net: add TestSendfileParts

Add test for freebsd issue #25809.

This test also fails on my Windows 10 Version 1803.
My hope is that adding new test will break one of our builders.

Updates #25722
Updates #25809

Change-Id: Ia103bc708b8fa3b9af57613acc44893f90b3fa18
Reviewed-on: https://go-review.googlesource.com/117775
Run-TryBot: Alex Brainman <alex.brainman@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/sendfile_test.go

@@ -7,11 +7,13 @@
 package net
 
 import (
+	"bytes"
 	"crypto/sha256"
 	"encoding/hex"
 	"fmt"
 	"io"
 	"os"
+	"runtime"
 	"testing"
 )
 
@@ -90,3 +92,65 @@ func TestSendfile(t *testing.T) {
 		t.Error(err)
 	}
 }
+
+func TestSendfileParts(t *testing.T) {
+	if runtime.GOOS == "freebsd" {
+		t.Skipf("skipping on %s (see golang.org/issue/25809 for details)", runtime.GOOS)
+	}
+
+	ln, err := newLocalListener("tcp")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer ln.Close()
+
+	errc := make(chan error, 1)
+	go func(ln Listener) {
+		// Wait for a connection.
+		conn, err := ln.Accept()
+		if err != nil {
+			errc <- err
+			close(errc)
+			return
+		}
+
+		go func() {
+			defer close(errc)
+			defer conn.Close()
+
+			f, err := os.Open(twain)
+			if err != nil {
+				errc <- err
+				return
+			}
+			defer f.Close()
+
+			for i := 0; i < 3; i++ {
+				// Return file data using io.CopyN, which should use
+				// sendFile if available.
+				_, err = io.CopyN(conn, f, 3)
+				if err != nil {
+					errc <- err
+					return
+				}
+			}
+		}()
+	}(ln)
+
+	c, err := Dial("tcp", ln.Addr().String())
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer c.Close()
+
+	buf := new(bytes.Buffer)
+	buf.ReadFrom(c)
+
+	if want, have := "Produced ", buf.String(); have != want {
+		t.Errorf("unexpected server reply %q, want %q", have, want)
+	}
+
+	for err := range errc {
+		t.Error(err)
+	}
+}