Make Result#fields return interned strings in Ruby 3+ (#1181)

Author: casperisfine

ext/mysql2/extconf.rb

@@ -31,6 +31,8 @@ def add_ssl_defines(header)
 # Missing in RBX (https://github.com/rubinius/rubinius/issues/3771)
 have_func('rb_wait_for_single_fd')
 
+have_func("rb_enc_interned_str", "ruby.h")
+
 # borrowed from mysqlplus
 # http://github.com/oldmoe/mysqlplus/blob/master/ext/extconf.rb
 dirs = ENV.fetch('PATH').split(File::PATH_SEPARATOR) + %w[

ext/mysql2/result.c

@@ -171,11 +171,18 @@ static VALUE rb_mysql_result_fetch_field(VALUE self, unsigned int idx, int symbo
       rb_field = rb_intern3(field->name, field->name_length, rb_utf8_encoding());
       rb_field = ID2SYM(rb_field);
     } else {
-      rb_field = rb_str_new(field->name, field->name_length);
-      rb_enc_associate(rb_field, conn_enc);
-      if (default_internal_enc) {
+#ifdef HAVE_RB_ENC_INTERNED_STR
+      rb_field = rb_enc_interned_str(field->name, field->name_length, conn_enc);
+      if (default_internal_enc && default_internal_enc != conn_enc) {
+        rb_field = rb_str_to_interned_str(rb_str_export_to_enc(rb_field, default_internal_enc));
+      }
+#else
+      rb_field = rb_enc_str_new(field->name, field->name_length, conn_enc);
+      if (default_internal_enc && default_internal_enc != conn_enc) {
         rb_field = rb_str_export_to_enc(rb_field, default_internal_enc);
       }
+      rb_obj_freeze(rb_field);
+#endif
     }
     rb_ary_store(wrapper->fields, idx, rb_field);
   }

spec/mysql2/result_spec.rb

@@ -118,6 +118,13 @@
       result = @client.query "SELECT 'a', 'b', 'c'"
       expect(result.fields).to eql(%w[a b c])
     end
+
+    it "should return an array of frozen strings" do
+      result = @client.query "SELECT 'a', 'b', 'c'"
+      result.fields.each do |f|
+        expect(f).to be_frozen
+      end
+    end
   end
 
   context "#field_types" do