From 52e3c99cfb32e5ff91a2a15d506d2148af8fc813 Mon Sep 17 00:00:00 2001
From: Rob Pike <r@golang.org>
Date: Wed, 13 Oct 2010 17:12:43 -0700
Subject: [PATCH] bufio: bulletproof UnreadRune After a fill(), there is
 nothing to back up.  Make sure UnreadRune recognizes the situation.

Fixes #1137.
(Stops the crash, but doesn't make UnreadRune usable after a Peek()).

R=rsc
CC=golang-dev
https://golang.org/cl/2498041
---
 src/pkg/bufio/bufio.go      | 2 +-
 src/pkg/bufio/bufio_test.go | 9 +++++++++
 2 files changed, 10 insertions(+), 1 deletion(-)

diff --git a/src/pkg/bufio/bufio.go b/src/pkg/bufio/bufio.go
index 8c951903a1..70caf5dae8 100644
--- a/src/pkg/bufio/bufio.go
+++ b/src/pkg/bufio/bufio.go
@@ -226,7 +226,7 @@ func (b *Reader) ReadRune() (rune int, size int, err os.Error) {
 // regard it is stricter than UnreadByte, which will unread the last byte
 // from any read operation.)
 func (b *Reader) UnreadRune() os.Error {
-	if b.lastRuneSize < 0 {
+	if b.lastRuneSize < 0 || b.r == 0 {
 		return ErrInvalidUnreadRune
 	}
 	b.r -= b.lastRuneSize
diff --git a/src/pkg/bufio/bufio_test.go b/src/pkg/bufio/bufio_test.go
index fb0ed045ae..fe04b91691 100644
--- a/src/pkg/bufio/bufio_test.go
+++ b/src/pkg/bufio/bufio_test.go
@@ -564,3 +564,12 @@ func TestPeek(t *testing.T) {
 		t.Fatalf("want EOF got %v", err)
 	}
 }
+
+func TestPeekThenUnreadRune(t *testing.T) {
+	// This sequence used to cause a crash.
+	r := NewReader(strings.NewReader("x"))
+	r.ReadRune()
+	r.Peek(1)
+	r.UnreadRune()
+	r.ReadRune() // Used to panic here
+}