From e77e6e9d1237a6999a1e1bb6dcd2c7fa8e6187b4 Mon Sep 17 00:00:00 2001
From: "Yukihiro \"Matz\" Matsumoto" <matz@ruby.or.jp>
Date: Tue, 28 Apr 2020 12:05:14 +0900
Subject: Fix `_read_buf` to be more efficient; fix #4982

The bug was introduced by #4712. The `getc' problem resurrected.
It should be addressed soon.
---
 mrbgems/mruby-io/mrblib/io.rb | 9 ++-------
 1 file changed, 2 insertions(+), 7 deletions(-)

diff --git a/mrbgems/mruby-io/mrblib/io.rb b/mrbgems/mruby-io/mrblib/io.rb
index c0cfdc403..ce33f2367 100644
--- a/mrbgems/mruby-io/mrblib/io.rb
+++ b/mrbgems/mruby-io/mrblib/io.rb
@@ -170,13 +170,8 @@ class IO
   end
 
   def _read_buf
-    return @buf if @buf && @buf.bytesize >= 4 # maximum UTF-8 character is 4 bytes
-    @buf ||= ""
-    begin
-      @buf += sysread(BUF_SIZE)
-    rescue EOFError => e
-      raise e if @buf.empty?
-    end
+    return @buf if @buf && @buf.bytesize > 0
+    sysread(BUF_SIZE, @buf)
   end
 
   def ungetc(substr)
-- 
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