[Scons-users] writing your own decider

[Dirk Bächle]

Hi Tom,

On 03.04.2014 18:07, Tom Tanner (BLOOMBERG/ LONDON) wrote:

> I've put some trace in my Decider function and found it can be called 

> twice for the same dependency/target pair (at least on 2.2). Is that 

> intentional? And if so, why is it needed? Is it safe to cache the 

> results if the check is expensive?

>

the decider function is called from within changed(), which itself may 
get called several times during a build. I tried to cache the result of 
changed() for my memory reducing optimizations, but had to step it back 
a little, see:

http://two.pairlist.net/pipermail/scons-dev/2014-January/001034.html

(start of thread at 
http://two.pairlist.net/pipermail/scons-dev/2014-January/001009.html ). 
So, yes this is intentional and caching in the wrong places may lead to 
spurious rebuilds. However, if you can guarantee that neither the file 
(once it exists) nor one of its implicit dependencies changes anymore 
during the build, you should be able to cache the result of the decider.

Best regards,

Dirk

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