Support downloading & installing Swift toolchain for Ubuntu/RHEL hosts when on Linux

[xtremekforever]

Right now, if you do the following on Ubuntu 22.04:

swift run swift-sdk-generator make-linux-sdk --swift-version 5.10.1-RELEASE --host x86_64-unknown-linux-gnu

The generator will still download the OSX package for the Swift 5.10.1, resulting in an SDK that is unusable on Ubuntu. If you pass --host-swift-package-path /path/to/swift/installation to the generator, then it will use the correct version of the Swift toolchain and package it in the generated SDK. However, there are errors involved since the Swift distribution for Linux includes extra libraries:

~/.swiftpm/swift-sdks/5.10.1-RELEASE_ubuntu_jammy_aarch64.artifactbundle/5.10.1-RELEASE_ubuntu_jammy_aarch64/aarch64-unknown-linux-gnu/ubuntu-jammy.sdk/usr/include/CoreFoundation/module.map:1:8: error: redefinition of module 'CoreFoundation'
module CoreFoundation [extern_c] [system] {
       ^
~/.swiftpm/swift-sdks/5.10.1-RELEASE_ubuntu_jammy_aarch64.artifactbundle/5.10.1-RELEASE_ubuntu_jammy_aarch64/aarch64-unknown-linux-gnu/swift.xctoolchain/usr/lib/swift/CoreFoundation/module.map:1:8: note: previously defined here
module CoreFoundation [extern_c] [system] {
       ^
~/.swiftpm/swift-sdks/5.10.1-RELEASE_ubuntu_jammy_aarch64.artifactbundle/5.10.1-RELEASE_ubuntu_jammy_aarch64/aarch64-unknown-linux-gnu/ubuntu-jammy.sdk/usr/include/dispatch/module.modulemap:1:8: error: redefinition of module 'Dispatch'
module Dispatch {
       ^
~/.swiftpm/swift-sdks/5.10.1-RELEASE_ubuntu_jammy_aarch64.artifactbundle/5.10.1-RELEASE_ubuntu_jammy_aarch64/aarch64-unknown-linux-gnu/swift.xctoolchain/usr/lib/swift/dispatch/module.modulemap:1:8: note: previously defined here
module Dispatch {

This can be resolved by simply removing these extra libraries (dispatch and CoreFoundation) from the swift.xctoolchain directory. However, it would be nice if the generator could do all this for us and correctly pull the Swift distribution for Linux automatically, if running on a Ubuntu or RHEL host.

[xtremekforever]

@MaxDesiatov @euanh question for you guys. Do you think it would be acceptable/okay for the swift-sdk-generator to have an option to generate an SDK with just the sysroot for the target and no version of Swift included, like the static Linux SDKs?

I would like to use the SDK generator to build some SDKs for various targets, but I do not want Swift included so that they can be used on any host (macOS, Linux, etc).

[euanh]

I think this would be a nice option. It was not possible before #155 because, as you noticed, we were copying some of the target SDK files into the toolchain. After #155 the SDK and toolchain should be completely separate, so you should be able to remove the toolchain and still have a working SDK.

⚠️ If you don't include a toolchain in the SDK then on macOS you will need to install a matching version of the OSS Swift toolchain rfom https://www.swift.org/install/macos/package_installer in order to use the SDK. This is the same situation as the Static Linux SDK: https://www.swift.org/documentation/articles/static-linux-getting-started.html#installing-the-sdk

[xtremekforever]

Indeed. I think that's an acceptable compromise for an SDK that is compatible with a wider range of hosts, as long as you have the exact version of Swift installed :). Thank you, I shall have a look here and see if I can add an option. Do I need to file a ticket for this as well?