path: fix path.relative() for prefixes at root

Fixes #5485

PR-URL: https://github.com/nodejs/node/pull/5490
Reviewed-By: Roman Reiss <me@silverwind.io>
Reviewed-By: Brian White <mscdex@mscdex.net>

lib/path.js

@@ -618,7 +618,7 @@ const win32 = {
             // 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) {
+          } else if (i === 2) {
             // We get here if `from` is the device root.
             // For example: from='C:\\'; to='C:\\foo'
             return toOrig.slice(toStart + i);
@@ -629,7 +629,7 @@ const win32 = {
             // 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) {
+          } else if (i === 2) {
             // We get here if `to` is the device root.
             // For example: from='C:\\foo\\bar'; to='C:\\'
             lastCommonSep = 3;
@@ -1299,16 +1299,26 @@ const posix = {
     var i = 0;
     for (; i <= length; ++i) {
       if (i === length) {
-        if (lastCommonSep === -1) {
-          lastCommonSep = i;
-        } else if (toLen > length && to.charCodeAt(i + 1) === 47/*/*/) {
-          // We get here if `from` is the exact base path for `to`.
-          // For example: from='/foo/bar'; to='/foo/bar/baz'
-          return to.slice(i + 2);
-        } else if (fromLen > length && from.charCodeAt(i + 1) === 47/*/*/) {
-          // We get here if `to` is the exact base path for `from`.
-          // For example: from='/foo/bar/baz'; to='/foo/bar'
-          lastCommonSep = i;
+        if (toLen > length) {
+          if (to.charCodeAt(toStart + i) === 47/*/*/) {
+            // We get here if `from` is the exact base path for `to`.
+            // For example: from='/foo/bar'; to='/foo/bar/baz'
+            return to.slice(toStart + i + 1);
+          } else if (i === 0) {
+            // We get here if `from` is the root
+            // For example: from='/'; to='/foo'
+            return to.slice(toStart + i);
+          }
+        } else if (fromLen > length) {
+          if (from.charCodeAt(fromStart + i) === 47/*/*/) {
+            // We get here if `to` is the exact base path for `from`.
+            // For example: from='/foo/bar/baz'; to='/foo/bar'
+            lastCommonSep = i;
+          } else if (i === 0) {
+            // We get here if `to` is the root.
+            // For example: from='/foo'; to='/'
+            lastCommonSep = 0;
+          }
         }
         break;
       }

test/parallel/test-path.js

@@ -476,7 +476,11 @@ const relativeTests = [
      ['\\\\foo\\bar', '\\\\foo\\bar\\baz', 'baz'],
      ['\\\\foo\\bar\\baz', '\\\\foo\\bar', '..'],
      ['\\\\foo\\bar\\baz-quux', '\\\\foo\\bar\\baz', '..\\baz'],
-     ['\\\\foo\\bar\\baz', '\\\\foo\\bar\\baz-quux', '..\\baz-quux']
+     ['\\\\foo\\bar\\baz', '\\\\foo\\bar\\baz-quux', '..\\baz-quux'],
+     ['C:\\baz-quux', 'C:\\baz', '..\\baz'],
+     ['C:\\baz', 'C:\\baz-quux', '..\\baz-quux'],
+     ['\\\\foo\\baz-quux', '\\\\foo\\baz', '..\\baz'],
+     ['\\\\foo\\baz', '\\\\foo\\baz-quux', '..\\baz-quux']
     ]
   ],
   [ path.posix.relative,
@@ -490,7 +494,9 @@ const relativeTests = [
      ['/foo/test', '/foo/test/bar/package.json', 'bar/package.json'],
      ['/Users/a/web/b/test/mails', '/Users/a/web/b', '../..'],
      ['/foo/bar/baz-quux', '/foo/bar/baz', '../baz'],
-     ['/foo/bar/baz', '/foo/bar/baz-quux', '../baz-quux']
+     ['/foo/bar/baz', '/foo/bar/baz-quux', '../baz-quux'],
+     ['/baz-quux', '/baz', '../baz'],
+     ['/baz', '/baz-quux', '../baz-quux']
     ]
   ]
 ];