RE: [expath] Re: Finalizing the File Module

I don't think operating system support is required here. The EXPath implementation could simply maintain its own cwd.
As long as it is not changeable, multitasking shouldn't cause any problems either I think.


MikeDavid Lee <dlee@calldei.com> wrote:I would advise against anything relying heavily on the concept of "the current directory".
This is a concept that is simply not reliably portable.  As tantalizing as it is to believe in "cwd" its a fantasy of epic proportions to bet on it.
 
Reasons:
Some operating systems simply do not have the concept at all - it simply doesnt exist.
In systems where "cwd" is a OS feature not all portions of a program have the same concept,
for example in windows and unix the "cwd" is a process-wide setting.   Thats simply not convenient for many uses such as multi-tenancy and multi-threaded applications ... so the concept is often virtualized to support those applications.  The problem is the virtualization is not always complete - because to virtualize the CWD usually implies that different components can independently set it ... yet this is not universally supported.   Even in OS's where the "cwd" is well defined (at a process level) its amazing and surprising to discover that many libraries will actually change it on you without your knowledge - sometimes in a thread-unsafe way.   A common (legacy) method of deterring the absolute path of the current directory on unix is to keep doing a chdir("..") and storing the inodes of "." until you cant anymore then unwind the stack.   Try opening a file in a different thread via "." while this happens ... :)
 ( just a FYI on unix/linux the cwd is not stored as a path its an inode and may have multiple equivalent paths).
 
Java implementations, for example, recognize that modifying the CWD is not supportable so simply don't expose any method at all to change it.  In fact the Java JRE doesnt directly support the concept of a CWD at all ... but that doesn't stop people from pretending (such as trying to get the absolute path of ".")   In xmlsh I attempt to support changing the cwd on a per-shell (which may or may not be a separate thread).  This largely but not entirely works.  It all depends on what API you use ... because I cant set the "real" CWD its virtualized ..  and only if you go through codepaths that know about this will it work.
 
All in all ... if you want a reliable and portable file path handling I suggest you never use the current directory explicitly but rather force all file references to be resolved against an absolute path ... that is generally a supportable feature.  You can set this path in various places in your module and "call it" the "current directory" but you need to be  very careful of all the places you make assumptions about resolving paths ... do they take the system CWD (if such a thing exists? ) do they take your set CWD ? at what snapshot in "time" ? can different modules have different settings for the cwd ? Is it part of a pushed/popped environment or globally set ? What scope ?  
Its very compelling to pretend that the problem is easy ... but its not.
 
-David
 
 
 
 
 
 
Hi Christian, 

I find both functions useful, especially current-dir(). It stands to reason that an XQuery application may be very interested in the current directory, in order to be able to resolve relative paths against the current dir. It should also be noted that file:list resolves relative paths against the current dir, so that subsequent use of the listed file names as URI argument in doc() simply MUST explicitly resolve against the current directory, as the implicit resolving performed by doc() is against the base URI of the query, which means the documents are not found (unless the query is contained by the current directory).

Also noteworthy is that retrieval of the current directoy via file:parent requires a different argument than '.', as using '.' yields the parent directory of the current directory, rather than the current directory. This in turn means that retrieval of the current directory via file:parent requires the use of some arbitrary (and possibly fake) file name as argument, like so

   file:parent('dummy')

It is very desirable that retrieval of the current directory can be achieved in a straightforward way, rather than the "tricky" way via file:parent.

Kind regards,
Hans-Jürgen



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