[armedbear-devel] undesirable behavior in #'directory ?

[Alan Ruttenberg]

There are, I would offer, several arguments suggesting that the
current behavior should not be the default.

1) Exceptions: Generally, listing the contents of a directory is not
considered an exception unless the directory structure is corrupted.
In some cases (e.g. unix "rm") you can't even do anything to the
target of the link. So ABCL signals an exception when most would not
expect it. Consider the case where the link points to a file on a
device that may or not be mounted. If the device is taken offline,
nothing in the directory changes, and yet the behavior does. It seems
that this situation is more properly handled by an exception when
opening the file.

The doc for truename says, about conditions: "An error of type
file-error is signaled if an appropriate file cannot be located within
the file system for the given file spec or if the file system cannot
perform the requested operation.". Whether or not the symbolic link is
a file or not. Certainly for some cases it is, for example when
handled by archiving or certain version control systems, or in the
cases I list in (4). In the case of "rm", the answer is yes, it is a
file, implicit from the documentation: "The rm utility attempts to
remove the non-directory type files specified on the command line.
[...] The rm utility removes symbolic links, not the files referenced
by the links. In ABCL, (directory "/") -> (#P"/"). This means that a)
ABCL is inconsistent in that it sometimes returns truenames for things
that can not be "opened" or b) ABCL admits that there can be truenames
for entities other than files, which makes it's treatment of symbolic
links inconsistent.

2) Pragmatism: With the current default, the only possible
programmatic repair in the common case that you don't care about these
entries - for example if you are looking for a file whose name matches
a pattern not expressible using the directory wild card expressivity -
is to use an implementation-specific keyword. CCL's implementation
also has this property, which I also consider to be a fail. In
practical terms this means that the unsuspecting programmer must wrap
all calls to directory with a catch and have the handler respond in an
implementation-specific way. It is much more common to protect
accesses to a file than accesses to a directory.

3) Truth in advertising:  The exception happens independent of the
value of the :resolve-symlinks keyword. If the function is told not to
resolve symlinks, one would expect it doesn't resolve symlinks. Yet it
does, since that's the only way that it could figure out that the
"file does not exist".

4) File system operations other than opening: There are legitimate
operations on unresolvable links. For example, such files can be
removed, renamed, and there are retrievable dates and other metadata
retrievable about them. One would not expect directory to balk in the
case that you are retrieving file names for one of these purposes.

My conclusion:
- If resolve-symlinks is false, the behavior should be either that of
SBCL's (return the name) or CCL's (don't return the name) depending on
which answer the implementation takes towards the question "is a
symbolic link a file?".
- If resolve-symlinks is true then signal an error.
- The default should be :resolve-symlinks nil, because cognate
directory operations in every operation's default case that I'm aware
of is to not consider this case an exception.

Regarding CCL's application-specific reference to emacs, I simply
think the argument is named poorly. In fact directory will return
paths to any links that do not resolve, not only emacs lock files.  A
better argname would be :include-unresolvable-links.

This email was tool long :)

-Alan

On Fri, Jan 10, 2014 at 6:44 AM, Mark Evenson <evenson at panix.com> wrote:
>
> On Jan 7, 2014, at 19:40, Alan Ruttenberg <alanruttenberg at gmail.com> wrote:
>
>> Well, SBCL returns the name of the directory entries(without resolving
>> symbolic links) when I use the directory function and truename returns
>> the directory entry unmolested for the  problematic entry. I guess
>> sbcl precedes Faré ;-)
>>
>> Note this is despite the fact that sbcl also has a :resolve-symlinks
>> argument to directory, and that it also defaults to true.
>>
>> I think this should be the behavior for ABCL, with the resolving and
>> probing separated altogether from the truename and directory code.
>>
>
> It is [quite clear from the Hyperspec][DIRECTORY] that DIRECTORY "returns a fresh list of pathnames corresponding to the truenames of those files.”
>
> And it is [quite clear as well][TRUENAME] that calling TRUENAME on a non-existent file should signal a file error.
>
> What is not clear is whether the “fresh list of pathnames” returned by DIRECTORY is the actual result of calling TRUENAME on the filespec.  I guess it doesn’t have to be, which is what SBCL and CCL seem to be implementing.
>
> I guess I am leaning towards adding a further arg to DIRECTORY “:RESOLVE-ENTRIES-VIA-TRUENAME” (terrible name, should be shorter) which defaults to NIL, which would preserve the current behavior.  Otherwise, we would treat symbolic links in the same manner as SBCL:  populate the PATHNAME with what would be used in making a TRUENAME call, but don’t actually make the call.
>
> Adding an application specific (“ignore Emacs backup files”) mechanism like CCL to the Common Lisp part of things smells wrong: one should at least implement the rudiments of an extensible API that would allow other filespecs to be added to the list of directory entries to be treated in this manner.
>
> But I do think that the ANSI spec intends that once one has the results of the DIRECTORY call, and the relevant parts of the filesystem are not changed in the meantime, applications expect that the results remain valid TRUENAMEs (i.e. they can be accessed).
>
> In practice, I guess this is why everyone has their own toolsets for dealing with filesystems.
>
> Comments?
>
>
> [DIRECTORY]: http://www.lispworks.com/documentation/HyperSpec/Body/f_dir.htm
> [TRUENAME]: http://www.lispworks.com/documentation/HyperSpec/Body/f_tn.htm
>
> --
> "A screaming comes across the sky.  It has happened before but there is nothing
> to compare to it now."
>
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