[Boost-build] Cache-file path handling bug on Windows

Subject: [Boost-build] Cache-file path handling bug on Windows
From: Jurko Gospodnetiæ (jurko.gospodnetic_at_[hidden])
Date: 2013-05-16 11:23:54

   Hi all.

   I encountered a slight problem when using the latest Boost Build HEAD 
on Windows.

   Not really sure what the project-cache.jam file created in the build 
folder is, but I found a use case when creating it on Windows fails due 
to a badly constructed file name.

   To reproduce do the following:
     1. Prepare an empty project folder.
     2. Add a jamroot.jam file containing just 'project XXX ;'.
     3. Open a command-prompt in that project folder.
     4. Run bjam with the following parameter:
         '"--build-dir=%CD%\xxx"'.

   This will cause output like:

> C:\Users\Jurko\ggg>bjam.exe "--build-dir=%CD%\xxx"
> ...found 1 target...
> ...updating 1 target...
> config-cache.write /C:/Users/Jurko/ggg/xxx/XXX/project-cache.jam
> The filename, directory name, or volume label syntax is incorrect.
>
>     type "C:\Users\Jurko\AppData\Local\Temp\jam15e4f4de.000" > "/C:/Users/Jurko/ggg/xxx/XXX/project-cache.jam"
>
> ...failed config-cache.write /C:/Users/Jurko/ggg/xxx/XXX/project-cache.jam...
> ...failed updating 1 target...
> ...found 1 target...

   The issue itself is not really important and does not have any 
serious repercussions on my projects. I just thought it would be better 
for Boost Build if such an ugly error message did not pop up.

   Problem seems to be caused by the jam target's path being constructed 
with an additional leading slash separator, i.e. using internal Boost 
Build instead of native OS path representation.

   The problem can be patched by tweaking the save() rule in 
config-cache.jam a bit. Instead of:
     local target = <new-cache-file>$(.cache-file) ;
do:
     local cache-file-native = [ path.native $(.cache-file) ] ;
     local target = <new-cache-file>$(cache-file-native) ;

   However, from my quick-glance through the code, I am not really sure 
this is a clean solution. I do not understand why the <new-cache-file> 
prefix is used nor did I check to see if other Boost Jam targets in 
Boost Build are constructed using their full path information or not.

   Could someone more familiar with this code (Steven?) take a look? Or 
should I go and dig deeper?

   Also, some test for this project-cache.py functionality should be 
added to the Boost Build test suite. I can add it if someone can explain 
to be what it is supposed to do.

   Best regards,
     Jurko Gospodnetiæ