[Scons-users] env[X'] and signatures

[Tom Tanner]

On 29/12/16 17:50, Bill Deegan wrote:
> As a workaround, perhaps add an action string which does nothing, but 
> uses the variables you depend on?
> Just a thought.
>
>
> Are the builders external python scripts which get run from the shell 
> by SCons? or are the builders python logic run inside scons?
>

I don't know about the original poster, but ours are generally executing 
other programs and if I can see an environment variable lurking, I make 
sure it's used on the command line as all command lines currently go via 
the shell, so putting 'MYVAR=$(MYVAR) ...' on the command line sorts 
that (at the cost of scons not recognising the actual script you're 
running being a dependency unless you tell it to). But there are those 
that get missed and then as I previously commented it can cause fun

Having to pass the environment explicitly to an action would make it 
clearer to people what they were doing (case in point: we have one 
program that needs to have HOME set and I'm pretty sure I'm dealing with 
that incorrectly, but it's slightly invisible and I'm not sure how to 
pass what I need in an action, though I suspect I'm missing something 
obvious)
> On Thu, Dec 29, 2016 at 8:54 AM, Tom Tanner <trtanner at btinternet.com 
> <mailto:trtanner at btinternet.com>> wrote:
>
>     On 27/12/16 18:22, Bill Deegan wrote:
>
>         If every builder depends on all values in env['ENV'], then any
>         change will rebuild everything.
>         This is almost never warranted.
>
>
>     But then why does scons clear out the supplied environment? It is
>     precisely because everything behaves in a way that is unclear and
>     not reproducable if you don't know exactly what the environment is.
>
>         I think, ideally, a builder (Action really), would specify
>         what parts of the Environment() it's sensitive too.
>
>
>     This would be better. If every builder specified which parts of
>     env['ENV'] it is interested in, and they (and nothing else) got
>     passed through and added to the build signature
>
>
>         Remember, while many users are only building source code which
>         is fairly quick, SCons is used in other "build" systems where
>         steps can take considerable time and resources so extraneous
>         rebuilds are quite costly for them.
>
>         -Bill
>
>
>     I agree but specifying the correct dependencies are how you avoid
>     costly rebuilds. Not rebuilding something that has changed can be
>     just as costly because instead of finding out about it at the
>     build stage, you find out about it when it is on a production
>     machine.
>
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