unix: add Pipe2 on dragonfly

The pipe2 syscall is available since DragonflyBSD 4.2, see
https://www.dragonflybsd.org/release42/

Change-Id: I94631fd64ee731e416049cac5f3d15544f1337dd
Reviewed-on: https://go-review.googlesource.com/c/sys/+/271177
Trust: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Matt Layher <mdlayher@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>

unix/syscall_dragonfly.go

diff --git a/unix/syscall_dragonfly.go b/unix/syscall_dragonfly.go
index 842ab5a..a4f2944 100644
--- a/unix/syscall_dragonfly.go
+++ b/unix/syscall_dragonfly.go
@@ -105,6 +105,19 @@
 	return
 }
 
+//sysnb	pipe2(p *[2]_C_int, flags int) (err error)
+
+func Pipe2(p []int, flags int) error {
+	if len(p) != 2 {
+		return EINVAL
+	}
+	var pp [2]_C_int
+	err := pipe2(&pp, flags)
+	p[0] = int(pp[0])
+	p[1] = int(pp[1])
+	return err
+}
+
 //sys	extpread(fd int, p []byte, flags int, offset int64) (n int, err error)
 func Pread(fd int, p []byte, offset int64) (n int, err error) {
 	return extpread(fd, p, 0, offset)

unix/zsyscall_dragonfly_amd64.go

diff --git a/unix/zsyscall_dragonfly_amd64.go b/unix/zsyscall_dragonfly_amd64.go
index aebfe51..1aaccd3 100644
--- a/unix/zsyscall_dragonfly_amd64.go
+++ b/unix/zsyscall_dragonfly_amd64.go
@@ -362,6 +362,16 @@
 
 // THIS FILE IS GENERATED BY THE COMMAND AT THE TOP; DO NOT EDIT
 
+func pipe2(p *[2]_C_int, flags int) (err error) {
+	_, _, e1 := RawSyscall(SYS_PIPE2, uintptr(unsafe.Pointer(p)), uintptr(flags), 0)
+	if e1 != 0 {
+		err = errnoErr(e1)
+	}
+	return
+}
+
+// THIS FILE IS GENERATED BY THE COMMAND AT THE TOP; DO NOT EDIT
+
 func extpread(fd int, p []byte, flags int, offset int64) (n int, err error) {
 	var _p0 unsafe.Pointer
 	if len(p) > 0 {