os: use wait6 to avoid wait/kill race on freebsd

This change is a followup to https://go-review.googlesource.com/23967
for FreeBSD.

Updates #13987.
Updates #16028.

Change-Id: I0f0737372fce6df89d090fe9847305749b79eb4c
Reviewed-on: https://go-review.googlesource.com/24021
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/os/wait_unimp.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build dragonfly freebsd nacl netbsd openbsd solaris
+// +build dragonfly nacl netbsd openbsd solaris
 
 package os
 

src/os/wait_wait6.go

@@ -0,0 +1,46 @@
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build freebsd
+
+package os
+
+import (
+	"runtime"
+	"syscall"
+)
+
+const _P_PID = 0
+
+// blockUntilWaitable attempts to block until a call to p.Wait will
+// succeed immediately, and returns whether it has done so.
+// It does not actually call p.Wait.
+func (p *Process) blockUntilWaitable() (bool, error) {
+	var errno syscall.Errno
+	switch runtime.GOARCH {
+	case "386", "arm":
+		// The arguments on 32-bit FreeBSD look like the
+		// following:
+		// - freebsd32_wait6_args{ idtype, id1, id2, status, options, wrusage, info } or
+		// - freebsd32_wait6_args{ idtype, pad, id1, id2, status, options, wrusage, info } when PAD64_REQUIRED=1 on MIPS or PowerPC
+		_, _, errno = syscall.Syscall9(syscall.SYS_WAIT6, _P_PID, 0, uintptr(p.Pid), 0, syscall.WEXITED|syscall.WNOWAIT, 0, 0, 0, 0)
+	default:
+		_, _, errno = syscall.Syscall6(syscall.SYS_WAIT6, _P_PID, uintptr(p.Pid), 0, syscall.WEXITED|syscall.WNOWAIT, 0, 0)
+	}
+	if errno != 0 {
+		// The wait6 system call is supported only on FreeBSD
+		// 9.3 and above, so it may return an ENOSYS error.
+		// Also the system call may return an ECHILD error
+		// when the child process has not finished the
+		// transformation using execve system call.
+		// In both cases, we just leave the care of child
+		// process to the following wait4 system call in
+		// Process.wait.
+		if errno == syscall.ENOSYS || errno == syscall.ECHILD {
+			return false, nil
+		}
+		return false, NewSyscallError("wait6", errno)
+	}
+	return true, nil
+}