From f85e5e01bafeca387e833b9d79cab43a8b22aa3d Mon Sep 17 00:00:00 2001
From: nagachika <nagachika@ruby-lang.org>
Date: Sun, 16 Mar 2025 20:10:00 +0900
Subject: merge revision(s) f423f6e10c0c226dfed98e7cb7a5d489191dfa35: [Backport
 #21131]

	Ensure IO.copy_stream buffer is an independent string

	Otherwise, changes to the buffer by the destination write method
	could result in data changing for supposedly independent strings.

	Fixes [Bug #21131]
---
 test/ruby/test_io.rb | 28 ++++++++++++++++++++++++++++
 1 file changed, 28 insertions(+)

(limited to 'test/ruby')

diff --git a/test/ruby/test_io.rb b/test/ruby/test_io.rb
index 51c9f2b83c..16de2e8a76 100644
--- a/test/ruby/test_io.rb
+++ b/test/ruby/test_io.rb
@@ -1116,6 +1116,34 @@ class TestIO < Test::Unit::TestCase
     }
   end
 
+  def test_copy_stream_dup_buffer
+    bug21131 = '[ruby-core:120961] [Bug #21131]'
+    mkcdtmpdir do
+      dst_class = Class.new do
+        def initialize(&block)
+          @block = block
+        end
+
+        def write(data)
+          @block.call(data.dup)
+          data.bytesize
+        end
+      end
+
+      rng = Random.new(42)
+      body = Tempfile.new("ruby-bug", binmode: true)
+      body.write(rng.bytes(16_385))
+      body.rewind
+
+      payload = []
+      IO.copy_stream(body, dst_class.new{|cls| payload << cls})
+      body.rewind
+      assert_equal(body.read, payload.join, bug21131)
+    ensure
+      body&.close
+    end
+  end
+
   def test_copy_stream_write_in_binmode
     bug8767 = '[ruby-core:56518] [Bug #8767]'
     mkcdtmpdir {
-- 
cgit v1.2.3