fs: defer path resolution in realpath functions

In `fs.realpath` and `fs.realpathSync`, path resolution is now performed
after symlink traversal instead of before, ensuring correct behavior
when dealing with parent directory references (`..`) in paths containing
symlinks.

Fixes: #60779

Author: Xstoudi

lib/fs.js

@@ -2681,7 +2681,6 @@ function realpathSync(p, options) {
     p += '';
   }
   validatePath(p);
-  p = pathModule.resolve(p);
 
   const cache = options[realpathCacheKey];
   const maybeCachedResult = cache?.get(p);
@@ -2800,6 +2799,8 @@ function realpathSync(p, options) {
     }
   }
 
+  p = pathModule.resolve(p);
+
   cache?.set(original, p);
   return encodeRealpathResult(p, options);
 }
@@ -2842,7 +2843,6 @@ function realpath(p, options, callback) {
     p += '';
   }
   validatePath(p);
-  p = pathModule.resolve(p);
 
   const seenLinks = new SafeMap();
   const knownHard = new SafeSet();
@@ -2875,6 +2875,7 @@ function realpath(p, options, callback) {
   function LOOP() {
     // Stop if scanned past end of path
     if (pos >= p.length) {
+      p = pathModule.resolve(p);
       return callback(null, encodeRealpathResult(p, options));
     }
 
@@ -2896,6 +2897,7 @@ function realpath(p, options, callback) {
     if (knownHard.has(base)) {
       if (isFileType(statValues, S_IFIFO) ||
           isFileType(statValues, S_IFSOCK)) {
+        p = pathModule.resolve(p);
         return callback(null, encodeRealpathResult(p, options));
       }
       return process.nextTick(LOOP);

test/parallel/test-fs-realpath.js

@@ -85,6 +85,7 @@ function asynctest(testBlock, args, callback, assertBlock) {
 
 // sub-tests:
 function test_simple_error_callback(realpath, realpathSync, cb) {
+  console.log('test_simple_error_callback');
   realpath('/this/path/does/not/exist', common.mustCall(function(err, s) {
     assert(err);
     assert(!s);
@@ -93,6 +94,7 @@ function test_simple_error_callback(realpath, realpathSync, cb) {
 }
 
 function test_simple_error_cb_with_null_options(realpath, realpathSync, cb) {
+  console.log('test_simple_error_cb_with_null_options');
   realpath('/this/path/does/not/exist', null, common.mustCall(function(err, s) {
     assert(err);
     assert(!s);
@@ -382,6 +384,43 @@ function test_non_symlinks(realpath, realpathSync, callback) {
   });
 }
 
+function test_parent_after_symlink(realpath, realpathSync, callback) {
+  console.log('test_parent_after_symlink');
+  if (skipSymlinks) {
+    common.printSkipMessage('symlink test (no privs)');
+    return callback();
+  }
+
+  const folder = tmp('folder');
+  const nested = path.join(folder, 'nested');
+  const symlinkDir = tmp('symlink-dir');
+
+  try {
+    fs.mkdirSync(folder);
+    fs.mkdirSync(nested);
+  } catch {
+    // Continue regardless of error.
+  }
+
+  try { fs.unlinkSync(symlinkDir); } catch {
+    // Continue regardless of error.
+  }
+
+  fs.symlinkSync(nested, symlinkDir, 'dir');
+  unlink.push(symlinkDir);
+  unlink.push(nested);
+  unlink.push(folder);
+
+  const a = realpathSync(`${symlinkDir}/..`);
+  const b = realpathSync(folder);
+
+  assertEqualPath(a, b);
+
+  asynctest(realpath, [`${symlinkDir}/..`], callback, (err, res) => {
+    assertEqualPath(res, b);
+  });
+}
+
 const upone = path.join(process.cwd(), '..');
 function test_escape_cwd(realpath, realpathSync, cb) {
   console.log('test_escape_cwd');
@@ -579,6 +618,7 @@ const tests = [
   test_up_multiple_with_null_options,
   test_root,
   test_root_with_null_options,
+  test_parent_after_symlink,
 ];
 const numtests = tests.length;
 let testsRun = 0;