vfs: reset state after failed mount

A failed mount registration should leave the VirtualFileSystem instance
unmounted so it does not report stale mount state or handle paths under
the rejected mount point.

Signed-off-by: Kamat, Trivikram <16024985+trivikr@users.noreply.github.com>
Assisted-by: openai:gpt-5.5

Author: trivikr

lib/internal/vfs/file_system.js

@@ -143,7 +143,13 @@ class VirtualFileSystem {
     this[kMounted] = true;
     debug('mount %s', this[kMountPoint]);
     loadVfsSetup();
-    registerVFS(this);
+    try {
+      registerVFS(this);
+    } catch (err) {
+      this[kMountPoint] = null;
+      this[kMounted] = false;
+      throw err;
+    }
     return this;
   }
 

test/parallel/test-vfs-mount-errors.js

@@ -16,6 +16,11 @@ const { vfsState } = require('internal/fs/utils');
 const baseMountPoint = path.resolve('/tmp/vfs-mount-errors-' + process.pid);
 let mountCounter = 0;
 const nextMount = () => baseMountPoint + '-' + (mountCounter++);
+function assertUnmounted(instance, mountPoint) {
+  assert.strictEqual(instance.mounted, false);
+  assert.strictEqual(instance.mountPoint, null);
+  assert.strictEqual(instance.shouldHandle(mountPoint), false);
+}
 
 // EXDEV: rename across two different VFS instances
 {
@@ -130,13 +135,15 @@ const nextMount = () => baseMountPoint + '-' + (mountCounter++);
   const b = vfs.create();
   a.mount(parent);
   assert.throws(() => b.mount(child), { code: 'ERR_INVALID_STATE' });
+  assertUnmounted(b, child);
   a.unmount();
 
   // Reverse direction: child first, then parent rejected
   const c = vfs.create();
   const d = vfs.create();
   c.mount(child);
   assert.throws(() => d.mount(parent), { code: 'ERR_INVALID_STATE' });
+  assertUnmounted(d, parent);
   c.unmount();
 }
 
@@ -147,6 +154,7 @@ const nextMount = () => baseMountPoint + '-' + (mountCounter++);
   const b = vfs.create();
   a.mount(m);
   assert.throws(() => b.mount(m), { code: 'ERR_INVALID_STATE' });
+  assertUnmounted(b, m);
   a.unmount();
 }