unix: add Send on Linux

Added Send function which in turn calls existing Sendto.

Fixes golang/go#47288

Change-Id: I5c928a19cc4f10961a9e6d2802e197cc3b799438
Reviewed-on: https://go-review.googlesource.com/c/sys/+/336569
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Matt Layher <mdlayher@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Matt Layher <mdlayher@gmail.com>

unix/syscall_unix.go

@@ -313,6 +313,10 @@ func Recvfrom(fd int, p []byte, flags int) (n int, from Sockaddr, err error) {
 	return
 }
 
+func Send(s int, buf []byte, flags int) (err error) {
+	return sendto(s, buf, flags, nil, 0)
+}
+
 func Sendto(fd int, p []byte, flags int, to Sockaddr) (err error) {
 	ptr, n, err := to.sockaddr()
 	if err != nil {

unix/syscall_unix_test.go

@@ -8,6 +8,7 @@
 package unix_test
 
 import (
+	"bytes"
 	"flag"
 	"fmt"
 	"io/ioutil"
@@ -890,6 +891,44 @@ func TestUtimesNanoAt(t *testing.T) {
 	}
 }
 
+func TestSend(t *testing.T) {
+	ec := make(chan error, 2)
+	ts := []byte("HELLO GOPHER")
+
+	fds, err := unix.Socketpair(unix.AF_LOCAL, unix.SOCK_STREAM, 0)
+	if err != nil {
+		t.Fatalf("Socketpair: %v", err)
+	}
+	defer unix.Close(fds[0])
+	defer unix.Close(fds[1])
+
+	go func() {
+		data := make([]byte, len(ts))
+
+		_, _, err := unix.Recvfrom(fds[1], data, 0)
+		if err != nil {
+			ec <- err
+		}
+		if !bytes.Equal(ts, data) {
+			ec <- fmt.Errorf("data sent != data received. Received %q", data)
+		}
+		ec <- nil
+	}()
+	err = unix.Send(fds[0], ts, 0)
+	if err != nil {
+		ec <- err
+	}
+
+	select {
+	case err = <-ec:
+		if err != nil {
+			t.Fatalf("Send: %v", err)
+		}
+	case <-time.After(2 * time.Second):
+		t.Fatal("Send: nothing received after 2 seconds")
+	}
+}
+
 // mktmpfifo creates a temporary FIFO and provides a cleanup function.
 func mktmpfifo(t *testing.T) (*os.File, func()) {
 	err := unix.Mkfifo("fifo", 0666)