Implement rb_io_descriptor()

* Avoids an issue if the fallback is declared with the same name:
  ruby/io-console@d1d9aef
* Specs change from https://github.com/ruby/ruby/pull/6511/files

Author: eregon

CHANGELOG.md

@@ -145,6 +145,7 @@ Compatibility:
 * Add `rb_io_maybe_wait_readable`, `rb_io_maybe_wait_writable` and `rb_io_maybe_wait` functions (#2733, @andrykonchin).
 * `StringIO#set_encoding` should coerce the argument to an Encoding (#2954, @eregon).
 * Implement changes of Ruby 3.0 to `IO#wait` (#2953, @larskanis).
+* Implement `rb_io_descriptor()` (@eregon).
 
 Performance:
 

lib/cext/ABI_version.txt

@@ -1 +1 @@
-6
+7

lib/truffle/truffle/cext.rb

@@ -1254,6 +1254,10 @@ def rb_io_puts(out, args)
     Truffle::IOOperations.puts out, args
   end
 
+  def rb_io_descriptor(io)
+    Primitive.io_fd(io)
+  end
+
   def rb_equal(a, b)
     Primitive.same_or_equal?(a, b)
   end

spec/ruby/optional/capi/ext/io_spec.c

@@ -28,9 +28,13 @@ static int set_non_blocking(int fd) {
 }
 
 static int io_spec_get_fd(VALUE io) {
+#ifdef RUBY_VERSION_IS_3_1
+  return rb_io_descriptor(io);
+#else
   rb_io_t* fp;
   GetOpenFile(io, fp);
   return fp->fd;
+#endif
 }
 
 VALUE io_spec_GetOpenFile_fd(VALUE self, VALUE io) {

spec/ruby/optional/capi/io_spec.rb

@@ -175,13 +175,18 @@
     end
   end
 
-  describe "GetOpenFile" do
+  describe "rb_io_descriptor or GetOpenFile" do
     it "allows access to the system fileno" do
       @o.GetOpenFile_fd($stdin).should == 0
       @o.GetOpenFile_fd($stdout).should == 1
       @o.GetOpenFile_fd($stderr).should == 2
       @o.GetOpenFile_fd(@io).should == @io.fileno
     end
+
+    it "raises IOError if the IO is closed" do
+      @io.close
+      -> { @o.GetOpenFile_fd(@io) }.should raise_error(IOError, "closed stream")
+    end
   end
 
   describe "rb_io_binmode" do

src/main/c/cext/io.c

@@ -14,6 +14,14 @@
 
 // IO, rb_io_*
 
+int rb_io_descriptor(VALUE io) {
+  int fd = polyglot_as_i32(RUBY_CEXT_INVOKE_NO_WRAP("rb_io_descriptor", io));
+  if (fd < 0) {
+    rb_raise(rb_eIOError, "closed stream");
+  }
+  return fd;
+}
+
 void rb_io_check_writable(rb_io_t *io) {
   if (!rb_tr_writable(io->mode)) {
     rb_raise(rb_eIOError, "not opened for writing");