From 290ec2d92bca6472fdabce51fc331b3bcaa7129d Mon Sep 17 00:00:00 2001
From: Ian Lance Taylor <iant@golang.org>
Date: Sun, 26 Jan 2025 21:52:58 -0800
Subject: 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 | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

(limited to 'src/bufio/bufio.go')

diff --git a/src/bufio/bufio.go b/src/bufio/bufio.go
index d589701e19..5244ce2e0c 100644
--- a/src/bufio/bufio.go
+++ b/src/bufio/bufio.go
@@ -519,9 +519,11 @@ func (b *Reader) WriteTo(w io.Writer) (n int64, err error) {
 	b.lastByte = -1
 	b.lastRuneSize = -1
 
-	n, err = b.writeBuf(w)
-	if err != nil {
-		return
+	if b.r < b.w {
+		n, err = b.writeBuf(w)
+		if err != nil {
+			return
+		}
 	}
 
 	if r, ok := b.rd.(io.WriterTo); ok {
-- 
cgit v1.3-5-g9baa