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

[Dirk Bächle]

Hi all,


On 26.12.2016 20:43, William Blevins wrote:
> It makes sense that the env should be the same for building and 
> cleaning because the dependency tree is based on the environment. 
> Explicit library dependencies must come from where libraries are 
> listed. The value node is admittedly more obscure.
>
> Glad you figured it out.
>

the canonical solution would be to use the "varlist" keyword for the 
Action class, I guess. Since the Builder wrapper "Command()" doesn't 
propagate any keywords to the underlying Action() constructor, one has 
to setup a complete Builder on its own:

   import SCons.Action
   import SCons.Builder

   myAction = SCons.Action.Action('./create.sh', varlist=['MYENVVAR'])
   myBuilder = SCons.Builder.Builder(myAction, ...)

(untested and from the top of my head). Note how instead of the 
"create.sh" string an actual Python function may be used too.
In the MAN page (search for the "varlist" keyword) there is also the 
short form described, where all additional arguments (idx > 2) for the 
Action() constructor are evaluated as "varlist entries":

    # Use positional arguments.
    a = Action(build_it, '$STRINGIT', ['XXX'])

    # Alternatively, use a keyword argument.
    a = Action(build_it, varlist=['XXX'])

When the signature of the Action gets computed to decide whether a 
content change has occurred, SCons will subst() all values in the 
"varlist" and add the result to the cached signature.
This should have the desired effect of triggering a rebuild when the 
content of the environment variable "MYENVVAR" changes.

Best regards,

Dirk

> On Dec 26, 2016 1:52 PM, "Fred Wright" <fw at fwright.net 
> <mailto:fw at fwright.net>> wrote:
>
>
>     On Mon, 26 Dec 2016, William Blevins wrote:
>
>     > I do also, but at one point, I got the impression (from him)
>     that this
>     > changed in SCons. I don't think this was supported implicitly,
>     but it could
>     > have been supported explicitly by adding the variable to the
>     hash or other.
>     > This is why I wanted to know the SCons version that "broke" this.
>
>     If the details of this behavior are well-documented, it's certainly
>     escaped me. :-)
>
>     I've actually been burned recently in both directions by this
>     issue.  On
>     the one hand, I found that if LIBPATH isn't set up the same way for
>     cleaning as it is for building (*), then some targets don't get
>     cleaned.
>     On the other hand, a Python builder taking an environment variable
>     as an
>     input doesn't get rerun if the value changes.  To fix that, I added a
>     Value() node derived from the environment variable, and added it as an
>     explicit dependency.
>
>     * The Configure section is so slow and verbose that it's useful to
>     suppress most of it when "cleaning or helping".  But one has to be
>     careful
>     not to perturb the definitions of cleanable targets.
>
>     Fred Wright
>
>     > On Dec 26, 2016 3:44 AM, "Tom Tanner (BLOOMBERG/ LONDON)" <
>     > ttanner2 at bloomberg.net <mailto:ttanner2 at bloomberg.net>> wrote:
>     >
>     > > I think he's talking the situation where you set
>     env.env['MYVAR'], which
>     > > means for something like this
>     > >
>     > > env.env['MYVAR'] = 'Fish'
>     > > env.Command('some_random_script < $SOURCE > $TARGET', sources,
>     targets)
>     > >
>     > > and 'some_random_script' does
>     > >
>     > > #!/bin/sh
>     > > echo $MYVAR
>     > >
>     > > and you then change the SConscript file and set
>     env.env['MYVAR'] to
>     > > 'Chips', it won't cause a rebuild.
>     [...]
>
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