[racket-dev] Symlink trouble

From: Tobias Hammer (tobias.hammer at dlr.de)
Date: Wed Apr 17 11:19:58 EDT 2013

On Wed, 17 Apr 2013 16:39:22 +0200, Matthew Flatt <mflatt at cs.utah.edu>  

> Yes, I think Racket should use PWD --- if the expansion of soft links
> produces the same path as getcwd(), which seems to be what "/bin/pwd"
> does.

That check is even better than the one i had in mind. That should prevent  
possible ambiguities.

> Should Racket also set PWD (optionally, but by default) when it creates
> a subprocess? I think probably so.

Yes, i think that is a sensible default. Setting only the os cwd, as
racket currently does would cause the same problems for any subprocesses
that uses this information (at least most shells and, in the future,  

> To make sure we're all on the same page:
> The general problem is that there can be more than one filesystem path
> that reaches a file. It would be great if we could normalize every path
> to a canonical form, but path normalization in general seems to
> intractable due to the possibilities of soft links, hard links,
> multiple mount points, case-sensitivity choices, and probably other
> twists that I'm forgetting. We have therefore settled on different
> definitions of "same file", depending on the context.


> For module paths, "same file" involves only syntactic normalizations of
> the pathname (e.g., no checking for soft links). Various pieces of the
> system are carefully implemented to be consistent with syntactic
> normalization. For example, suppose that PLTCOLLECTS is set to
> "/home/mflatt/plt", but "/home/mflatt" is a symlink to "/Users/mflatt";
> pathnames associated to modules that are accessed via collection will
> consistently use "/home/mflatt", and not somehow hop over to
> "/Users/mflatt". As long as a user is similarly consistent when
> supplying paths, it all works out.

That matches my observations. Files accessed via collection always keep  
paths 'as is'. But it is enough to start a program via racket <file>  
instead of
racket -l what/ever to break this. Therefore it seems to be the normal  
case that
it fails (at least for console racket).

> Unfortunately, `current-directory' is a place where you don't get to
> choose the path. You might say "/home/mflatt/plt" to get to a Racket
> installation, but to initialize `current-directory', the path gets
> turned into an inode and back to a path via getcwd() --- exactly the
> sort of thing that breaks a syntactic view of "same".


> The PWD environment variable addresses the problem with getcwd(): nice
> shells set PWD based on a syntactic derivation of the current
> directory, instead of an inode-based derivation.
> So, Racket should take advantage of the information that nice shells
> provide. Probably it should also act as a nice shell by default.

What exactly do you mean by acting as a nice shell? Setting PWD for  
In that sense it should definitely be nice (by default).

> (As it happens, I use "csh" on Mac OS X, and it's not nice in the above
> sense. That helps explain why I never got PWD vs. cwd() before.)

Just tried bash, csh and ksh on linux and they all seem to set PWD. But i  
can't tell
if thats the default or specific to my installation.

> At Wed, 17 Apr 2013 12:06:29 +0200, Tobias Hammer wrote:
>> Hi,
>> i am currently implementing an application that heavily relies on  
>> rackets
>> great serialize functionality to exchange data between racket processes  
>> on
>> different computers. That works well until i stumbled over a very
>> confusion behavior of rackets filesystem and module path resolution.
>> I will explain first, what i observed and then why this causes some
>> trouble:
>> * relative (module) paths are resolved with something like (or
>> (current-load-directory) (current-directory))
>> * collection paths are resolved with
>>   (find-executable-path (find-system-path 'exec-file) (find-system-path
>> 'collects-dir)) for the system collection and with the given path for  
>> the
>> others
>> * you can require a module relative and via collection, if they resolve  
>> to
>> the same name, there is no error
>> serialize stores the module path and symbol where the deserialize  
>> function
>> can be found. It's interesting how this module path is determined
>> * If the file containing the deserialize identifier (if implemented by
>> hand or the file where e.g serializable-stuct is used) is loaded via
>> collection, then the serialized stream contains a collection path
>> (determined via identifier binding and mpi magic)
>> * If this file is loaded relative, the fallback method with
>> current-(load)-directory is used
>> Nothing special so far, but the fun starts with how current-directory is
>> initialized. It uses (on *nix systems) getcwd() but this function  
>> returns
>> the path with all symbolic links resolved (getcwd is only a thin
>> OS-wrapper, and the OS provides nothing else).
>> This little detail can easily break the serialization framework (and  
>> maybe
>> other things too).
>> The scenario is a file that is in a path containing a symlink and that  
>> is
>> in the current collections, e.g
>> /abc/symlink/more/def/file.rkt
>> and PLTCOLLECTS="/abc/symlink/more:"
>> and file.rkt contains a serializable-struct definition.
>> Now one racket process loads "file.rkt" relative, serializes a struct
>> instance and sends it to another racket process. The other process loads
>> def/file via collection and deserialies the struct. The receiver now  
>> has a
>> struct that is of a different type and that he can't access.
>> This fails because the serialized data contains the absolute  
>> symlink-free
>> path that differs from the path the receiver used to load file.rkt
>> (because for collection dirs symlinks are not resolved).
>> The same happens of course when the data is send to another computer  
>> that
>> has a symlink in the path to file.rkt, even if they both load the same  
>> way.
>> The confusing thing is that from the users point of view everything is
>> consistent. His working directory and collections all point to the same
>> location.
>> It is clear that this behavior is by far not limited to racket as nearly
>> all programming languages use getcwd internally. A quick google search  
>> for
>> getcwd and symlinks gives a lot of results...
>> I came up with a few solutions but i would like to get some feedback on
>> them. They all more or less use that the shell keeps track of the 'real'
>> (better: visible) working directory. Most *nix shells set 'PWD' in the
>> environment but it is not guaranteed and can of cause be altered by the
>> user.
>> - The quick and very dirty hack is to set the current-directoy before  
>> any
>> use code is executed
>> racket -e '(current-directory (or (getenv "PWD") (current-directory)))'
>> program.rkt
>> Too ugly to really use it...
>> - A better fix would be to change how the current-directory parameter is
>> initialized during the startup. It could be some heuristic that tries to
>> use the env-variable if it is a complete and existing path and falls  
>> back
>> to getcwd otherwise. As far as i can tell this won't break anything
>> because after this one time at startup the C-sides cwd and rackets
>> parameter are completely decoupled.
>> - A more conservative solution would be a command line argument to  
>> racket
>> to set the initial value for current-directory. One could then populate  
>> it
>> with env's PWD or from `pwd` or whatever suits.
>> I would appreciate any feedback on how i can work around this behavior
>> (except don't use symlinks ...) or if i missed something obvious. If  
>> not,
>> would any of the two real solutions be viable? They shouldn't be too  
>> hard
>> to implement i could create a patch if one of them seems ok.
>> Tobias
>> --
>> ---------------------------------------------------------
>> Tobias Hammer
>> DLR / Robotics and Mechatronics Center (RMC)
>> Muenchner Str. 20, D-82234 Wessling
>> Tel.: 08153/28-1487
>> Mail: tobias.hammer at dlr.de
>> _________________________
>>   Racket Developers list:
>>   http://lists.racket-lang.org/dev

Tobias Hammer
DLR / Robotics and Mechatronics Center (RMC)
Muenchner Str. 20, D-82234 Wessling
Tel.: 08153/28-1487
Mail: tobias.hammer at dlr.de

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