gh-96290: Support partial/invalid UNC drives in ntpath.normpath() and splitdrive() (GH-100351)

This brings the Python implementation of `ntpath.normpath()` in line with the C implementation added in 99fcf15

Co-authored-by: Eryk Sun <eryksun@gmail.com>

1 files changed, 21 insertions, 34 deletions

diff --git a/Lib/ntpath.py b/Lib/ntpath.py
index 265eaa8..cd7fb58 100644
--- a/Lib/ntpath.py
+++ b/Lib/ntpath.py
@@ -87,16 +87,20 @@ except ImportError:
 def isabs(s):
     """Test whether a path is absolute"""
     s = os.fspath(s)
-    # Paths beginning with \\?\ are always absolute, but do not
-    # necessarily contain a drive.
     if isinstance(s, bytes):
-        if s.replace(b'/', b'\\').startswith(b'\\\\?\\'):
-            return True
+        sep = b'\\'
+        altsep = b'/'
+        colon_sep = b':\\'
     else:
-        if s.replace('/', '\\').startswith('\\\\?\\'):
-            return True
-    s = splitdrive(s)[1]
-    return len(s) > 0 and s[0] and s[0] in _get_bothseps(s)
+        sep = '\\'
+        altsep = '/'
+        colon_sep = ':\\'
+    s = s[:3].replace(altsep, sep)
+    # Absolute: UNC, device, and paths with a drive and root.
+    # LEGACY BUG: isabs("/x") should be false since the path has no drive.
+    if s.startswith(sep) or s.startswith(colon_sep, 1):
+        return True
+    return False
 
 
 # Join two (or more) paths.
@@ -172,34 +176,26 @@ def splitdrive(p):
             sep = b'\\'
             altsep = b'/'
             colon = b':'
-            unc_prefix = b'\\\\?\\UNC'
+            unc_prefix = b'\\\\?\\UNC\\'
         else:
             sep = '\\'
             altsep = '/'
             colon = ':'
-            unc_prefix = '\\\\?\\UNC'
+            unc_prefix = '\\\\?\\UNC\\'
         normp = p.replace(altsep, sep)
-        if (normp[0:2] == sep*2) and (normp[2:3] != sep):
-            # is a UNC path:
-            # vvvvvvvvvvvvvvvvvvvv drive letter or UNC path
-            # \\machine\mountpoint\directory\etc\...
-            #           directory ^^^^^^^^^^^^^^^
-            if normp[:8].upper().rstrip(sep) == unc_prefix:
-                start = 8
-            else:
-                start = 2
+        if normp[0:2] == sep * 2:
+            # UNC drives, e.g. \\server\share or \\?\UNC\server\share
+            # Device drives, e.g. \\.\device or \\?\device
+            start = 8 if normp[:8].upper() == unc_prefix else 2
             index = normp.find(sep, start)
             if index == -1:
-                return p[:0], p
+                return p, p[:0]
             index2 = normp.find(sep, index + 1)
-            # a UNC path can't have two slashes in a row
-            # (after the initial two)
-            if index2 == index + 1:
-                return p[:0], p
             if index2 == -1:
-                index2 = len(p)
+                return p, p[:0]
             return p[:index2], p[index2:]
         if normp[1:2] == colon:
+            # Drive-letter drives, e.g. X:
             return p[:2], p[2:]
     return p[:0], p
 
@@ -523,20 +519,11 @@ except ImportError:
             altsep = b'/'
             curdir = b'.'
             pardir = b'..'
-            special_prefixes = (b'\\\\.\\', b'\\\\?\\')
         else:
             sep = '\\'
             altsep = '/'
             curdir = '.'
             pardir = '..'
-            special_prefixes = ('\\\\.\\', '\\\\?\\')
-        if path.startswith(special_prefixes):
-            # in the case of paths with these prefixes:
-            # \\.\ -> device names
-            # \\?\ -> literal paths
-            # do not do any normalization, but return the path
-            # unchanged apart from the call to os.fspath()
-            return path
         path = path.replace(altsep, sep)
         prefix, path = splitdrive(path)