path: fix relative on Windows

PR-URL: https://github.com/nodejs/node/pull/53991
Fixes: https://github.com/nodejs/node/issues/27534
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: James M Snell <jasnell@gmail.com>

benchmark/path/relative-win32.js

@@ -9,6 +9,7 @@ const bench = common.createBenchmark(main, {
     ['C:\\foo\\bar\\baz', 'C:\\foo\\bar\\baz'].join('|'),
     ['C:\\foo\\BAR\\BAZ', 'C:\\foo\\bar\\baz'].join('|'),
     ['C:\\foo\\bar\\baz\\quux', 'C:\\'].join('|'),
+    ['c:\\İ\\a\\İ', 'c:\\İ\\b\\İ\\test.txt', '..\\..\\b\\İ\\test.txt'].join('|'),
   ],
   n: [1e5],
 });

lib/path.js

@@ -22,12 +22,16 @@
 'use strict';
 
 const {
+  ArrayPrototypeJoin,
+  ArrayPrototypeSlice,
   FunctionPrototypeBind,
   StringPrototypeCharCodeAt,
   StringPrototypeIndexOf,
   StringPrototypeLastIndexOf,
+  StringPrototypeRepeat,
   StringPrototypeReplace,
   StringPrototypeSlice,
+  StringPrototypeSplit,
   StringPrototypeToLowerCase,
 } = primordials;
 
@@ -539,6 +543,42 @@ const win32 = {
     if (from === to)
       return '';
 
+    if (fromOrig.length !== from.length || toOrig.length !== to.length) {
+      const fromSplit = StringPrototypeSplit(fromOrig, '\\');
+      const toSplit = StringPrototypeSplit(toOrig, '\\');
+      if (fromSplit[fromSplit.length - 1] === '') {
+        fromSplit.pop();
+      }
+      if (toSplit[toSplit.length - 1] === '') {
+        toSplit.pop();
+      }
+
+      const fromLen = fromSplit.length;
+      const toLen = toSplit.length;
+      const length = fromLen < toLen ? fromLen : toLen;
+
+      let i;
+      for (i = 0; i < length; i++) {
+        if (StringPrototypeToLowerCase(fromSplit[i]) !== StringPrototypeToLowerCase(toSplit[i])) {
+          break;
+        }
+      }
+
+      if (i === 0) {
+        return toOrig;
+      } else if (i === length) {
+        if (toLen > length) {
+          return ArrayPrototypeJoin(ArrayPrototypeSlice(toSplit, i), '\\');
+        }
+        if (fromLen > length) {
+          return StringPrototypeRepeat('..\\', fromLen - 1 - i) + '..';
+        }
+        return '';
+      }
+
+      return StringPrototypeRepeat('..\\', fromLen - i) + ArrayPrototypeJoin(ArrayPrototypeSlice(toSplit, i), '\\');
+    }
+
     // Trim any leading backslashes
     let fromStart = 0;
     while (fromStart < from.length &&

test/parallel/test-path-relative.js

@@ -32,6 +32,11 @@ const relativeTests = [
      ['\\\\foo\\baz', '\\\\foo\\baz-quux', '..\\baz-quux'],
      ['C:\\baz', '\\\\foo\\bar\\baz', '\\\\foo\\bar\\baz'],
      ['\\\\foo\\bar\\baz', 'C:\\baz', 'C:\\baz'],
+     ['c:\\a\\İ', 'c:\\a\\İ\\test.txt', 'test.txt'],
+     ['c:\\İ\\a\\İ', 'c:\\İ\\b\\İ\\test.txt', '..\\..\\b\\İ\\test.txt'],
+     ['c:\\İ\\a\\i̇', 'c:\\İ\\b\\İ\\test.txt', '..\\..\\b\\İ\\test.txt'],
+     ['c:\\i̇\\a\\İ', 'c:\\İ\\b\\İ\\test.txt', '..\\..\\b\\İ\\test.txt'],
+     ['c:\\ß\\a\\ß', 'c:\\ß\\b\\ß\\test.txt', '..\\..\\b\\ß\\test.txt'],
     ],
   ],
   [ path.posix.relative,