path.relative() wrong result in v5.7.0 Windows

[frickt]

Version: <= 5.6.0:

$ node
> process.version
'v5.6.0'
> path.relative('C:\\git\\foo\\bootstrap-loader','C:\\git\\foo\\bootstrap')
'..\\bootstrap'

Version: 5.7.0

$ node
> process.version
process.version
'v5.7.0'
> path.relative('C:\\git\\foo\\bootstrap-loader','C:\\git\\foo\\bootstrap')
'..'

'..' needs to be '..\bootstrap'

[rvagg]

dupe of #5383 fixed in #5389, this is a regression that'll be fixed in a 5.7.1 which is due out shortly, thanks for reporting

[targos]

It's not a dupe of #5383. The result is really wrong here and I'm quite sure #5389 did not fix this.

cc @mscdex

[rvagg]

right, sorry for my hastiness, confirmed that it's still borked with #5389

[omsmith]

Played with it a bit, the following diff passes, but really not sure about it.

diff --git a/lib/path.js b/lib/path.js
index d01e9b6..e012441 100644
--- a/lib/path.js
+++ b/lib/path.js
@@ -603,14 +603,28 @@ const win32 = {
     var i = 0;
     for (; i <= length; ++i) {
       if (i === length) {
-        if (lastCommonSep > 2 && // ignore separator match following device root
-            toLen > length &&
-            to.charCodeAt(i) === 92/*\*/) {
-          // We get here if `from` is the exact base path for `to`.
-          // For example: from='C:\\foo\\bar'; to='C:\\foo\\bar\\baz'
-          return toOrig.slice(i + 1);
+        if (toLen > length) {
+          if (to.charCodeAt(toStart + i) === 92/*\*/) {
+            // We get here if `from` is the exact base path for `to`.
+            // For example: from='C:\\foo\\bar'; to='C:\\foo\\bar\\baz'
+            return toOrig.slice(toStart + i + 1);
+          } else if (lastCommonSep === 2) {
+            // We get here if `from` is the device root.
+            // For example: from='C:\\'; to='C:\\foo'
+            return toOrig.slice(toStart + i);
+          }
+        }
+        if (fromLen > length) {
+          if (from.charCodeAt(fromStart + i) === 92/*\*/) {
+            // We get here if `to` is the exact base path for `from`.
+            // For example: from:='C:\\foo\\bar'; to='C:\\foo'
+            lastCommonSep = i;
+          } else if (lastCommonSep === 2) {
+            // We get here if `to` is the device root.
+            // For example: from='C:\\foo\\bar'; to='C:\\'
+            lastCommonSep = 3;
+          }
         }
-        lastCommonSep = i;
         break;
       }
       var fromCode = from.charCodeAt(fromStart + i);
diff --git a/test/parallel/test-path.js b/test/parallel/test-path.js
index aeb6ec0..02b280e 100644
--- a/test/parallel/test-path.js
+++ b/test/parallel/test-path.js
@@ -470,7 +470,9 @@ const relativeTests = [
      ['c:/AaAa/bbbb', 'c:/aaaa/bbbb', ''],
      ['c:/aaaaa/', 'c:/aaaa/cccc', '..\\aaaa\\cccc'],
      ['C:\\foo\\bar\\baz\\quux', 'C:\\', '..\\..\\..\\..'],
-     ['C:\\foo\\test', 'C:\\foo\\test\\bar\\package.json', 'bar\\package.json']
+     ['C:\\foo\\test', 'C:\\foo\\test\\bar\\package.json', 'bar\\package.json'],
+     ['C:\\foo\\bar\\baz-quux', 'C:\\foo\\bar\\baz', '..\\baz'],
+     ['C:\\foo\\bar', 'C:\\baz\\quux', '..\\..\\baz\\quux']
     ]
   ],
   [ path.posix.relative,

[mscdex]

@omsmith That LGTM, mind making a PR?

[omsmith]

@mscdex sure thing