unix: add bindings for kinfo_proc on Darwin

On Linux, we can extract a list of all the processes on the system by
calling readdir() against /proc. On BSD-like systems, this information
needs to be extracted from sysctl in the form of kinfo_proc structures.

This change adds bindings for this structure and adds a method for
reading an array of these structures from sysctl.

Change-Id: Iaaed27cdbbf13d7c2cc6a6787667ac04d65bf41c
GitHub-Last-Rev: 34926f847495e7584bb8b68bbf24c5a0764ca861
GitHub-Pull-Request: golang/sys#111
Reviewed-on: https://go-review.googlesource.com/c/sys/+/328169
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

unix/syscall_darwin.go

@@ -13,6 +13,7 @@
 package unix
 
 import (
+	"fmt"
 	"runtime"
 	"syscall"
 	"unsafe"
@@ -398,6 +399,38 @@ func GetsockoptXucred(fd, level, opt int) (*Xucred, error) {
 	return x, err
 }
 
+func SysctlKinfoProcSlice(name string) ([]KinfoProc, error) {
+	mib, err := sysctlmib(name)
+	if err != nil {
+		return nil, err
+	}
+
+	// Find size.
+	n := uintptr(0)
+	if err := sysctl(mib, nil, &n, nil, 0); err != nil {
+		return nil, err
+	}
+	if n == 0 {
+		return nil, nil
+	}
+	if n%SizeofKinfoProc != 0 {
+		return nil, fmt.Errorf("sysctl() returned a size of %d, which is not a multiple of %d", n, SizeofKinfoProc)
+	}
+
+	// Read into buffer of that size.
+	buf := make([]KinfoProc, n/SizeofKinfoProc)
+	if err := sysctl(mib, (*byte)(unsafe.Pointer(&buf[0])), &n, nil, 0); err != nil {
+		return nil, err
+	}
+	if n%SizeofKinfoProc != 0 {
+		return nil, fmt.Errorf("sysctl() returned a size of %d, which is not a multiple of %d", n, SizeofKinfoProc)
+	}
+
+	// The actual call may return less than the original reported required
+	// size so ensure we deal with that.
+	return buf[:n/SizeofKinfoProc], nil
+}
+
 //sys	sendfile(infd int, outfd int, offset int64, len *int64, hdtr unsafe.Pointer, flags int) (err error)
 
 /*