merge revision(s) ef525b012a709077ea2797e8642fae0b61234063,dc9a13abeef5a2b936fbb55edc112b8b382a05e7: [Backport #18775]

	Explicit handling of frozen strings in `IO::Buffer#for`. (#5892)

	---
	 io_buffer.c                 | 122 +++++++++++++++++++++++++++++++++++---------
	 test/ruby/test_io_buffer.rb |  36 +++++++------
	 2 files changed, 117 insertions(+), 41 deletions(-)

	Fix rdoc of IO::Buffer [ci skip]

	---
	 io_buffer.c | 15 +--------------
	 1 file changed, 1 insertion(+), 14 deletions(-)

1 files changed, 20 insertions, 16 deletions

diff --git a/test/ruby/test_io_buffer.rb b/test/ruby/test_io_buffer.rb
index 7e3b467ed5..e3f7021b26 100644
--- a/test/ruby/test_io_buffer.rb
+++ b/test/ruby/test_io_buffer.rb
@@ -88,30 +88,34 @@ class TestIOBuffer < Test::Unit::TestCase
   def test_string_mapped
     string = "Hello World"
     buffer = IO::Buffer.for(string)
-    refute buffer.readonly?
-
-    # Cannot modify string as it's locked by the buffer:
-    assert_raise RuntimeError do
-      string[0] = "h"
-    end
-
-    buffer.set_value(:U8, 0, "h".ord)
-
-    # Buffer releases it's ownership of the string:
-    buffer.free
-
-    assert_equal "hello World", string
-    string[0] = "H"
-    assert_equal "Hello World", string
+    assert buffer.readonly?
   end
 
   def test_string_mapped_frozen
     string = "Hello World".freeze
     buffer = IO::Buffer.for(string)
-
     assert buffer.readonly?
   end
 
+  def test_string_mapped_mutable
+    string = "Hello World"
+    IO::Buffer.for(string) do |buffer|
+      refute buffer.readonly?
+
+      # Cannot modify string as it's locked by the buffer:
+      assert_raise RuntimeError do
+        string[0] = "h"
+      end
+
+      buffer.set_value(:U8, 0, "h".ord)
+
+      # Buffer releases it's ownership of the string:
+      buffer.free
+
+      assert_equal "hello World", string
+    end
+  end
+
   def test_non_string
     not_string = Object.new