[Scons-users] Running all external commands under a wrapper

[Andrew C. Morrow]

I'd like to ensure that all spawned commands are run under the icerun
wrapper program. I expected this to be not so hard given the availability
of [P]SPAWN and SHELL. However, I ran into some surprises that made it
harder than I expected.

- SPAWN is substituted without access to `target` and`source`: In
CommandAction.execute, the SPAWN variable is substituted, however the
`target` and `source` keyword arguments are not provided. This means that
you can't, for instance, make SPAWN into a generator that behaves
differently based on targets or sources. Independently of my suggestions
below, I think this should just be fixed so that generators can be used as
the value of SPAWN.

- SPAWN needs to be set to a function with a particular signature. There is
no easy way to interpose on SPAWN without grabbing the existing definitions
out of the Environment and wrapping them. And there isn't any way to
"accumulate" modifications to SPAWN. If a sub-environment wanted to inject
yet another wrapper, it would need to access my wrapper function around the
original wrapper, if that makes sense.

- Finally, and confusingly, the `env` argument inside a spawn function
invocation isn't the SCons Environment, it is the ENV block. Though, this
may not matter if the above issue about providing target and source when
substituting SPAWN is addressed.

At that point, I started thinking that SPAWN wasn't the right mechanism.
But SHELL isn't quite the right thing either:

- You can't make the SHELL variable into a list, so I can't set it, to, for
instance `SHELL=['sh', 'icerun']. And I'd still have the issue, like with
SPAWN, that if I customize it I need to capture the value of SHELL at the
time of customization, which means sub-environment changes to SHELL would
lose my changes.

- We probably want, at some point, to not need to shell out at all, for
commands that don't need shell behavior. But I still would want the
icerun wrapper in play.

I wonder if something else is required entirely, like an
EXTERNAL_COMMAND_WRAPPERS list, which, abusing some notation, gets expanded
like:

$SPAWN($SHELL $EXTERNAL_COMMAND_WRAPPERS $COMMAND)

Then, I could just say:

env.PrependUnique(EXTERNAL_COMMAND_WRAPPERS=['icerun'])

Thoughts on the above? Worth opening GitHub issue(s)?

Thanks,
Andrew
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