From b555e659c4974acc423083b71b1bd5ec6a926046 Mon Sep 17 00:00:00 2001
From: Nobuyoshi Nakada <nobu@ruby-lang.org>
Date: Sun, 5 Dec 2021 18:08:55 +0900
Subject: Do not use `fcopyfile` if appending to non-empty file [Bug #18388]

`fcopyfile` appends `src` to `to` and then truncates `to` to it's
original size.
---
 test/ruby/test_io.rb | 12 ++++++++++++
 1 file changed, 12 insertions(+)

(limited to 'test')

diff --git a/test/ruby/test_io.rb b/test/ruby/test_io.rb
index 96e572b98d..71a8911fbd 100644
--- a/test/ruby/test_io.rb
+++ b/test/ruby/test_io.rb
@@ -476,6 +476,18 @@ class TestIO < Test::Unit::TestCase
     }
   end
 
+  def test_copy_stream_append_to_nonempty
+    with_srccontent("foobar") {|src, content|
+      preface = 'preface'
+      File.write('dst', preface)
+      File.open('dst', 'ab') do |dst|
+        ret = IO.copy_stream(src, dst)
+        assert_equal(content.bytesize, ret)
+        assert_equal(preface + content, File.read("dst"))
+      end
+    }
+  end
+
   def test_copy_stream_smaller
     with_srccontent {|src, content|
 
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