Fix Marshal.dump(closed_io) to raise TypeError and allow encoding on closed IO

Mashalling a closed IO object raised "closed stream (IOError)" before instead of TypeError.
This changes IO#(in|ex)ternal_encoding to still return the encoding even if the underlying FD is closed.

Fixes bug #18077

io.c

@@ -12092,9 +12092,8 @@ rb_io_s_copy_stream(int argc, VALUE *argv, VALUE io)
 static VALUE
 rb_io_external_encoding(VALUE io)
 {
-    rb_io_t *fptr;
+    rb_io_t *fptr = RFILE(rb_io_taint_check(io))->fptr;
 
-    GetOpenFile(io, fptr);
     if (fptr->encs.enc2) {
 	return rb_enc_from_encoding(fptr->encs.enc2);
     }
@@ -12117,9 +12116,8 @@ rb_io_external_encoding(VALUE io)
 static VALUE
 rb_io_internal_encoding(VALUE io)
 {
-    rb_io_t *fptr;
+    rb_io_t *fptr = RFILE(rb_io_taint_check(io))->fptr;
 
-    GetOpenFile(io, fptr);
     if (!fptr->encs.enc2) return Qnil;
     return rb_enc_from_encoding(io_read_encoding(fptr));
 }

spec/ruby/core/io/external_encoding_spec.rb

@@ -94,8 +94,10 @@ describe "IO#external_encoding" do
     rm_r @name
   end
 
-  it "raises an IOError on closed stream" do
-    -> { IOSpecs.closed_io.external_encoding }.should raise_error(IOError)
+  ruby_version_is '3.1' do
+    it "can be retrieved from a closed stream" do
+      IOSpecs.closed_io.external_encoding.should equal(Encoding.default_external)
+    end
   end
 
   describe "with 'r' mode" do

spec/ruby/core/io/internal_encoding_spec.rb

@@ -113,8 +113,10 @@ describe "IO#internal_encoding" do
     Encoding.default_internal = @internal
   end
 
-  it "raises an IOError on closed stream" do
-    -> { IOSpecs.closed_io.internal_encoding }.should raise_error(IOError)
+  ruby_version_is '3.1' do
+    it "can be retrieved from a closed stream" do
+      IOSpecs.closed_io.internal_encoding.should equal(Encoding.default_internal)
+    end
   end
 
   describe "with 'r' mode" do

test/ruby/test_io.rb

@@ -3991,6 +3991,18 @@ __END__
     }
   end
 
+  def test_marshal_closed_io
+    bug18077 = '[ruby-core:104927] [Bug #18077]'
+    r, w = IO.pipe
+    r.close; w.close
+    assert_raise(TypeError, bug18077) {Marshal.dump(r)}
+
+    class << r
+      undef_method :closed?
+    end
+    assert_raise(TypeError, bug18077) {Marshal.dump(r)}
+  end
+
   def test_stdout_to_closed_pipe
     EnvUtil.invoke_ruby(["-e", "loop {puts :ok}"], "", true, true) do
       |in_p, out_p, err_p, pid|