Author eryksun
Recipients eryksun, paul.moore, steve.dower, tim.golden, zach.ware
Date 2019-08-19.21:20:14
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Here are two additional differences between mount points and symlinks:

(1) A mount point in a remote path is always evaluated on the server and restricted to devices that are local to the server. So if we handle a mount point as if it's a POSIX symlink that works with readlink(), then what are we to do with the server's drive "Z:"? Genuine symlinks are evaluated on the client, so readlink() always makes sense. (Though if we resolve a symlink manually, then we're bypassing the system's R2L symlink policy.)

(2) A mount point has its own security that's checked in addition to the security on the target directory when it's reparsed. In contrast, security set on a symlink is not checked when the link is reparsed, which is why icacls.exe implicitly resolves a symlink when setting and viewing security unless the /L option is used.

>  - if it's a directory junction, call os.stat instead and return that > (???)

I wanted lstat in Windows to traverse mount points by default (but I gave up on this), as it does in Unix, because a mount point behaves like a hard name grafting in a path. This is important for relative symlinks that use ".." components to traverse above their parent directory. The result is different from a directory symlink that targets the same path.

A counter-argument (in favor of winlinks) is that a mount point is still ultimately a name-surrogate reparse point, so, unlike a hard link, its existence doesn't prevent the directory from being deleted. It's left in place as a dangling link if the target is deleted or the device is removed from the system. Trying to follow it fails with ERROR_PATH_NOT_FOUND or ERROR_FILE_NOT_FOUND. 

Also, handling a mount point as a directory by default would require an additional parameter because in some cases we need to be able to open a junction instead of traversing it, such as to implement shutil.rmtree to behave like CMD's `rmdir /s`. 

Another place identifying a mount point is required, unfortunately, is in realpath(). Ideally we would be able to handle mount points as just directories. The problem is that NT allows a mount point to target a symlink, something that's not allowed in Unix. Traversing the mount point is effectively the same as traversing the symlink. So we have to read the mount-point target, and if it's a symlink, we have to read and evaluate it. (Consequently it seems that getting the real path for a remote path is an intractable problem when mount points are involved. We can only get the final path.)

---

Even without the addition of a new parameter, we may still want to limit the definition of 'link' in Windows lstat to name-surrogate reparse points, i.e. winlinks. Reparse points that aren't name surrogates don't behave like links. They behave like the file itself, and reparsing may automatically replace the reparse point with the real file. Some of them are even directories that have the directory bit (28) set in the tag value, which means they're allowed to contain other files. (Without the directory tag bit, setting a reparse point on a non-empty directory should fail.)

The counter-argument to changing lstat to only open winlinks is that changing the meaning of 'link' in lstat is too disruptive to existing software that may depend on the old behavior, i.e. opening any reparse point. I think the use cases for opening non-links are rare enough that it's not beyond the pale to change this behavior in 3.8 or 3.9.

> Right, but is that because they deliberately want the junction 
> to be treated like a file? Or because they want it to be treated 
> like the directory is really right there?

For copytree it makes sense to traverse a mount point as a directory. We can't reliably copy a mount point. In Unix, even when a volume mount or bind mount can be detected, there's no standard way to clone it to a new mount point, and even if there were, that would require super-user access. In Windows, we could wrap CreateDirectorExW, which can copy a mount point, but it requires administrator access to copy a volume mount point (i.e. "\\\\?\\Volume{...}\\"), for which it calls SetVolumeMountPointW in order to update the mount-point manager in the kernel. 

We also have a limited ability to create mount points via _winapi.CreateJunction, but it's buggy in corner cases and incomplete. It suffices for the reason it was added -- testing the ability to delete a junction via os.remove(). 

> os.rmdir() already does special things to behave like a junction 
> rather than the real directory, 

This is similar in spirit to Unix, except Unix refuses to delete a mount point. For example, if we have a Unix bind mount to a non-empty directory, rmdir() fails with EBUSY. On the other hand, rmdir() on the real directory fails with ENOTEMPTY. If Unix handled the mount point as if it's just the mounted directory, I'd expect the error to be the same. 

It's not particularly special in Windows unless it's a volume mount point. Then RemoveDirectoryW tries to call DeleteVolumeMountPointW. This could be a case where it would fail to remove a mount point, just like Unix. But the internal DeleteVolumeMountPointW call is allowed to fail if the caller doesn't have access to update the mount-point manager, in which case it removes the junction anyway.

The consequence of failing to update the mount-point manager is that GetFinalPathNameByHandleW calls will subsequently return a non-existing path for a volume that was mounted only in the deleted folder (i.e. the volume isn't also assigned a drive letter). Thus we can't assume the result from GetFinalPathNameByHandleW exists. This just pertains to volume mount points, which are special to the mount-point manager because it uses them to translate a native device path into a canonical DOS path. Bind mount points have no special significance to the mount-point manager. 

> the islink/readlink/symlink process is going to be problematic on 
> Windows since most users can't create symlinks. 

Then copying the symlink fails, which I think is better than silently transforming the behavior from a mount point to a symlink. Defensive code can fall back on physically copying the target file or directory. 

The latter is the default behavior for copytree. It's only an issue if code calls copytree(src, dst, symlinks=True). 

However, it's always a concern with shutil.move(), which attempts to move a file via os.rename. This fails for a cross-volume rename. Then if islink() is true, it falls back on os.symlink(os.readlink(src), real_dst) and os.unlink(src). 

(On my own systems, I grant the symlink privilege to the Authenticated Users group, which allows symlink creation by standard users and administrators -- elevated or not. But in general, a fear of symlinks is warranted, even in Unix.)

> I'm proposing to fix the inconsistency by fixing the flags. Your
> proposal is to fix the inconsistency by generating a new error in 
> unlink()? (Just clarifying.)

unlink() didn't used to remove junctions prior to 3.5 (see issue 18314). Instead of rolling back the change, or conflating the meaning of S_IFLNK, a counter-proposal is to harmonize unlink with the proposed change to lstat, i.e. to allow removing all name-surrogate directories. A name-surrogate directory cannot have children in the directory itself, so allowing it for os.unlink is in the spirit of the function, even if doing so is inconsistent with the literal specification. 

This is documented in ntifs.h:

    D [bit 28] is the directory bit. When set to 1, indicates that any
    directory with this reparse tag can have children. Has no special
    meaning when used on a non-directory file. Not compatible with the
    name surrogate bit [bit 29].

Regarding the directory bit, the registered tags with this bit are IO_REPARSE_TAG_CLOUD*, IO_REPARSE_TAG_WCI_1, and IO_REPARSE_TAG_PROJFS (for projected file systems).

> Currently Windows shutil.rmtree traverses into junctions and deletes 
> everything, though it then succeeds to delete the junction. 

That's like Unix mount-point behavior, except Windows allows a volume mount point to be deleted (not just a bind mount point), despite negative consequences to API functions such as GetFinalPathNameByHandleW if the user isn't allowed to update the system database of volume mount points.

An issue here, and with all code that walks a tree (especially destructively), is the link behavior of mount points. Bind mount points have the same problem in both Unix and Windows. For example, shutil.rmtree will fail to remove a mount point that targets a directory that it already removed. It's a different OSError in Unix vs Windows (EBUSY vs ENOENT or ERROR_PATH_NOT_FOUND), but an error all the same. That in itself is not an argument to handle a junction as a symlink, because it's still a mount point that behaves as such, even if someone is using it as a symlink. However, it is an argument for special handling of winlinks, which would allow the Windows implementation to behave better than Unix, IMO, in addition to helping Windows users that are forced to use mount points instead of symlinks.

> With my change, rmtree() directly on a junction now raises (could be 
> fixed?) but rmtree on a directory containing a junction will remove 
> the junction without touching the target directory. So I think we're 
> both happy about this one.

Changing rmtree to work on a target directory that claims to be a symlink would require special casing Windows in shutil.rmtree. But in general this is a problem that affects all code that looks for symlinks, not just code in the standard library.

If the meaning of S_IFLNK remains the same, then existing code has the option of being upgraded to delete directory winlinks without traversing them, but nothing is forced on them. In this case, for example, we could wrap the os.scandir call:

    if not _WINDOWS:
        _rmtree_unsafe_scandir = os.scandir
    else:
        import contextlib

        def _rmtree_unsafe_scandir(path):
            try:
                st = os.lstat(path)
                attr, tag = st.st_file_attributes, st.st_reparse_tag
            except OSError:
                attr = tag = 0
            if (attr & stat.FILE_ATTRIBUTE_DIRECTORY
                  and attr & stat.FILE_ATTRIBUTE_REPARSE_POINT
                  and tag & 0x2000_0000): # IsReparseTagNameSurrogate
                return contextlib.nullcontext([])
            else:
                return os.scandir(path)

For a directory winlink, the above _rmtree_unsafe_scandir function returns a context manager that yields an empty list, so _rmtree_unsafe skips to os.rmdir(path). This reproduces the behavior of CMD's `rmdir /s`, which will not traverse any name-surrogate reparse point (it checks the tag for the name-surrogate bit) even if the reparse point is the target directory.
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Date User Action Args
2019-08-19 21:20:15eryksunsetrecipients: + eryksun, paul.moore, tim.golden, zach.ware, steve.dower
2019-08-19 21:20:15eryksunsetmessageid: <1566249615.56.0.734355637631.issue37834@roundup.psfhosted.org>
2019-08-19 21:20:15eryksunlinkissue37834 messages
2019-08-19 21:20:14eryksuncreate