bpo-37834: Normalise handling of reparse points on Windows (GH-15231)

bpo-37834: Normalise handling of reparse points on Windows
* ntpath.realpath() and nt.stat() will traverse all supported reparse points (previously was mixed)
* nt.lstat() will let the OS traverse reparse points that are not name surrogates (previously would not traverse any reparse point)
* nt.[l]stat() will only set S_IFLNK for symlinks (previous behaviour)
* nt.readlink() will read destinations for symlinks and junction points only

bpo-1311: os.path.exists('nul') now returns True on Windows
* nt.stat('nul').st_mode is now S_IFCHR (previously was an error)

Author: zooba

Doc/library/os.rst

@@ -1858,6 +1858,12 @@ features:
    .. versionchanged:: 3.6
       Accepts a :term:`path-like object` for *src* and *dst*.
 
+   .. versionchanged:: 3.8
+      On Windows, now opens reparse points that represent another path
+      (name surrogates), including symbolic links and directory junctions.
+      Other kinds of reparse points are resolved by the operating system as
+      for :func:`~os.stat`.
+
 
 .. function:: mkdir(path, mode=0o777, *, dir_fd=None)
 
@@ -2039,6 +2045,10 @@ features:
    This function can also support :ref:`paths relative to directory descriptors
    <dir_fd>`.
 
+   When trying to resolve a path that may contain links, use
+   :func:`~os.path.realpath` to properly handle recursion and platform
+   differences.
+
    .. availability:: Unix, Windows.
 
    .. versionchanged:: 3.2
@@ -2053,6 +2063,11 @@ features:
    .. versionchanged:: 3.8
       Accepts a :term:`path-like object` and a bytes object on Windows.
 
+   .. versionchanged:: 3.8
+      Added support for directory junctions, and changed to return the
+      substitution path (which typically includes ``\\?\`` prefix) rather
+      than the optional "print name" field that was previously returned.
+
 .. function:: remove(path, *, dir_fd=None)
 
    Remove (delete) the file *path*.  If *path* is a directory, an
@@ -2366,7 +2381,8 @@ features:
 
       On Unix, this method always requires a system call. On Windows, it
       only requires a system call if *follow_symlinks* is ``True`` and the
-      entry is a symbolic link.
+      entry is a reparse point (for example, a symbolic link or directory
+      junction).
 
       On Windows, the ``st_ino``, ``st_dev`` and ``st_nlink`` attributes of the
       :class:`stat_result` are always set to zero. Call :func:`os.stat` to
@@ -2403,6 +2419,17 @@ features:
    This function can support :ref:`specifying a file descriptor <path_fd>` and
    :ref:`not following symlinks <follow_symlinks>`.
 
+   On Windows, passing ``follow_symlinks=False`` will disable following all
+   name-surrogate reparse points, which includes symlinks and directory
+   junctions. Other types of reparse points that do not resemble links or that
+   the operating system is unable to follow will be opened directly. When
+   following a chain of multiple links, this may result in the original link
+   being returned instead of the non-link that prevented full traversal. To
+   obtain stat results for the final path in this case, use the
+   :func:`os.path.realpath` function to resolve the path name as far as
+   possible and call :func:`lstat` on the result. This does not apply to
+   dangling symlinks or junction points, which will raise the usual exceptions.
+
    .. index:: module: stat
 
    Example::
@@ -2427,6 +2454,14 @@ features:
    .. versionchanged:: 3.6
       Accepts a :term:`path-like object`.
 
+   .. versionchanged:: 3.8
+      On Windows, all reparse points that can be resolved by the operating
+      system are now followed, and passing ``follow_symlinks=False``
+      disables following all name surrogate reparse points. If the operating
+      system reaches a reparse point that it is not able to follow, *stat* now
+      returns the information for the original path as if
+      ``follow_symlinks=False`` had been specified instead of raising an error.
+
 
 .. class:: stat_result
 
@@ -2578,7 +2613,7 @@ features:
 
       File type.
 
-   On Windows systems, the following attribute is also available:
+   On Windows systems, the following attributes are also available:
 
    .. attribute:: st_file_attributes
 
@@ -2587,6 +2622,12 @@ features:
       :c:func:`GetFileInformationByHandle`. See the ``FILE_ATTRIBUTE_*``
       constants in the :mod:`stat` module.
 
+   .. attribute:: st_reparse_tag
+
+      When :attr:`st_file_attributes` has the ``FILE_ATTRIBUTE_REPARSE_POINT``
+      set, this field contains the tag identifying the type of reparse point.
+      See the ``IO_REPARSE_TAG_*`` constants in the :mod:`stat` module.
+
    The standard module :mod:`stat` defines functions and constants that are
    useful for extracting information from a :c:type:`stat` structure. (On
    Windows, some items are filled with dummy values.)
@@ -2614,6 +2655,14 @@ features:
    .. versionadded:: 3.7
       Added the :attr:`st_fstype` member to Solaris/derivatives.
 
+   .. versionadded:: 3.8
+      Added the :attr:`st_reparse_tag` member on Windows.
+
+   .. versionchanged:: 3.8
+      On Windows, the :attr:`st_mode` member now identifies special
+      files as :const:`S_IFCHR`, :const:`S_IFIFO` or :const:`S_IFBLK`
+      as appropriate.
+
 .. function:: statvfs(path)
 
    Perform a :c:func:`statvfs` system call on the given path.  The return value is

Doc/library/shutil.rst

@@ -304,6 +304,10 @@ Directory and files operations
       Added a symlink attack resistant version that is used automatically
       if platform supports fd-based functions.
 
+   .. versionchanged:: 3.8
+      On Windows, will no longer delete the contents of a directory junction
+      before removing the junction.
+
    .. attribute:: rmtree.avoids_symlink_attacks
 
       Indicates whether the current platform and implementation provides a

Doc/library/stat.rst

@@ -425,3 +425,13 @@ for more detail on the meaning of these constants.
           FILE_ATTRIBUTE_VIRTUAL
 
    .. versionadded:: 3.5
+
+On Windows, the following constants are available for comparing against the
+``st_reparse_tag`` member returned by :func:`os.lstat`. These are well-known
+constants, but are not an exhaustive list.
+
+.. data:: IO_REPARSE_TAG_SYMLINK
+          IO_REPARSE_TAG_MOUNT_POINT
+          IO_REPARSE_TAG_APPEXECLINK
+
+   .. versionadded:: 3.8

Doc/whatsnew/3.8.rst

@@ -808,6 +808,21 @@ A new :func:`os.memfd_create` function was added to wrap the
 ``memfd_create()`` syscall.
 (Contributed by Zackery Spytz and Christian Heimes in :issue:`26836`.)
 
+On Windows, much of the manual logic for handling reparse points (including
+symlinks and directory junctions) has been delegated to the operating system.
+Specifically, :func:`os.stat` will now traverse anything supported by the
+operating system, while :func:`os.lstat` will only open reparse points that
+identify as "name surrogates" while others are opened as for :func:`os.stat`.
+In all cases, :attr:`stat_result.st_mode` will only have ``S_IFLNK`` set for
+symbolic links and not other kinds of reparse points. To identify other kinds
+of reparse point, check the new :attr:`stat_result.st_reparse_tag` attribute.
+
+On Windows, :func:`os.readlink` is now able to read directory junctions. Note
+that :func:`~os.path.islink` will return ``False`` for directory junctions,
+and so code that checks ``islink`` first will continue to treat junctions as
+directories, while code that handles errors from :func:`os.readlink` may now
+treat junctions as links.
+
 
 os.path
 -------
@@ -824,6 +839,9 @@ characters or bytes unrepresentable at the OS level.
 environment variable and does not use :envvar:`HOME`, which is not normally set
 for regular user accounts.
 
+:func:`~os.path.isdir` on Windows no longer returns true for a link to a
+non-existent directory.
+
 :func:`~os.path.realpath` on Windows now resolves reparse points, including
 symlinks and directory junctions.
 
@@ -912,6 +930,9 @@ format for new archives to improve portability and standards conformance,
 inherited from the corresponding change to the :mod:`tarfile` module.
 (Contributed by C.A.M. Gerlach in :issue:`30661`.)
 
+:func:`shutil.rmtree` on Windows now removes directory junctions without
+recursively removing their contents first.
+
 
 ssl
 ---

Include/fileutils.h

@@ -84,6 +84,7 @@ struct _Py_stat_struct {
     time_t st_ctime;
     int st_ctime_nsec;
     unsigned long st_file_attributes;
+    unsigned long st_reparse_tag;
 };
 #else
 #  define _Py_stat_struct stat

Lib/shutil.py

@@ -452,7 +452,14 @@ def _copytree(entries, src, dst, symlinks, ignore, copy_function,
         dstname = os.path.join(dst, srcentry.name)
         srcobj = srcentry if use_srcentry else srcname
         try:
-            if srcentry.is_symlink():
+            is_symlink = srcentry.is_symlink()
+            if is_symlink and os.name == 'nt':
+                # Special check for directory junctions, which appear as
+                # symlinks but we want to recurse.
+                lstat = srcentry.stat(follow_symlinks=False)
+                if lstat.st_reparse_tag == stat.IO_REPARSE_TAG_MOUNT_POINT:
+                    is_symlink = False
+            if is_symlink:
                 linkto = os.readlink(srcname)
                 if symlinks:
                     # We can't just leave it to `copy_function` because legacy
@@ -537,6 +544,37 @@ def copytree(src, dst, symlinks=False, ignore=None, copy_function=copy2,
                          ignore_dangling_symlinks=ignore_dangling_symlinks,
                          dirs_exist_ok=dirs_exist_ok)
 
+if hasattr(stat, 'FILE_ATTRIBUTE_REPARSE_POINT'):
+    # Special handling for directory junctions to make them behave like
+    # symlinks for shutil.rmtree, since in general they do not appear as
+    # regular links.
+    def _rmtree_isdir(entry):
+        try:
+            st = entry.stat(follow_symlinks=False)
+            return (stat.S_ISDIR(st.st_mode) and not
+                (st.st_file_attributes & stat.FILE_ATTRIBUTE_REPARSE_POINT
+                 and st.st_reparse_tag == stat.IO_REPARSE_TAG_MOUNT_POINT))
+        except OSError:
+            return False
+
+    def _rmtree_islink(path):
+        try:
+            st = os.lstat(path)
+            return (stat.S_ISLNK(st.st_mode) or
+                (st.st_file_attributes & stat.FILE_ATTRIBUTE_REPARSE_POINT
+                 and st.st_reparse_tag == stat.IO_REPARSE_TAG_MOUNT_POINT))
+        except OSError:
+            return False
+else:
+    def _rmtree_isdir(entry):
+        try:
+            return entry.is_dir(follow_symlinks=False)
+        except OSError:
+            return False
+
+    def _rmtree_islink(path):
+        return os.path.islink(path)
+
 # version vulnerable to race conditions
 def _rmtree_unsafe(path, onerror):
     try:
@@ -547,11 +585,7 @@ def _rmtree_unsafe(path, onerror):
         entries = []
     for entry in entries:
         fullname = entry.path
-        try:
-            is_dir = entry.is_dir(follow_symlinks=False)
-        except OSError:
-            is_dir = False
-        if is_dir:
+        if _rmtree_isdir(entry):
             try:
                 if entry.is_symlink():
                     # This can only happen if someone replaces
@@ -681,7 +715,7 @@ def onerror(*args):
             os.close(fd)
     else:
         try:
-            if os.path.islink(path):
+            if _rmtree_islink(path):
                 # symlinks to directories are forbidden, see bug #1669
                 raise OSError("Cannot call rmtree on a symbolic link")
         except OSError: