Make ntpath compress multiple slashes between drive letter and the rest of the

path.  Also clarifies UNC handling and adds appropriate tests.

Applies patch #988607 to fix bug #980327.  Thanks Paul Moore.

Lib/ntpath.py

@@ -440,9 +440,25 @@ def normpath(path):
     """Normalize path, eliminating double slashes, etc."""
     path = path.replace("/", "\\")
     prefix, path = splitdrive(path)
-    while path[:1] == "\\":
-        prefix = prefix + "\\"
-        path = path[1:]
+    # We need to be careful here. If the prefix is empty, and the path starts
+    # with a backslash, it could either be an absolute path on the current
+    # drive (\dir1\dir2\file) or a UNC filename (\\server\mount\dir1\file). It
+    # is therefore imperative NOT to collapse multiple backslashes blindly in
+    # that case.
+    # The code below preserves multiple backslashes when there is no drive
+    # letter. This means that the invalid filename \\\a\b is preserved
+    # unchanged, where a\\\b is normalised to a\b. It's not clear that there
+    # is any better behaviour for such edge cases.
+    if prefix == '':
+        # No drive letter - preserve initial backslashes
+        while path[:1] == "\\":
+            prefix = prefix + "\\"
+            path = path[1:]
+    else:
+        # We have a drive letter - collapse initial backslashes
+        if path.startswith("\\"):
+            prefix = prefix + "\\"
+            path = path.lstrip("\\")
     comps = path.split("\\")
     i = 0
     while i < len(comps):