merge revision(s) b555e659c4974acc423083b71b1bd5ec6a926046: [Backport #18388]

	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.
	---
	 io.c                 |  7 +++++++
	 test/ruby/test_io.rb | 12 ++++++++++++
	 2 files changed, 19 insertions(+)

1 files changed, 12 insertions, 0 deletions

diff --git a/test/ruby/test_io.rb b/test/ruby/test_io.rb
index 5b84fd6a4d..e178e7579b 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|