[Scons-users] Bug in FunctionAction: not allowing custom get_contents

[Bill Deegan]

Interesting.
I'd say you're the first person to try this, thus the existing code doesn't
function as you were expecting..

File and Issue (or a PR ) on github?

On Fri, Sep 18, 2020 at 11:31 PM Jeremy Elson <jelson at gmail.com> wrote:

> Hi! First, let me just say congratulations on making a wonderful build
> system. I've been using it to manage a very complex build system and it has
> worked wonderfully.
>
> I believe I have found a bug in a somewhat obscure area of scons. I'm
> creating a custom Builder with a FunctionAction, i.e. an action that calls
> a Python function. I want to override get_contents so that my action can
> tell scons when it needs to be re-invoked. There isn't anything in the
> documentation on this topic, but from reading the scons sources it appears
> I need to make a callable object that has a get_contents attribute, like
> this:
>
> class MyCustomAction:
>     def __call__(self, env, target, source):
>         <....my code that generates targets from source....>
>
>     def get_contents(self, env, target, source):
>         <... my code that computes a signature based on relevant parts of
> source>
>
> env.Append(BUILDERS={'MyCustomAction': Builder(
>     action=MyCustomAction()
> )})
>
> This does succeed in generating the targets from source, but scons is not
> properly deciding when the target needs to be rebuilt based on the source.
> In the course of my debugging, I added "assert(False)" to get_contents and
> discovered it was never being called.
>
> I believe the source of the bug is in the scons implementation of
> FunctionAction (in Action.py). Specifically, the __init__ function, which I
> have quoted in part below. It appears to try to determine the static
> contents of the function first, by calling _callable_contents. Only if that
> fails does it check if the callable contains its own get_contents function.
>
> Shouldn't the check to see if get_contents is a defined attribute come
> first?
>
>         self.execfunction = execfunction
>         try:
>             self.funccontents = _callable_contents(execfunction)
>         except AttributeError:
>             try:
>                 # See if execfunction will do the heavy lifting for us.
>                 self.gc = execfunction.get_contents
>             except AttributeError:
>                 # This is weird, just do the best we can.
>                 self.funccontents = _object_contents(execfunction)
>
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