[Build][Runtime] Add a new threading library.

[al45tair]

A fair bit of this is really shuffling things around — that's largely true for the swift::Mutex, which was an existing thing. swift::Thread is really a placeholder for when we need more support — right now, we don't need any of the functionality that e.g. std::thread has, but we do need some functionality it doesn't have. It's also desirable to be able to choose which threading package to use, especially for embedded use or for testing purposes, and if we make everything defer to the C++ library we'll only be able to use whatever the C++ library is using.

The main new things are:

• You can ask to use a specific threading package.
• The threading code is in its own library, so no more #includes of implementation files in order to bring them in to the standard library or concurrency code.
• swift::Lazy now uses swift::once rather than having its own implementation that uses a macro.
• New swift::once implementations for Linux and Windows that should be somewhat faster on those platforms.
• Improved error handling, so the threading code raising errors can hook into the error handling used in the places it's being used.
• Adjusted the TLS support code slightly, to avoid trying to put TLS keys in template arguments, as that isn't guaranteed to work.

[al45tair]

@swift-ci Please test

[grynspan]

I haven't taken a deep dive yet, but I'm wondering about the necessity of the platform-specific implementations. Since Linux and Windows both have C++'s std::thread (and std::mutex and and and…), would it make sense to just… use that for every non-Apple platform? I can buy that Darwin may need to be specialized.

Given Swift's lack of official support for platforms other than Darwin and Linux, it seems to me like we should lean on the STL as much as possible rather than rolling our own equivalent implementations.

[al45tair]

I've updated the comment at the top with some notes on the changes that are hopefully useful for reviewers.

[al45tair]

@swift-ci Please test

[gottesmm]

@al45tair can you make sure the standalone minimal test still passes? I think it uses the single threaded flag.

[al45tair]

preset=stdlib_S_standalone_minimal_macho_x86_64,build,test
@swift-ci please test with toolchain and preset

[al45tair]

Hmmm. "Can't find CMake" seems like not my fault. I'll give it another go, in case it's just a host issue, then go look what the problem might be.

[al45tair]

preset=stdlib_S_standalone_minimal_macho_x86_64,build,test
@swift-ci please test with toolchain and preset

[kubamracek]

LGTM (sorry, it took a while to go over the large diff).

Given that the PR testing for "stdlib minimal" seems to have some infrastructure issues, I am going to run the "preset testing" locally to ensure we're not breaking it, I'll report back.

[al45tair]

@swift-ci Please test

[al45tair]

preset=stdlib_S_standalone_minimal_macho_x86_64,build,test
@swift-ci please test with toolchain and preset

[al45tair]

This passed local testing of the minimal preset just now, FWIW:

Testing Time: 456.15s
  Excluded         : 14471
  Unsupported      :   230
  Passed           :   289
  Expectedly Failed:     1

49 warning(s) in tests
-- check-swift-validation-only_executable-freestanding-x86_64 finished --
--- Finished tests for swift ---

[al45tair]

Looks like preset testing in CI is still broken.

[gottesmm]

@shahmishal can you prioritize fixing preset testing in CI? We need it to work to test PRs like this?

[gottesmm]

If the preset testing worked locally... I am happy.

[al45tair]

@swift-ci Please test Linux platform

[al45tair]

@swift-ci Please test

[al45tair]

@swift-ci Please test

[al45tair]

The Linux issue has to do with the fact that I'd not appreciated that SWIFT_BUILD_STATIC_STDLIB and SWIFT_BUILD_DYNAMIC_STDLIB weren't mutually exclusive. Hopefully the latest change fixes that.

Local testing of the preset passes still:

Testing Time: 452.37s
  Excluded         : 14471
  Unsupported      :   230
  Passed           :   289
  Expectedly Failed:     1

49 warning(s) in tests
-- check-swift-validation-only_executable-freestanding-x86_64 finished --
--- Finished tests for swift ---

[kateinoigakukun]

Thank you @al45tair for your great work!! The none thread variant is really helpful for us Wasm support!

[al45tair]

@kateinoigakukun Glad to know it's helpful. It's been reverted for now because of a few CI failures (the Linux problem you sent a PR for, plus some simulator related problems). I'll get it re-submitted ASAP.