
+  myVfs.renameSync('/d', '/d');
+  assert.strictEqual(myVfs.readFileSync('/d/keep.txt', 'utf8'), 'keep');
+}
+
+// Renaming a hard link onto another link to the same file is a no-op
+{
+  const myVfs = vfs.create();
+  myVfs.writeFileSync('/a.txt', 'a');
+  myVfs.linkSync('/a.txt', '/b.txt');
+
+  myVfs.renameSync('/a.txt', '/b.txt');
+  assert.strictEqual(myVfs.existsSync('/a.txt'), true);
+  assert.strictEqual(myVfs.existsSync('/b.txt'), true);
+  assert.strictEqual(myVfs.statSync('/a.txt').nlink, 2);
+}