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Date: Tue, 14 Feb 2006 07:18:41 -0700
From: Eric Blake <ebb9 AT byu DOT net>
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To: Larrie Carr <larrie AT telus DOT net>
CC: "John W. Eaton" <jwe AT bevo DOT che DOT wisc DOT edu>, cygwin AT cygwin DOT com
Subject: Re: [octave ] LOADPATH recurses only one level of subdirectories  (on network drives)
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Ugh - top-posting reformatted http://cygwin.com/acronyms/#TOFU

According to Larrie Carr on 2/11/2006 11:25 PM:
>>> Probably the code you are looking for is the function do_subdir in
>>> liboctave/kpse.cc.  This file contains a stripped-down version of the
>>> kpathsearch library.  Most modifications were to remove TeX-specific
>>> stuff and to convert it to use std::string instead of plain C strings
>>> which historically leaked memory.  In any case, that function may use
>>> an optimization to decide when to check for subdirectories.  The
>>> optimization looks at the link count of the current directory.  If it
>>> is 2, then the assumption is that the current directory does not
>>> contain any subdirectories.  That seems to work fine for Unixy
>>> systems.  Does that assumption not hold for Cygwin?  If so, then I
>>> think the fix is fairly simple as there is also Windows-specific code
>>> in that function.  Whether the optimization is performed depends on
>>> what is #defined at compile time, so you'll probably have to do some
>>> checking on a Cygwin system to see what is really going on.
>>

> So the punch line is that octave will not work with network drives due
> to the difference on how "stat" returns the number of hard links.
> Octave uses stat to determine if the directory is recusible.  But you
> can replicate the problem with using stat on the command line.
>
> $stat -c "%h %f" /cygdrive/c/test
> 2 41c0
>
> $stat -c "%h %f" /usr/share/octave
> 1 41ed
>
> $stat -c "%h %f" /cygdrive/x/cygwin/usr/share/octave
> 1 41ed
>
> $ls -l /usr/share/octave
> total 0
> drwxr-xr-x 1 larrie mkpasswd 0 Feb   8 23.31 2.1.72
> drwxr-xr-x 1 larrie mkpasswd 0 Feb   8 23.31 site
>
> The code checks for two links (the %h) given that a subdirectory should
> have a "." and a ".." entry.  But for some reason, network drives
> created using windows within cygwin report 1.

Because it is too expensive for cygwin to report an accurate link count
(stat() of the remote dir would effectively have to stat every file in
that dir to see how many subdirectories there were, which would be quite
expensive over the network).

>
> So I'm at the edge of my understanding - should cygwin be reporting 2 or
> 1 or is octave using a method that works on every other system except
> cygwin.

find has the same optimization of using the directory link count to decide
in advance how many subdirectories it should expect from readdir(), so
that it can quit the directory search early once all the subdirs have been
found.  However, find takes care that if the link count of a directory is
less than 2, then the optimization must not be performed.  POSIX does not
guarantee the link count of directories (and both mount points, and
systems that allow directory hard links [although cygwin does not allow
directory hard links] can mess up the traditional semantics), so it is a
bug in octave if it is mis-optimizing traversal in the presence of a
directory link count of 1.  It might make sense, though, for cygwin to set
the link count to 0 on remote directories (rather than 1), to make it
obvious that the link count really is unknown, but this still does not
take the blame off of octave for the mis-optimization.

- --
Life is short - so eat dessert first!

Eric Blake             ebb9 AT byu DOT net
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