os, syscall: use wait6 to avoid wait/kill race on dragonfly

Follow CL 23967 and CL 24021 which did the same on linux and freebsd,
respectively.

Updates #13987
Updates #16028

Change-Id: Ia30ef8b5cffd8f9eb75c29ee5fe350dac2be6d44
Reviewed-on: https://go-review.googlesource.com/c/go/+/315279
Trust: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/os/wait_unimp.go

@@ -2,8 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build aix || darwin || dragonfly || (js && wasm) || netbsd || openbsd || solaris
-// +build aix darwin dragonfly js,wasm netbsd openbsd solaris
+//go:build aix || darwin || (js && wasm) || netbsd || openbsd || solaris
+// +build aix darwin js,wasm netbsd openbsd solaris
 
 package os
 

src/os/wait_wait6.go

@@ -2,8 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build freebsd
-// +build freebsd
+//go:build dragonfly || freebsd
+// +build dragonfly freebsd
 
 package os
 
@@ -23,9 +23,9 @@ func (p *Process) blockUntilWaitable() (bool, error) {
 		// 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 ARM, MIPS or PowerPC
-		if runtime.GOARCH == "386" {
+		if runtime.GOOS == "freebsd" && runtime.GOARCH == "386" {
 			_, _, errno = syscall.Syscall9(syscall.SYS_WAIT6, _P_PID, uintptr(p.Pid), 0, 0, syscall.WEXITED|syscall.WNOWAIT, 0, 0, 0, 0)
-		} else if runtime.GOARCH == "arm" {
+		} else if runtime.GOOS == "freebsd" && runtime.GOARCH == "arm" {
 			_, _, errno = syscall.Syscall9(syscall.SYS_WAIT6, _P_PID, 0, uintptr(p.Pid), 0, 0, syscall.WEXITED|syscall.WNOWAIT, 0, 0, 0)
 		} else {
 			_, _, errno = syscall.Syscall6(syscall.SYS_WAIT6, _P_PID, uintptr(p.Pid), 0, syscall.WEXITED|syscall.WNOWAIT, 0, 0)
@@ -35,7 +35,9 @@ func (p *Process) blockUntilWaitable() (bool, error) {
 		}
 	}
 	runtime.KeepAlive(p)
-	if errno != 0 {
+	if errno == syscall.ENOSYS {
+		return false, nil
+	} else if errno != 0 {
 		return false, NewSyscallError("wait6", errno)
 	}
 	return true, nil