https://git.ruby-lang.org/ruby.git/commit/?id=b555e659c4

From b555e659c4974acc423083b71b1bd5ec6a926046 Mon Sep 17 00:00:00 2001
From: Nobuyoshi Nakada <nobu@r...>
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.
---
 io.c                 |  7 +++++++
 test/ruby/test_io.rb | 12 ++++++++++++
 2 files changed, 19 insertions(+)

diff --git a/io.c b/io.c
index b2ca5a69e37..bc64938648b 100644
--- a/io.c
+++ b/io.c
@@ -11483,6 +11483,13 @@ nogvl_fcopyfile(struct copy_stream_struct *stp) https://github.com/ruby/ruby/blob/trunk/io.c#L11483
         return 0;
     if (lseek(stp->dst_fptr->fd, 0, SEEK_CUR) > (off_t)0) /* if dst IO was already written */
         return 0;
+    if (fcntl(stp->dst_fptr->fd, F_GETFL) & O_APPEND) {
+        /* fcopyfile(3) appends src IO to dst IO and then truncates
+         * dst IO to src IO's original size. */
+        off_t end = lseek(stp->dst_fptr->fd, 0, SEEK_END);
+        lseek(stp->dst_fptr->fd, 0, SEEK_SET);
+        if (end > (off_t)0) return 0;
+    }
 
     if (src_offset > (off_t)0) {
         off_t r;
diff --git a/test/ruby/test_io.rb b/test/ruby/test_io.rb
index 96e572b98d7..71a8911fbd0 100644
--- a/test/ruby/test_io.rb
+++ b/test/ruby/test_io.rb
@@ -476,6 +476,18 @@ class TestIO < Test::Unit::TestCase https://github.com/ruby/ruby/blob/trunk/test/ruby/test_io.rb#L476
     }
   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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