From 917ef1e51131d734f92efc946a0ab5ca4ff69be6 Mon Sep 17 00:00:00 2001
From: Martin Garton <garton@gmail.com>
Date: Tue, 27 Jun 2017 18:02:23 +0100
Subject: [PATCH] bufio: make Reader.Peek invalidate Unreads

Since Reader.Peek potentially reads from the underlying io.Reader,
discarding previous buffers, UnreadRune and UnreadByte cannot
necessarily work.  Change Peek to invalidate the unread buffers in all
cases (as allowed according to the documentation) and thus prevent
hiding bugs in the caller.

(This change was previoiusly merged and then reverted due concern about
being too close to a release)

Fixes #18556
---
 src/bufio/bufio.go      |  3 +++
 src/bufio/bufio_test.go | 18 ++++++++++++++++++
 2 files changed, 21 insertions(+)

diff --git a/src/bufio/bufio.go b/src/bufio/bufio.go
index 46df619228..ffb278ad9e 100644
--- a/src/bufio/bufio.go
+++ b/src/bufio/bufio.go
@@ -128,6 +128,9 @@ func (b *Reader) Peek(n int) ([]byte, error) {
 		return nil, ErrNegativeCount
 	}
 
+	b.lastByte = -1
+	b.lastRuneSize = -1
+
 	for b.w-b.r < n && b.w-b.r < len(b.buf) && b.err == nil {
 		b.fill() // b.w-b.r < len(b.buf) => buffer is not full
 	}
diff --git a/src/bufio/bufio_test.go b/src/bufio/bufio_test.go
index 34d70312f7..f7a0682e70 100644
--- a/src/bufio/bufio_test.go
+++ b/src/bufio/bufio_test.go
@@ -285,6 +285,24 @@ func TestUnreadRune(t *testing.T) {
 	}
 }
 
+func TestNoUnreadRuneAfterPeek(t *testing.T) {
+	br := NewReader(strings.NewReader("example"))
+	br.ReadRune()
+	br.Peek(1)
+	if err := br.UnreadRune(); err == nil {
+		t.Error("UnreadRune didn't fail after Peek")
+	}
+}
+
+func TestNoUnreadByteAfterPeek(t *testing.T) {
+	br := NewReader(strings.NewReader("example"))
+	br.ReadByte()
+	br.Peek(1)
+	if err := br.UnreadByte(); err == nil {
+		t.Error("UnreadByte didn't fail after Peek")
+	}
+}
+
 func TestUnreadByte(t *testing.T) {
 	segments := []string{"Hello, ", "world"}
 	r := NewReader(&StringReader{data: segments})