windows: fix normalization of UNC paths

lib/path.js

@@ -59,7 +59,7 @@ if (isWindows) {
   // Regex to split a windows path into three parts: [*, device, slash,
   // tail] windows-only
   var splitDeviceRe =
-      /^([a-zA-Z]:|[\\\/]{2}[^\\\/]+[\\\/][^\\\/]+)?([\\\/])?([\s\S]*?)$/;
+      /^([a-zA-Z]:|[\\\/]{2}[^\\\/]+[\\\/]+[^\\\/]+)?([\\\/])?([\s\S]*?)$/;
 
   // Regex to split the tail part of the above into [*, dir, basename, ext]
   var splitTailRe =
@@ -80,6 +80,10 @@ if (isWindows) {
     return [device, dir, basename, ext];
   };
 
+  var normalizeUNCRoot = function(device) {
+    return '\\\\' + device.replace(/^[\\\/]+/, '').replace(/[\\\/]+/g, '\\');
+  }
+
   // path.resolve([from ...], to)
   // windows version
   exports.resolve = function() {
@@ -138,8 +142,11 @@ if (isWindows) {
       }
     }
 
-    // Replace slashes (in UNC share name) by backslashes
-    resolvedDevice = resolvedDevice.replace(/\//g, '\\');
+    // Convert slashes to backslashes when `resolvedDevice` points to an UNC
+    // root. Also squash multiple slashes into a single one where appropriate.
+    if (isUnc) {
+      resolvedDevice = normalizeUNCRoot(resolvedDevice);
+    }
 
     // At this point the path should be resolved to a full absolute path,
     // but handle relative paths to be safe (might happen when process.cwd()
@@ -180,7 +187,10 @@ if (isWindows) {
     }
 
     // Convert slashes to backslashes when `device` points to an UNC root.
-    device = device.replace(/\//g, '\\');
+    // Also squash multiple slashes into a single one where appropriate.
+    if (isUnc) {
+      device = normalizeUNCRoot(device);
+    }
 
     return device + (isAbsolute ? '\\' : '') + tail;
   };
@@ -194,11 +204,21 @@ if (isWindows) {
     var paths = Array.prototype.filter.call(arguments, f);
     var joined = paths.join('\\');
 
-    // Make sure that the joined path doesn't start with two slashes
-    // - it will be mistaken for an unc path by normalize() -
-    // unless the paths[0] also starts with two slashes
-    if (/^[\\\/]{2}/.test(joined) && !/^[\\\/]{2}/.test(paths[0])) {
-      joined = joined.substr(1);
+    // Make sure that the joined path doesn't start with two slashes, because
+    // normalize() will mistake it for an UNC path then.
+    //
+    // This step is skipped when it is very clear that the user actually
+    // intended to point at an UNC path. This is assumed when the first
+    // non-empty string arguments starts with exactly two slashes followed by
+    // at least one more non-slash character.
+    //
+    // Note that for normalize() to treat a path as an UNC path it needs to
+    // have at least 2 components, so we don't filter for that here.
+    // This means that the user can use join to construct UNC paths from
+    // a server name and a share name; for example:
+    //   path.join('//server', 'share') -> '\\\\server\\share\')
+    if (!/^[\\\/]{2}[^\\\/]/.test(paths[0])) {
+      joined = joined.replace(/^[\\\/]{2,}/, '\\');
     }
 
     return exports.normalize(joined);

test/simple/test-path.js

@@ -172,9 +172,67 @@ var joinTests =
      [[' ', '.'], ' '],
      [[' ', '/'], ' /'],
      [[' ', ''], ' '],
+     [['/', 'foo'], '/foo'],
+     [['/', '/foo'], '/foo'],
+     [['/', '//foo'], '/foo'],
+     [['/', '', '/foo'], '/foo'],
+     [['', '/', 'foo'], '/foo'],
+     [['', '/', '/foo'], '/foo'],
      // filtration of non-strings.
      [['x', true, 7, 'y', null, {}], 'x/y']
     ];
+
+// Windows-specific join tests
+if (isWindows) {
+  joinTests = joinTests.concat(
+    [// UNC path expected
+     [['//foo/bar'], '//foo/bar/'],
+     [['\\/foo/bar'], '//foo/bar/'],
+     [['\\\\foo/bar'], '//foo/bar/'],
+     // UNC path expected - server and share separate
+     [['//foo', 'bar'], '//foo/bar/'],
+     [['//foo/', 'bar'], '//foo/bar/'],
+     [['//foo', '/bar'], '//foo/bar/'],
+     // UNC path expected - questionable
+     [['//foo', '', 'bar'], '//foo/bar/'],
+     [['//foo/', '', 'bar'], '//foo/bar/'],
+     [['//foo/', '', '/bar'], '//foo/bar/'],
+     // UNC path expected - even more questionable
+     [['', '//foo', 'bar'], '//foo/bar/'],
+     [['', '//foo/', 'bar'], '//foo/bar/'],
+     [['', '//foo/', '/bar'], '//foo/bar/'],
+     // No UNC path expected (no double slash in first component)
+     [['\\', 'foo/bar'], '/foo/bar'],
+     [['\\', '/foo/bar'], '/foo/bar'],
+     [['', '/', '/foo/bar'], '/foo/bar'],
+     // No UNC path expected (no non-slashes in first component - questionable)
+     [['//', 'foo/bar'], '/foo/bar'],
+     [['//', '/foo/bar'], '/foo/bar'],
+     [['\\\\', '/', '/foo/bar'], '/foo/bar'],
+     [['//'], '/'],
+     // No UNC path expected (share name missing - questionable).
+     [['//foo'], '/foo'],
+     [['//foo/'], '/foo/'],
+     [['//foo', '/'], '/foo/'],
+     [['//foo', '', '/'], '/foo/'],
+     // No UNC path expected (too many leading slashes - questionable)
+     [['///foo/bar'], '/foo/bar'],
+     [['////foo', 'bar'], '/foo/bar'],
+     [['\\\\\\/foo/bar'], '/foo/bar'],
+     // Drive-relative vs drive-absolute paths. This merely describes the
+     // status quo, rather than being obviously right
+     [['c:'], 'c:.'],
+     [['c:.'], 'c:.'],
+     [['c:', ''], 'c:.'],
+     [['', 'c:'], 'c:.'],
+     [['c:.', '/'], 'c:./'],
+     [['c:.', 'file'], 'c:file'],
+     [['c:', '/'], 'c:/'],
+     [['c:', 'file'], 'c:/file']
+    ]);
+}
+
+// Run the join tests.
 joinTests.forEach(function(test) {
   var actual = path.join.apply(path, test[0]);
   var expected = isWindows ? test[1].replace(/\//g, '\\') : test[1];
@@ -215,7 +273,13 @@ if (isWindows) {
        [['c:/ignore', 'c:/some/file'], 'c:\\some\\file'],
        [['d:/ignore', 'd:some/dir//'], 'd:\\ignore\\some\\dir'],
        [['.'], process.cwd()],
-       [['//server/share', '..', 'relative\\'], '\\\\server\\share\\relative']];
+       [['//server/share', '..', 'relative\\'], '\\\\server\\share\\relative'],
+       [['c:/', '//'], 'c:\\'],
+       [['c:/', '//dir'], 'c:\\dir'],
+       [['c:/', '//server/share'], '\\\\server\\share\\'],
+       [['c:/', '//server//share'], '\\\\server\\share\\'],
+       [['c:/', '///some//dir'], 'c:\\some\\dir']
+      ];
 } else {
   // Posix
   var resolveTests =