Amazon Linux 2 Support for Swift Build

[cmcgee1024]

Amazon Linux 2 is a little more challenging to support than other Linux distributions. In particular, there is a missing implementation of posix_spawn_file_actions_addchdir for the version of glibc available there. There could be other issues with Swift Build support for that platform too, but this is the most immediate problem.

[jakepetroules]

The main thing in Swift Build is our use of Foundation Process (NSTask), which has an implementation of setting the working directory which is NOT thread safe on platforms lacking posix_spawn_file_actions_addchdir (Amazon Linux 2, OpenBSD, QNX, and older Android versions). To solve this we can adopt swift-subprocess, which has a fork/exec fallback path for environments where posix_spawn_file_actions_addchdir is unavailable.

The other one is llbuild's process spawning code, which has a hard dependency on posix_spawn_file_actions_addchdir, and needs to be tracked separately on the llbuild project.

[cmcgee1024]

See swiftlang/swift-llbuild#980 for the llbuild portion.

[jakepetroules]

The underlying llbuild issue was resolved, and #545 (#551 for 6.2) unskipped all of the relevant tests.

There are a 4 tests which are still skipped because they use Foundation.Process instead of llbuild, but those are test-only issues and don't impact build operations.

There is also a workaround in place for parts of task construction which also go through Foundation.Process rather than llbuild:

swift-build/Sources/SWBCore/ProcessExecutionCache.swift

Line 20 in 9626e28

// FIXME: Work around lack of thread-safe working directory support in Foundation (Amazon Linux 2, OpenBSD). Executing processes in the current working directory is less deterministic, but all of the clients which use this class are generally not expected to be sensitive to the working directory anyways. This workaround can be removed once we drop support for Amazon Linux 2 and/or adopt swift-subprocess and/or Foundation.Process's working directory support is made thread safe.

I think I'll leave this open until those pieces are fully resolved too.