Eagerly defrost chilled strings

[Feature #20390]

Co-authored-by: Jean Boussier <byroot@ruby-lang.org>

ext/stringio/stringio.c

@@ -15,6 +15,8 @@
 static const char *const
 STRINGIO_VERSION = "3.1.1";
 
+#include <stdbool.h>
+
 #include "ruby.h"
 #include "ruby/io.h"
 #include "ruby/encoding.h"
@@ -49,6 +51,13 @@ 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 bool
+rb_str_chilled_p(VALUE str)
+{
+    return false;
+}
+#endif
 
 static struct StringIO *
 strio_alloc(void)
@@ -166,8 +175,14 @@ writable(VALUE strio)
 static void
 check_modifiable(struct StringIO *ptr)
 {
-    if (OBJ_FROZEN(ptr->string)) {
-	rb_raise(rb_eIOError, "not modifiable string");
+    if (NIL_P(ptr->string)) {
+        /* Null device StringIO */
+    }
+    else if (rb_str_chilled_p(ptr->string)) {
+        rb_str_modify(ptr->string);
+    }
+    else if (OBJ_FROZEN_RAW(ptr->string)) {
+        rb_raise(rb_eIOError, "not modifiable string");
     }
 }
 
@@ -287,7 +302,8 @@ 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);
 	}
@@ -481,7 +497,7 @@ strio_set_string(VALUE self, VALUE string)
     rb_io_taint_check(self);
     ptr->flags &= ~FMODE_READWRITE;
     StringValue(string);
-    ptr->flags = OBJ_FROZEN(string) ? FMODE_READABLE : FMODE_READWRITE;
+    ptr->flags = OBJ_FROZEN(string) && !rb_str_chilled_p(string) ? FMODE_READABLE : FMODE_READWRITE;
     ptr->pos = 0;
     ptr->lineno = 0;
     RB_OBJ_WRITE(self, &ptr->string, string);

test/stringio/test_stringio.rb

@@ -978,6 +978,27 @@ 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
+      chilled_string = eval(%{""})
+      io = StringIO.new(chilled_string)
+      assert_warning(/literal string will be frozen/) { io << "test" }
+      assert_equal("test", io.string)
+      assert_same(chilled_string, io.string)
+    end
+
+    def test_chilled_string_string_set
+      io = StringIO.new
+      chilled_string = eval(%{""})
+      io.string = chilled_string
+      assert_warning(/literal string will be frozen/) { io << "test" }
+      assert_equal("test", io.string)
+      assert_same(chilled_string, io.string)
+    end
+  end
+
+  private
+
   def assert_string(content, encoding, str, mesg = nil)
     assert_equal([content, encoding], [str, str.encoding], mesg)
   end