Re: readdir/telldir/seekdir problem (i think)

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.

-- 
John Baldwin