ALWAYS + NOUPDATE?

From: Douglas Gregor (gregod_at_[hidden])
Date: 2003-03-31 19:32:39

I would like to have a target (an XML catalog) that is both ALWAYS and 
NOUPDATE, meaning that:
1) The action for this target is always executed (ALWAYS does this)
2) The action for this target is executed before anything that depends on it 
(i.e., any target built via XSLT) (the generated dependency does this)
3) Regeneration of this target does not result in regeneration of its 
dependencies (NOUPDATE does this)

I can get any two of three, but not all three. It seems that the combination 
of ALWAYS and NOUPDATE should do this.

Here's a short testcase: create a file foo.txt (or mark the target NOTFILE; 
doesn't matter) and execute bjam twice on this Jamfile:

rule make-bar ( target : source )
{
ALWAYS $(source) ;
NOUPDATE $(source) ;
DEPENDS $(target) : $(source) ;
make-bar-action $(target) : $(source) ;
}

actions make-bar-action
{
echo "Making a bar from a foo" > bar.txt
}

make-bar bar.txt : foo.txt ;
DEPENDS all : bar.txt ;

The first time through, bar.txt should be generated because it doesn't exist. 
The second time through, bar.txt should not be regenerated because it's only 
dependency, foo.txt, is marked "NOUPDATE". Note that the ALWAYS foo.txt 
doesn't make much sense here: in the real example, foo.txt is generated from 
module settings, so we need to generate it every time.

My questions are:
1) Is there a way to do this without hacking Jam?
2) Would a Jam patch be accepted to deal with ALWAYS + NOUPDATE with the 
semantics above?

Doug