Re: [boost] [filesystem] proposal: treat reparse files as regular files

Subject: Re: [boost] [filesystem] proposal: treat reparse files as regular files
From: Niall Douglas (s_sourceforge_at_[hidden])
Date: 2015-07-27 07:30:55

On 27 Jul 2015 at 10:55, Paul Harris wrote:

> > However, they all still look like symlinks to me. Just because the OS
> > magically replaces them with the real file on first access is
> > immaterial - the same thing could happen on Linux. If you don't treat
> > them as symlinks, there is no way of inspecting the link without
> > causing it to be auto-downloaded which could be catastrophic in some
> > use cases.
> >
> > I still vote for pseudo-symlinks to be reported by Filesystem as
> > symlinks.
> >
> 
> 
> I did think about that, but the design of these reparse points intends for
> these files to be treated as plain files by the client - as per MS
> documents.

This is like saying that POSIX symlinks are intended to be treated as 
their target, which is the whole point of using them.

Reparse points are the *technology* by which Microsoft implemented 
symlinks in NTFS. They offer a *family* of symlink implementations, 
all with varying semantics. Some of that family bear strong 
resemblence to the much more limited POSIX symlink, others are quite 
different.

If you weren't on NTFS, the technology used to implement symlinks is 
different. For example, the NT kernel provides its own non-persistent 
symlink implementation totally separate from NTFS.

> Plus, I understand it as: the reparse buffer is entirely driver-specific,
> and so you can't expect boost or any user program to be able to decode what
> is inside the reparse buffer and do anything intelligent.

Microsoft have published the structure for their reparse tag formats. 
Anyone can parse that structure (AFIO does).

>  AND the
> resolving is done by the driver on the server side.  Note that there are
> probably a dozen products out there that use these reparse buffers for
> their storage solution... its not just windows dedup.

The resolution varies actually. For example junction points are 
resolved server side, symlinks are resolved client side.

> So, I don't see how the client can't do anything intelligent with symlink
> knowledge,
> AND if boost library users are forced to treat them as symlinks, then you
> now have 2 kinds of symlinks:
> 
> 1) standard symlink, which you really want a shallow copy sometimes, and
> you have to be careful of loops ( A -> B -> A )
> 
> 2) reparse (but not symlink), which you cannot shallow-copy (as far as I
> understand), and loops are not possible.

You can copy the standard Microsoft reparse points as those are 
documented.

I see no reason why Filesystem's read_symlink(), create_symlink() and 
copy_symlink() all don't work just fine if upgraded to understand 
more reparse point types.

> * My software doesn't want to follow links, but now the new version will
> force me to specifically check if its just a reparse-file and then follow.

No, that depends on whatever the OS does with the symlink. Ordinarily 
I would assume it dereferences the link unless you specifically ask 
for it not to, same as on POSIX i.e. if you lstat() it, it returns 
the stat for the symlink, if you stat() it it returns the stat for 
the target.

> * Whole-disk backup software don't follow symlinks because they assume
> they'll get the real file later.  Reparse (nonsymlink) files do not have
> any other "real file" so those files are not being backed up at all right
> now.
> 
> So treating as a symlink causes more trouble than the helping the one edge
> case.
> 
> reparse-files-non-symlink is such a specialised case, I'd personally want a
> specialised get_reparse_info kind of function, so if I really need to care,
> then I can find that information.
> 
> Your thoughts?

I think Filesystem should provide what POSIX provides. Where Windows 
provides close enough to POSIX behaviours we should support that too.

However pages of special Windows support isn't what Boost does 
usually. We're here to abstract out the commonalities generally 
speaking.

I agree Filesystem (and AFIO) should recognise deduped files as 
something valid and can be worked with. Anything past that is up to 
the end user.

Niall

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