From f423f6e10c0c226dfed98e7cb7a5d489191dfa35 Mon Sep 17 00:00:00 2001
From: Jeremy Evans <code@jeremyevans.net>
Date: Mon, 17 Feb 2025 17:23:14 -0800
Subject: 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 999bbd9d2a..3668085d83 100644
--- a/test/ruby/test_io.rb
+++ b/test/ruby/test_io.rb
@@ -1141,6 +1141,34 @@ 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{payload << it})
+      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 {
-- 
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