Re: [Boost-build] bjam handling of temps

Subject: Re: [Boost-build] bjam handling of temps
From: Steven Watanabe (watanabesj_at_[hidden])
Date: 2017-11-03 16:43:55

AMDG

On 11/03/2017 09:35 AM, Stefan Seefeld via Boost-build wrote:
> 
> I'm investigating an issue with bjam's handling of temporaries. It seems I'm 
> still not understanding how bjam handles them. Consider this code snippet:
> 
> actions make {
>    touch $(<)
> }
> 
> actions cat {
>    cat $(>) > $(<)
> }
> 
> make A ;
> cat B : A ;
> cat C : A ;
> cat D : B C ;
> 
> TEMPORARY B ;
> 
> DEPENDS B : A ;
> DEPENDS D : B C ;
> 
> UPDATE D ;
> 
> 
> Running `bjam` on this the first time creates A, B, C, and D, as expected. 
> Calling it again updates D again, and I don't understand why. If I remove the 
> TEMPORARY flag on B, D isn't updated, so this forceful update of D seems to be 
> caused by B being a temp.
> 
> The second issue seems even more serious: if I remove B, and call bjam again, 
> nothing is updated, as expected. But if I remove B and C and call bjam, I get an 
> error as D is attempted to be updated after C is remade, but B wasn't updated 
> (it's marked "temporary stable" in the debug output). Why isn't B updated first ?
> 
> The logic in make.c suggest's that, despite B's binding is set to PARENT (i.e., 
> D), its fate is determined before that of D, and thus is found to be stable, 
> even though a little later D's fate is set to update. Shouldn't a temporary's 
> fate be determined *after* its parent ?
> 
> What am I missing ?
> 

Two things:

First of all, a TEMPORARY target is supposed to be
deleted:

actions RmTemps {
  rm $(>)
}
...
cat D : B C ;
RmTemps D : B ;

  Rebuilding when a temporary is present seems to be
deliberate, though I'm not quite clear on the logic
behind this.
#define T_FATE_SPOIL          4       /* >= SPOIL rebuilds parents */
#define T_FATE_ISTMP          4       /* unneeded temp target oddly
present */

Second, TEMPORARY is really designed to be used with the
`updated` actions flag:

actions piecemeal updated ar {
  ar rc $(<) $(>)
}

TEMPORARY x.o y.o z.o ;
ar x.a : x.o y.o z.o ;
RmTemps x.a : x.o y.o z.o ;

In this case, rebuilding x.a because of one changed source,
should not force the other object files to be created.

In Christ,
Steven Watanabe