[Scons-users] Do not add a Tool to an Environment multiple times

[Bill Deegan]

Mike,

Simplified logic

if 'this_tool' in env['TOOLS']:
   return.


It might be desirable to re-initialize a tool after changing environment
variables which affect it.
For example env['ENV']['PATH']

That said if there are tools which misbehave.. then those should probably
be addressed.

-Bill

On Fri, Dec 6, 2019 at 10:44 AM Mike Haboustak <haboustak at gmail.com> wrote:

> My various build systems make use of several SCons Tools to encapulate
> resuable functionality. In general, I try to ensure that a Tool's
> generate method is idempotent, using facilities like env.SetDefault(),
> so that it can be added to a single environment multiple times without
> ill affects. This has helped with tool dependencies, for example when
> tools B and C both depend on tool A.
>
> However many of the built-in tools are not idempotent, and I understand
> that it can be difficult to achieve idempotency for non-trivial tools.
>
> Here's an example SConstruct that demonstrates the issue by including
> the gcc tool twice. The '-Wall -Werror` flags are discarded when gcc
> is included a second time.
>
> --- SConstruct
> env = Environment(
>     tools=[
>         'gnulink', 'gcc', 'g++', 'gas', 'ar',
>         'lex', 'yacc', 'filesystem'
>     ])
> env.Append(
>     CFLAGS=['-Wall', '-Werror']
> )
> print('CFLAGS=', env['CFLAGS'])
>
> env.Tool('gcc')
> print(','.join(env['TOOLS']))
> print('CFLAGS=', env['CFLAGS'])
> ----
>
> The example uses the gcc tool because it includes the cc tool which
> stomps on the CFLAGS construction variable, but there are more reasonable
> examples of the problem in real projects/Tools.
>
> Does it make sense for the env.Tool() infrastructure to only add a Tool
> to an Environment once?
>
> I'm not sure I've seen a use case where including a Tool multiple times
> was intended behavior. I can deal with this within each Tool's
> generate() implementation, but it's not very easy to do. For example:
>
> def generate(env):
>     if len([t for t in env['TOOLS'] if t=='this_tool']) > 1:
>         return
>
> If tools can't generally be made load-once, maybe there's a facility to
> make load-once tools easier to write. I'd prefer the Tool define
> idempotency for the env.Tool() infrastructure, rather than an option on
> env.Tool() itself, e.g. env.Tool('gcc', if_not_exist=True).
>
> Thanks,
> Mike
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