fs: check subdir correctly in cpSync

src/node_file.cc

@@ -3193,7 +3193,8 @@ static void CpSyncCheckPaths(const FunctionCallbackInfo<Value>& args) {
 
   std::string dest_path_str = dest_path.string();
   // Check if dest_path is a subdirectory of src_path.
-  if (src_is_dir && dest_path_str.starts_with(src_path.string())) {
+  if (src_is_dir &&
+      dest_path.parent_path().string().starts_with(src_path.string())) {
     std::string message = "Cannot copy " + src_path.string() +
                           " to a subdirectory of self " + dest_path.string();
     return THROW_ERR_FS_CP_EINVAL(env, message.c_str());

test/parallel/test-fs-cp.mjs

@@ -24,8 +24,8 @@ import tmpdir from '../common/tmpdir.js';
 tmpdir.refresh();
 
 let dirc = 0;
-function nextdir() {
-  return tmpdir.resolve(`copy_${++dirc}`);
+function nextdir(dirname) {
+  return tmpdir.resolve(dirname || `copy_${++dirc}`);
 }
 
 // Synchronous implementation of copy.
@@ -312,6 +312,32 @@ function nextdir() {
   );
 }
 
+// It must not throw error if attempt is made to copy to dest
+// directory with same prefix as src directory
+// regression test for https://github.com/nodejs/node/issues/54285
+{
+  const src = nextdir('prefix');
+  const dest = nextdir('prefix-a');
+  mkdirSync(src);
+  mkdirSync(dest);
+  cpSync(src, dest, mustNotMutateObjectDeep({ recursive: true }));
+}
+
+// It throws error if attempt is made to copy src to dest
+// when src is parent directory of the parent of dest
+{
+  const src = nextdir('a');
+  const destParent = nextdir('a/b');
+  const dest = nextdir('a/b/c');
+  mkdirSync(src);
+  mkdirSync(destParent);
+  mkdirSync(dest);
+  assert.throws(
+    () => cpSync(src, dest, mustNotMutateObjectDeep({ recursive: true })),
+    { code: 'ERR_FS_CP_EINVAL' },
+  )
+}
+
 // It throws error if attempt is made to copy to subdirectory of self.
 {
   const src = './test/fixtures/copy/kitchen-sink';