Eagerly defrost chilled strings

[Bug #20390]

Co-authored-by: Jean Boussier <[email protected]>

ext/stringio/stringio.c

@@ -49,6 +49,13 @@ static long strio_write(VALUE self, VALUE str);
 #define IS_STRIO(obj) (rb_typeddata_is_kind_of((obj), &strio_data_type))
 #define error_inval(msg) (rb_syserr_fail(EINVAL, msg))
 #define get_enc(ptr) ((ptr)->enc ? (ptr)->enc : !NIL_P((ptr)->string) ? rb_enc_get((ptr)->string) : NULL)
+#ifndef HAVE_RB_STR_CHILLED_P
+static int
+rb_str_chilled_p(VALUE str)
+{
+    return 0;
+}
+#endif
 
 static struct StringIO *
 strio_alloc(void)
@@ -287,13 +294,17 @@ strio_init(int argc, VALUE *argv, struct StringIO *ptr, VALUE self)
     else if (!argc) {
 	string = rb_enc_str_new("", 0, rb_default_external_encoding());
     }
-    if (!NIL_P(string) && OBJ_FROZEN_RAW(string)) {
+
+    if (!NIL_P(string) && OBJ_FROZEN_RAW(string) && !rb_str_chilled_p(string)) {
 	if (ptr->flags & FMODE_WRITABLE) {
 	    rb_syserr_fail(EACCES, 0);
 	}
     }
     else {
 	if (NIL_P(vmode)) {
+            if (!NIL_P(string) && rb_str_chilled_p(string)) {
+                rb_str_modify(string);
+            }
 	    ptr->flags |= FMODE_WRITABLE;
 	}
     }
@@ -481,6 +492,9 @@ strio_set_string(VALUE self, VALUE string)
     rb_io_taint_check(self);
     ptr->flags &= ~FMODE_READWRITE;
     StringValue(string);
+    if (rb_str_chilled_p(string)) {
+        rb_str_modify(string);
+    }
     ptr->flags = OBJ_FROZEN(string) ? FMODE_READABLE : FMODE_READWRITE;
     ptr->pos = 0;
     ptr->lineno = 0;

test/stringio/test_stringio.rb

@@ -978,6 +978,39 @@ def test_coderange_after_overwrite
     assert_predicate(s.string, :ascii_only?)
   end
 
+  if eval(%{ "test".frozen? && !"test".equal?("test") }) # Ruby 3.4+ chilled strings
+    def test_chilled_string
+      silence_deprecation_warnings do
+        chilled_string = eval(%{""})
+        io = StringIO.new(chilled_string)
+        io << "test"
+        assert_equal("test", io.string)
+        assert_same(chilled_string, io.string)
+      end
+    end
+
+    def test_chilled_string_string_set
+      silence_deprecation_warnings do
+        io = StringIO.new
+        chilled_string = eval(%{""})
+        io.string = chilled_string
+        io << "test"
+        assert_equal("test", io.string)
+        assert_same(chilled_string, io.string)
+      end
+    end
+  end
+
+  private
+
+  def silence_deprecation_warnings
+    deprecated_was = Warning[:deprecated]
+    Warning[:deprecated] = false
+    yield
+  ensure
+    Warning[:deprecated] = deprecated_was
+  end
+
   def assert_string(content, encoding, str, mesg = nil)
     assert_equal([content, encoding], [str, str.encoding], mesg)
   end