From 1d47a1184a4718a34ab1df4d9bf05a284aba4c70 Mon Sep 17 00:00:00 2001
From: Keith Randall <khr@golang.org>
Date: Fri, 26 Nov 2021 07:36:09 -0800
Subject: [PATCH] bufio: mention that panic at slicing means underlying reader
 is broken

Fixes #49795

Change-Id: I2b4fd14f0ed36b643522559bebf5ce52b1d7b304
Reviewed-on: https://go-review.googlesource.com/c/go/+/367214
Trust: Keith Randall <khr@golang.org>
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
---
 src/bufio/bufio.go | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/src/bufio/bufio.go b/src/bufio/bufio.go
index 9ea058db3ed..7483946fc0a 100644
--- a/src/bufio/bufio.go
+++ b/src/bufio/bufio.go
@@ -244,6 +244,8 @@ func (b *Reader) Read(p []byte) (n int, err error) {
 	}
 
 	// copy as much as we can
+	// Note: if the slice panics here, it is probably because
+	// the underlying reader returned a bad count. See issue 49795.
 	n = copy(p, b.buf[b.r:b.w])
 	b.r += n
 	b.lastByte = int(b.buf[b.r-1])