stdlib: use stubs for unit tests as well

[compnerd]

What's in this pull request?

Resolved bug number: (SR-)

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This unifies the behaviour for imports across the private libc extras and the
public libc shims.

[gribozavr]

Did you mean to name the file Subprocess.cpp?

[compnerd]

No, I named it .c to avoid the extern "C" block. I don't have a strong opinion on that, and can change it you believe it is better.

[gribozavr]

@swift-ci Please test

[compnerd]

I don't understand the failure on the OS X target. Shouldn't /usr/include/crt_externs.h always exist on the build hosts? The header is used in TaskQueue.inc as well.

[gribozavr]

@swift-ci Please test

[compnerd]

Interesting, it seems that not all SDKs provide the header. Ive changed it to a forward declaration instead of the header to permit building against any of the SDKs.

[gribozavr]

@swift-ci Please test and merge

[compnerd]

Seems like another instance of the packaging issues:

swift-build: error: exit(1): which swiftc

during the package tests.

[gribozavr]

Sorry, it is more than that:

https://ci.swift.org/job/swift-PR-osx/732/console

swift/stdlib/private/SwiftPrivateLibcExtras/Subprocess.c:23:10: error: 'posix_spawn_file_actions_init' is unavailable: not available on tvOS
  return posix_spawn_file_actions_init(file_actions);

[compnerd]

Oh, sorry, I didn't see that when I looked through the failures. Is this really just a declaration issue? Since it would seem that if the function was truly unavailable, then the link would fail (since they are not declared weak).

[gribozavr]

Yes, it is just a declaration. If we can persuade the compiler to create a reference, everything will link just fine.

[compnerd]

Sorry about the delay. I believe that this version should work on tvOS as well now.

[compnerd]

ping?

[gribozavr]

@swift-ci Please test and merge