Fix the build-script --skip-build option.

[atrick]

This option configures the build directories without building any
targets. Splitting configuration from build allows for the decoupling
of build products. This decoupling is essential for the enlightened
way of developing Swift where the build-script is never actually used
to build anything, and build products can be independently
configured. When fully supported, this avoids many unnecessary
full/clean rebuilds and enables debugging by mixing-and-matching
configurations and rebuilding only select products after a change.

Sadly, the option has degraded, and a recent commit rendered it fully broken:

commit 34848e6
Author: Alex Langford [email protected]
Date: Wed Jan 22 19:27:44 2020

[build] Unify logic to skip building projects in build-script-impl

The breaking commit was itself a reasonable cleanup. The underlying
problem was the original --skip-build was implemented using hacks that
conflated configuration with build.

This fix reinstates a reasonable situation:

--skip-build has no effect on configuration, as documented. It merely
skips building the targets. This is how it must behave to work as
intended.

--skip-build-{product} and its inverse --build-{product} controls
which products will be configured. These options are in heavy use
throughout the scripts, so changing the name (e.g. to --skip-config)
would be disruptive and of questionable benefit.

None of this changes the fact that any required build logic that
people have dumped into build-script-impl still effectively breaks the
enlightened way of building Swift, particularly when building
toolchain components.

[atrick]

@swift-ci test

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - 632468e386cf88b560a99cea37d4eeb54c2165e3

[swift-ci]

Build failed
Swift Test Linux Platform
Git Sha - 632468e386cf88b560a99cea37d4eeb54c2165e3

[atrick]

@swift-ci test

[swift-ci]

Build failed
Swift Test Linux Platform
Git Sha - 632468e386cf88b560a99cea37d4eeb54c2165e3

[atrick]

For some reason, the unittests fail when I run locally (although this used to work). So, this PR is trial-and-error.
python 2.7.16

FAIL: Swift(macosx-x86_64) :: Python/build_swift.swift (1 of 1)
******************** TEST 'Swift(macosx-x86_64) :: Python/build_swift.swift' FAILED ********************
Script:
--
: 'RUN: at line 10';   /System/Library/Frameworks/Python.framework/Versions/2.7/Resources/Python.app/Contents/MacOS/Python /Users/atrick/s/sopt/swift/utils/build_swift/run_tests.py
--
Exit Code: 1

Command Output (stderr):
--
Traceback (most recent call last):
  File "/Users/atrick/s/sopt/swift/utils/build_swift/run_tests.py", line 19, in <module>
    import argparse
  File "/Users/atrick/s/sopt/swift/utils/build_swift/argparse/__init__.py", line 18, in <module>

ImportError: cannot import name ArgumentDefaultsHelpFormatter

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - 632468e386cf88b560a99cea37d4eeb54c2165e3

[Rostepher]

@atrick That test failure was caused by a stale Python cache.

[Rostepher]

Overall this seems relatively reasonable. Not sure I like the names of --build-{product} and --skip-build-{product} now. I would wait to merge this until @compnerd and @edymtt have had a chance to look at it.

[gottesmm]

@Rostepher I wish it was something like --only-configure. That is really what it is = /.

[Rostepher]

I'd be fine with eventually renaming all these arguments. We could deprecate the old ones in build-script and have their namespace destinations match the new arguments. Pretty sure we have a deprecation mechanism, we just need to make sure it puts up a warning. Since build-script-impl is an implementation detail we can change all these there.

[atrick]

@swift-ci smoke test

[atrick]

I don't particularly like the names, but this implementation in this PR now matches the existing documentation of those option names. This is completely blocking me, so any additional cleanup should be proposed in a separate PR.

[atrick]

@swift-ci smoke test

[compnerd]

What we discussed was that it would replace the old options - how come those are not removed?

[Rostepher]

We can remove them in follow-up PRs. This turned out to be blocking both @atrick and others.

[atrick]

@compnerd the existing --skip-build-XXX are simply the inverse of the --build-XXX options. These are generally important for controlling which products will be configured. They really haven't changed, it's just that they're no longer serving doing double duty as "configure but don't build" for some subset of the products. With the change in this PR, we no longer need to pass any --skip-build-XXX to the impl script for the normal default configuration. However, I intend to start using --skip-build-llvm to avoid constantly reconfiguring LLVM even when I don't intend to build it. Note that I share the same LLVM build across all my swift configurations and working branches.