bufio: don't do empty Write at start of WriteTo

The empty Write will cause the wrong thing to happen when using
io.Copy to copy to a package-based stream.

Fixes #71424

Change-Id: I046a27539447182692ac76a8bdd422327345dd8d
Reviewed-on: https://go-review.googlesource.com/c/go/+/644535
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@google.com>
Auto-Submit: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Mauri de Souza Meneguzzo <mauri870@gmail.com>
Reviewed-by: Damien Neil <dneil@google.com>

src/bufio/bufio.go

@@ -519,10 +519,12 @@ func (b *Reader) WriteTo(w io.Writer) (n int64, err error) {
 	b.lastByte = -1
 	b.lastRuneSize = -1
 
+	if b.r < b.w {
 		n, err = b.writeBuf(w)
 		if err != nil {
 			return
 		}
+	}
 
 	if r, ok := b.rd.(io.WriterTo); ok {
 		m, err := r.WriteTo(w)

src/bufio/bufio_test.go

@@ -1149,7 +1149,7 @@ func (w errorWriterToTest) Write(p []byte) (int, error) {
 var errorWriterToTests = []errorWriterToTest{
 	{1, 0, nil, io.ErrClosedPipe, io.ErrClosedPipe},
 	{0, 1, io.ErrClosedPipe, nil, io.ErrClosedPipe},
-	{0, 0, io.ErrUnexpectedEOF, io.ErrClosedPipe, io.ErrClosedPipe},
+	{0, 0, io.ErrUnexpectedEOF, io.ErrClosedPipe, io.ErrUnexpectedEOF},
 	{0, 1, io.EOF, nil, nil},
 }
 

src/bufio/net_test.go

@@ -0,0 +1,96 @@
+// Copyright 2025 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.
+
+//go:build unix
+
+package bufio_test
+
+import (
+	"bufio"
+	"io"
+	"net"
+	"path/filepath"
+	"strings"
+	"sync"
+	"testing"
+)
+
+// TestCopyUnixpacket tests that we can use bufio when copying
+// across a unixpacket socket. This used to fail due to an unnecessary
+// empty Write call that was interpreted as an EOF.
+func TestCopyUnixpacket(t *testing.T) {
+	tmpDir := t.TempDir()
+	socket := filepath.Join(tmpDir, "unixsock")
+
+	// Start a unixpacket server.
+	addr := &net.UnixAddr{
+		Name: socket,
+		Net:  "unixpacket",
+	}
+	server, err := net.ListenUnix("unixpacket", addr)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// Start a goroutine for the server to accept one connection
+	// and read all the data sent on the connection,
+	// reporting the number of bytes read on ch.
+	ch := make(chan int, 1)
+	var wg sync.WaitGroup
+	wg.Add(1)
+	go func() {
+		defer wg.Done()
+
+		tot := 0
+		defer func() {
+			ch <- tot
+		}()
+
+		serverConn, err := server.Accept()
+		if err != nil {
+			t.Error(err)
+			return
+		}
+
+		buf := make([]byte, 1024)
+		for {
+			n, err := serverConn.Read(buf)
+			tot += n
+			if err == io.EOF {
+				return
+			}
+			if err != nil {
+				t.Error(err)
+				return
+			}
+		}
+	}()
+
+	clientConn, err := net.DialUnix("unixpacket", nil, addr)
+	if err != nil {
+		// Leaves the server goroutine hanging. Oh well.
+		t.Fatal(err)
+	}
+
+	defer wg.Wait()
+	defer clientConn.Close()
+
+	const data = "data"
+	r := bufio.NewReader(strings.NewReader(data))
+	n, err := io.Copy(clientConn, r)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if n != int64(len(data)) {
+		t.Errorf("io.Copy returned %d, want %d", n, len(data))
+	}
+
+	clientConn.Close()
+	tot := <-ch
+
+	if tot != len(data) {
+		t.Errorf("server read %d, want %d", tot, len(data))
+	}
+}