[TCLCORE] TIP #402: General Platform UNC Support

[Jan N. <jan...@us...>]

 TIP #402: GENERAL PLATFORM UNC SUPPORT 
========================================
 Version:        $Revision: 1.1 $
 Author:         Jan Nijtmans <jan.nijtmans_at_users.sf.net>
 State:          Draft
 Type:           Project
 Tcl-Version:    8.6
 Vote:           Pending
 Created:        Saturday, 16 July 2011
 URL:            http://purl.org/tcl/tip/402.html
 WebEdit:        http://purl.org/tcl/tip/edit/402
 Discussions-To: Tcl Core list
 Post-History:   

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 ABSTRACT 
==========

 Both Windows and Cygwin interpret paths starting with *//* as a special 
 prefix, indicating that the path has the form: 
 *//server/share/file_path*. Windows has built-in handling of such paths 
 built-in. UNIX doesn't have this. It could be implemented through a VFS 
 extension, but there is one problem: File normalization on UNIX 
 collapes multiple slashes into a single slash, so this would convert 
 the UNC path into a normal path. This makes it impossible to implement 
 a VFS extension which uses *//* as prefix, implementing connection to a 
 Samba server using the UNC path format. 

 RATIONALE 
===========

 At the moment, Cygwin and Windows have built-in a special case that 
 paths starting with double-slash will not be collapsed into a single 
 slash. UNIX does not do that. This change will allow a single uniform 
 format for accessing (Samba) shares on external machines using the path 
 format *//server/share/file_path*. On Windows and Cygwin this already 
 works, because it is built-in Windows functionality. On UNIX a VFS 
 extension could be developed which does the same. 

 SPECIFICATION 
===============

 This document proposes: 

     * Extend the special case built-in for Windows and Cygwin to UNIX, 
       so paths starting with double-slash will no longer normalize to 
       paths starting with a single slash. 

       *POTENTIAL INCOMPATIBILITY* 

     * As *//* becomes a special prefix, the *file split* will return 
       *//* as its first list element when the original path starts with 
       a double slash. 

 COMPATIBILITY 
===============

 On UNIX, this means that paths like *//usr/bin/tclsh* might no longer 
 do what it did earlier, if a VFS exists which uses *//* as path prefix. 
 If no such VFS exists, it will probably still work, only comparing 
 normalized paths will no longer regard *//<foo>* as equal to */<foo>*. 

 Handling of multiple slashes in other locations of the string will not 
 change. So normalizing */foo//bar* will still give */foo/bar*. The most 
 likely cause of multiple slashes appearing in a path is because of 
 appending a file name to a path which already ends with a slash, e.g. 

   set dir "somedir/"
   set path $dir/filename

 Of course, the *file join* command does not have this danger: 

   set dir "somedir/"
   set path [file join $dir filename]

 On Windows and Cygwin, there is no change in behavior at all. 

 ALTERNATIVES 
==============

 Variations are possible in the handling of paths starting with 3 or 
 more slashes. The current TIP implementation collapes more than 2 
 slashes to exactly 2 slashes, as the current Windows and Cygwin 
 implementations do. 

 REFERENCE IMPLEMENTATION 
==========================

 A reference implementation is available at <URL:http://core.tcl.tk/tcl> 
 in branch /jn-unc-vfs/. 

 COPYRIGHT 
===========

 This document has been placed in the public domain. 

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