https://git.ruby-lang.org/ruby.git/commit/?id=7eaec9a6b3

From 7eaec9a6b355a685a3a9503a005b91f4b3cb4f39 Mon Sep 17 00:00:00 2001
From: NAKAMURA Usaku <usa@r...>
Date: Sat, 19 Mar 2022 22:41:16 +0900
Subject: 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(+)
---
 io.c                 |  7 +++++++
 test/ruby/test_io.rb | 12 ++++++++++++
 version.h            |  2 +-
 3 files changed, 20 insertions(+), 1 deletion(-)

diff --git a/io.c b/io.c
index 4d32e2ee3b..be6c6b637f 100644
--- a/io.c
+++ b/io.c
@@ -11022,6 +11022,13 @@ nogvl_fcopyfile(struct copy_stream_struct *stp) https://github.com/ruby/ruby/blob/trunk/io.c#L11022
         return 0;
     if (lseek(stp->dst_fd, 0, SEEK_CUR) > (off_t)0) /* if dst IO was already written */
         return 0;
+    if (fcntl(stp->dst_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_fd, 0, SEEK_END);
+        lseek(stp->dst_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 1e4d88ef1e..37f1477483 100644
--- a/test/ruby/test_io.rb
+++ b/test/ruby/test_io.rb
@@ -440,6 +440,18 @@ class TestIO < Test::Unit::TestCase https://github.com/ruby/ruby/blob/trunk/test/ruby/test_io.rb#L440
     }
   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|
 
diff --git a/version.h b/version.h
index 8afa4b53af..6fbb66953c 100644
--- a/version.h
+++ b/version.h
@@ -2,7 +2,7 @@ https://github.com/ruby/ruby/blob/trunk/version.h#L2
 # define RUBY_VERSION_MINOR RUBY_API_VERSION_MINOR
 #define RUBY_VERSION_TEENY 6
 #define RUBY_RELEASE_DATE RUBY_RELEASE_YEAR_STR"-"RUBY_RELEASE_MONTH_STR"-"RUBY_RELEASE_DAY_STR
-#define RUBY_PATCHLEVEL 215
+#define RUBY_PATCHLEVEL 216
 
 #define RUBY_RELEASE_YEAR 2022
 #define RUBY_RELEASE_MONTH 3
-- 
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