net: broken sendfile on SmartOS/Solaris

In the event of a partial write on Solaris and some BSDs, the offset
pointer passed to sendfile() will be updated even though the function
returns -1 if errno is set to EAGAIN/EINTR.  In that case, calculate the
bytes written based on the difference between the updated offset and the
original offset.  If no bytes were written, and errno is set to
EAGAIN/EINTR, ignore the errno.

Fixes #13892

Change-Id: I6334b5ef2edcbebdaa7db36fa4f7785967313c2d
Reviewed-on: https://go-review.googlesource.com/21769
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/sendfile_test.go

@@ -0,0 +1,90 @@
+// Copyright 2016 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.
+
+package net
+
+import (
+	"crypto/sha256"
+	"encoding/hex"
+	"fmt"
+	"io"
+	"os"
+	"testing"
+)
+
+const (
+	twain       = "../compress/testdata/Mark.Twain-Tom.Sawyer.txt"
+	twainLen    = 387851
+	twainSHA256 = "461eb7cb2d57d293fc680c836464c9125e4382be3596f7d415093ae9db8fcb0e"
+)
+
+func TestSendFile(t *testing.T) {
+	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()
+
+			// Return file data using io.Copy, which should use
+			// sendFile if available.
+			sbytes, err := io.Copy(conn, f)
+			if err != nil {
+				errc <- err
+				return
+			}
+
+			if sbytes != twainLen {
+				errc <- fmt.Errorf("sent %d bytes; expected %d", sbytes, twainLen)
+				return
+			}
+		}()
+	}(ln)
+
+	// Connect to listener to retrieve file and verify digest matches
+	// expected.
+	c, err := Dial("tcp", ln.Addr().String())
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer c.Close()
+
+	h := sha256.New()
+	rbytes, err := io.Copy(h, c)
+	if err != nil {
+		t.Error(err)
+	}
+
+	if rbytes != twainLen {
+		t.Errorf("received %d bytes; expected %d", rbytes, twainLen)
+	}
+
+	if res := hex.EncodeToString(h.Sum(nil)); res != twainSHA256 {
+		t.Error("retrieved data hash did not match")
+	}
+
+	for err := range errc {
+		t.Error(err)
+	}
+}