unix: add Sendfile test

There wasn't one. I was mucking around with the Sendfile internals in
CL 154663 so I need one to make sure I didn't break anything.

Change-Id: I38b25de4995831680fdac440dfa7f26542d2624f
Reviewed-on: https://go-review.googlesource.com/c/154664
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

unix/sendfile_test.go

+// Copyright 2018 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 darwin,amd64 darwin,386 dragonfly freebsd linux solaris
+
+package unix_test
+
+import (
+	"io/ioutil"
+	"net"
+	"os"
+	"path/filepath"
+	"testing"
+
+	"golang.org/x/sys/unix"
+)
+
+func TestSendfile(t *testing.T) {
+	// Set up source data file.
+	tempDir, err := ioutil.TempDir("", "TestSendfile")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.RemoveAll(tempDir)
+	name := filepath.Join(tempDir, "source")
+	const contents = "contents"
+	err = ioutil.WriteFile(name, []byte(contents), 0666)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	done := make(chan bool)
+
+	// Start server listening on a socket.
+	ln, err := net.Listen("tcp", "127.0.0.1:0")
+	if err != nil {
+		t.Skipf("listen failed: %s\n", err)
+	}
+	defer ln.Close()
+	go func() {
+		conn, err := ln.Accept()
+		if err != nil {
+			t.Fatal(err)
+		}
+		defer conn.Close()
+		b, err := ioutil.ReadAll(conn)
+		if string(b) != contents {
+			t.Errorf("contents not transmitted: got %s (len=%d), want %s", string(b), len(b), contents)
+		}
+		done <- true
+	}()
+
+	// Open source file.
+	src, err := os.Open(name)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// Send source file to server.
+	conn, err := net.Dial("tcp", ln.Addr().String())
+	if err != nil {
+		t.Fatal(err)
+	}
+	file, err := conn.(*net.TCPConn).File()
+	if err != nil {
+		t.Fatal(err)
+	}
+	var off int64
+	n, err := unix.Sendfile(int(file.Fd()), int(src.Fd()), &off, len(contents))
+	if err != nil {
+		t.Errorf("Sendfile failed %s\n", err)
+	}
+	if n != len(contents) {
+		t.Errorf("written count wrong: want %d, got %d", len(contents), n)
+	}
+	// Note: off is updated on some systems and not others. Oh well.
+	// Linux: increments off by the amount sent.
+	// Darwin: leaves off unchanged.
+	// It would be nice to fix Darwin if we can.
+	if off != 0 && off != int64(len(contents)) {
+		t.Errorf("offset wrong: god %d, want %d or %d", off, 0, len(contents))
+	}
+	// The cursor position should be unchanged.
+	pos, err := src.Seek(0, 1)
+	if err != nil {
+		t.Errorf("can't get cursor position %s\n", err)
+	}
+	if pos != 0 {
+		t.Errorf("cursor position wrong: got %d, want 0", pos)
+	}
+
+	file.Close() // Note: required to have the close below really send EOF to the server.
+	conn.Close()
+
+	// Wait for server to close.
+	<-done
+}