3 files changed, 30 insertions, 1 deletions
diff --git a/io.c b/io.c
index 17f8d5799a..7f23656049 100644
--- a/io.c
+++ b/io.c
@@ -13098,6 +13098,7 @@ copy_stream_fallback_body(VALUE arg)
     while (1) {
         long numwrote;
         long l;
+        rb_str_make_independent(buf);
         if (stp->copy_length < (rb_off_t)0) {
             l = buflen;
         }
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 {
diff --git a/version.h b/version.h
index 4b21ab9908..63f382a26d 100644
--- a/version.h
+++ b/version.h
@@ -11,7 +11,7 @@
 # define RUBY_VERSION_MINOR RUBY_API_VERSION_MINOR
 #define RUBY_VERSION_TEENY 7
 #define RUBY_RELEASE_DATE RUBY_RELEASE_YEAR_STR"-"RUBY_RELEASE_MONTH_STR"-"RUBY_RELEASE_DAY_STR
-#define RUBY_PATCHLEVEL 134
+#define RUBY_PATCHLEVEL 135
 
 #include "ruby/version.h"
 #include "ruby/internal/abi.h"