readdir/telldir/seekdir problem (i think)

[Rick Macklem]

John Baldwin wrote:
> On Thursday, April 23, 2015 05:02:08 PM Julian Elischer wrote:
> > On 4/23/15 11:20 AM, Julian Elischer wrote:
> > > I'm debugging a problem being seen with samba 3.6.
> > >
> > > basically  telldir/seekdir/readdir don't seem to work as
> > > advertised..
> > 
> > ok so it looks like readdir() (and friends) is totally broken in
> > the face
> > of deletes unless you read the entire directory at once or reset to
> > the
> > the first file before the deletes, or earlier.
> 
> I'm not sure that Samba isn't assuming non-portable behavior.  For
> example:
> 
> From
> http://pubs.opengroup.org/onlinepubs/009695399/functions/readdir_r.html
> 
> If a file is removed from or added to the directory after the most
> recent call
> to opendir() or rewinddir(), whether a subsequent call to readdir()
> returns an
> entry for that file is unspecified.
> 
> While this doesn't speak directly to your case, it does note that you
> will
> get inconsistencies if you scan a directory concurrent with add and
> remove.
> 
> UFS might kind of work actually since deletes do not compact the
> backing
> directory, but I suspect NFS and ZFS would not work.  In addition,
> our
> current NFS support for seekdir is pretty flaky and can't be fixed
> without
> changes to return the seek offset for each directory entry (I believe
> that
> the projects/ino64 patches include this since they are breaking the
> ABI of
> the relevant structures already).  The ABI breakage makes this a very
> non-trivial task.  However, even if you have that per-item cookie, it
> is
> likely meaningless in the face of filesystems that use any sort of
> more
> advanced structure than an array (such as trees, etc.) to store
> directory
> entries.  POSIX specifically mentions this in the rationale for
> seekdir:
> 
> http://pubs.opengroup.org/onlinepubs/009695399/functions/seekdir.html
> 
> One of the perceived problems of implementation is that returning to
> a given point in a directory is quite difficult to describe
> formally, in spite of its intuitive appeal, when systems that use
> B-trees, hashing functions, or other similar mechanisms to order
> their directories are considered. The definition of seekdir() and
> telldir() does not specify whether, when using these interfaces, a
> given directory entry will be seen at all, or more than once.
> 
> In fact, given that quote, I would argue that what Samba is doing is
> non-portable.  This would seem to indicate that a conforming seekdir
> could
> just change readdir to immediately return EOF until you call
> rewinddir.
> 
Btw, Linux somehow makes readdir()/unlink() work for NFS. I haven't looked,
but I strongly suspect that it reads the entire directory upon either opendir()
or the first readdir().

Oh, and I hate to say it, but I suspect Linux defines the "standard" on
this and not POSIX. (In other words, if it works on Linux, it isn't broken;-)

rick

> --
> John Baldwin
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