[build-script] Shell Module

[Rostepher]

Purpose

This PR introduces two new modules. First is the shell module which implements a CommandExecutor that wraps portions of the standard subprocess module in a context object that tracks common state and command history. The executor is extensible with a trivial action system that allows for users to add any callable to the queue of actions performed prior to executing a command. The intended use-case here is to have a centralized shell-object that is passed around through the program to build up a history and maintain state.

For example:

import os

from build_swift import shell

ENV = {
    'PATH': '/usr/bin',
}

sh = shell.CommandExecutor(env=ENV, stdout=open(os.devnull, 'w'))
sh.cp('/Applications/Xcode.app')
sh.mkdir('test')

sh.check_call(['./utils/build-script', '--reconfigure', '-t'])

sh.history()

It's possible to hook-in custom actions like so:

def callable(command):
    # Do something with the command
    log_command(command)

sh.add_action(callable)

Every action is performed before executing the command.

The second module is pretty small, wrapping the xcrun utility using the shell module. Both have testing, albeit not much in the way of xcrun.

This furthers the efforts for completing SR-237
rdar://25281853

[Rostepher]

@swift-ci please test

[rintaro]

I think NullExecutor should override cd andpushd as well.
Consider:

sh.mkdir("foo")
with sh.pushd("foo"):
    # something
    pass

Since foo isn't created, it fails with OSError: [Errno 2] No such file or directory: 'foo'

[Rostepher]

@rintaro I'll override cd and pushd in the NullExecutor.

[Rostepher]

@swift-ci please test

[swift-ci]

Build failed
Swift Test Linux Platform
Git Sha - ef793b4cf4ead4ede918ec4dd46cba4b91e32229

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - ef793b4cf4ead4ede918ec4dd46cba4b91e32229

[Rostepher]

@swift-ci please test

[Rostepher]

@swift-ci please build toolchain

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - 720788e28d7aadd742218e1ae8abcb4671b3671e

[swift-ci]

Build failed
Swift Test Linux Platform
Git Sha - 720788e28d7aadd742218e1ae8abcb4671b3671e

[Rostepher]

@swift-ci please test Linux

[Rostepher]

I'm going to close this PR, it's no longer the direction I want to take this sort of module.