path: fix win32 relative() for UNC paths

win32 normalize() will output a trailing '\' for some UNC paths. trim
them before processing

Change by @mscdex

Add basic UNC path tests to win32 relative()

PR-URL: #5456
Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: Roman Reiss <me@silverwind.io>

lib/path.js

@@ -585,7 +585,12 @@ const win32 = {
       if (from.charCodeAt(fromStart) !== 92/*\*/)
         break;
     }
+    // Trim trailing backslashes (applicable to UNC paths only)
     var fromEnd = from.length;
+    for (; fromEnd - 1 > fromStart; --fromEnd) {
+      if (from.charCodeAt(fromEnd - 1) !== 92/*\*/)
+        break;
+    }
     var fromLen = (fromEnd - fromStart);
 
     // Trim any leading backslashes
@@ -594,7 +599,12 @@ const win32 = {
       if (to.charCodeAt(toStart) !== 92/*\*/)
         break;
     }
+    // Trim trailing backslashes (applicable to UNC paths only)
     var toEnd = to.length;
+    for (; toEnd - 1 > toStart; --toEnd) {
+      if (to.charCodeAt(toEnd - 1) !== 92/*\*/)
+        break;
+    }
     var toLen = (toEnd - toStart);
 
     // Compare paths to find the longest common path from root
@@ -662,12 +672,12 @@ const win32 = {
     // Lastly, append the rest of the destination (`to`) path that comes after
     // the common path parts
     if (out.length > 0)
-      return out + toOrig.slice(toStart + lastCommonSep);
+      return out + toOrig.slice(toStart + lastCommonSep, toEnd);
     else {
       toStart += lastCommonSep;
       if (toOrig.charCodeAt(toStart) === 92/*\*/)
         ++toStart;
-      return toOrig.slice(toStart);
+      return toOrig.slice(toStart, toEnd);
     }
   },
 

test/parallel/test-path.js

@@ -471,7 +471,12 @@ const relativeTests = [
      ['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\\bar\\baz-quux', 'C:\\foo\\bar\\baz', '..\\baz']
+     ['C:\\foo\\bar\\baz-quux', 'C:\\foo\\bar\\baz', '..\\baz'],
+     ['C:\\foo\\bar\\baz', 'C:\\foo\\bar\\baz-quux', '..\\baz-quux'],
+     ['\\\\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']
     ]
   ],
   [ path.posix.relative,