syspath: correctly prefix Windows UNC paths

Identified while tackling #670, but this should actually solve some legitimate
problems with cataloging music on a network drive.

beets/util/__init__.py

@@ -372,8 +372,12 @@ def syspath(path, prefix=True):
             encoding = sys.getfilesystemencoding() or sys.getdefaultencoding()
             path = path.decode(encoding, 'replace')
 
-    # Add the magic prefix if it isn't already there
+    # Add the magic prefix if it isn't already there.
+    # http://msdn.microsoft.com/en-us/library/windows/desktop/aa365247.aspx
     if prefix and not path.startswith(WINDOWS_MAGIC_PREFIX):
+        if path.startswith(u'\\\\'):
+            # UNC path. Final path should look like \\?\UNC\...
+            path = u'UNC' + path[1:]
         path = WINDOWS_MAGIC_PREFIX + path
 
     return path

docs/changelog.rst

@@ -41,6 +41,8 @@ Fixes:
   info is in the verbose logs.
 * :doc:`/plugins/mbsync`: Fix application of album-level metadata. Due to a
   regression a few releases ago, only track-level metadata was being updated.
+* On Windows, paths on network shares (UNC paths) no longer cause "invalid
+  filename" errors.
 
 .. _enum34: https://pypi.python.org/pypi/enum34
 .. _enum: https://docs.python.org/3.4/library/enum.html

test/test_library.py

@@ -582,6 +582,15 @@ class PathConversionTest(_common.TestCase):
         self.assertTrue(isinstance(outpath, unicode))
         self.assertTrue(outpath.startswith(u'\\\\?\\'))
 
+    def test_syspath_windows_format_unc_path(self):
+        # The \\?\ prefix on Windows behaves differently with UNC
+        # (network share) paths.
+        path = '\\\\server\\share\\file.mp3'
+        with _common.platform_windows():
+            outpath = util.syspath(path)
+        self.assertTrue(isinstance(outpath, unicode))
+        self.assertEqual(outpath, u'\\\\?\\UNC\\server\\share\\file.mp3')
+
     def test_syspath_posix_unchanged(self):
         with _common.platform_posix():
             path = os.path.join('a', 'b', 'c')