[OpenBSD] Subprocess+Linux.swift does not build here

[3405691582]

Describe the bug
Building swift-subprocess on OpenBSD fails in Subprocess+Linux.swift (and there is no other spawn implementation in Subprocess+Unix.swift).

To Reproduce
Steps to reproduce the behavior:

1. swift build on OpenBSD.

Expected behavior
Package builds.

Environment (please complete the following information):

• OpenBSD
• Swift 6.2/HEAD-ish.

Additional context

/tmp/swift-subprocess/Sources/Subprocess/Platforms/Subprocess+Linux.swift:373:39: error: value of type 'siginfo_t' has no member 'si_pid'
371 |                 errno = 0
372 |                 if waitid(P_ALL, id_t(0), &siginfo, WEXITED | WNOWAIT) == 0 {
373 |                     let pid = siginfo.si_pid
    |                                       `- error: value of type 'siginfo_t' has no member 'si_pid'

This is because for whatever reason, siginfo_t on this platform has some internal fields that may be undergoing change and reference to si_pid is #defined as the internal field path, i.e., _data._proc._pid. Of course, Swift doesn't know how to paste those tokens together when importing macros from C.

We probably want some C shims for the platform or something.

As to not lose context, I'm just making note of this right now since it's not in the critical path, but might be in the future, otherwise I'd just dabble with this and fix it myself. (Feel free to assign it to me if you'd like.)

[jakepetroules]

A similar issue exists for Linux/Android. Note the following code:

swift-subprocess/Sources/Subprocess/Platforms/Subprocess+Linux.swift

Lines 328 to 350 in 0e853a9

	#if !os(FreeBSD) && !os(OpenBSD)
	private extension siginfo_t {
	var si_status: Int32 {
	#if canImport(Glibc)
	return _sifields._sigchld.si_status
	#elseif canImport(Musl)
	return __si_fields.__si_common.__second.__sigchld.si_status
	#elseif canImport(Bionic)
	return _sifields._sigchld._status
	#endif
	}
	var si_pid: pid_t {
	#if canImport(Glibc)
	return _sifields._sigchld.si_pid
	#elseif canImport(Musl)
	return __si_fields.__si_common.__first.__piduid.si_pid
	#elseif canImport(Bionic)
	return _sifields._kill._pid
	#endif
	}
	}
	#endif

FreeBSD doesn't define those fields as macros, so the workaround isn't needed, and I assumed that OpenBSD was set up the same way. But it sounds like we need to apply the same Linux workaround for OpenBSD. Based on a cursory review of https://github.com/openbsd/src/blob/master/sys/sys/siginfo.h, it looks like the following change should do the trick. Would you mind giving that a try?

-#if !os(FreeBSD) && !os(OpenBSD)
-#if !os(FreeBSD)
 private extension siginfo_t {
     var si_status: Int32 {
+        #if os(OpenBSD)
+        return _data._proc._pdata._cld._status
+        #elseif canImport(Glibc)
-        #if canImport(Glibc)
         return _sifields._sigchld.si_status
         #elseif canImport(Musl)
         return __si_fields.__si_common.__second.__sigchld.si_status
         #elseif canImport(Bionic)
         return _sifields._sigchld._status
         #endif
     }

     var si_pid: pid_t {
+        #if os(OpenBSD)
+        return _data._proc._pid
+        #elseif canImport(Glibc)
-        #if canImport(Glibc)
         return _sifields._sigchld.si_pid
         #elseif canImport(Musl)
         return __si_fields.__si_common.__first.__piduid.si_pid
         #elseif canImport(Bionic)
         return _sifields._kill._pid
         #endif
     }
 }
#endif

[3405691582]

Yes, this works, thanks! I'm a little worried that these fields might change, but at least the other platforms are implemented in the same way, so I guess it's not that big a deal for now.

(The other build problem is with snprintf: it should use the same shim as Linux. FreeBSD will probably have the same problem.)

[jakepetroules]

@iCharlesHu mentioned in #79 (comment) that the snprintf workaround was needed because the Glibc module doesn't expose that function on Linux, but I didn't have that problem on FreeBSD.

[3405691582]

FWIW: I don't know why the ptr: argument is there (error: extraneous argument label 'ptr:' in call); if I remove it, I get error: 'snprintf' is unavailable: Variadic function is unavailable.