[ruby/stringio] Define `StringIO::MAX_LENGTH`

https://github.com/ruby/stringio/commit/0205bd1c86

ext/stringio/stringio.c

@@ -1861,6 +1861,10 @@ Init_stringio(void)
 
     rb_include_module(StringIO, rb_mEnumerable);
     rb_define_alloc_func(StringIO, strio_s_allocate);
+
+    /* Maximum length that a StringIO instance can hold */
+    rb_define_const(StringIO, "MAX_LENGTH", LONG2NUM(LONG_MAX));
+
     rb_define_singleton_method(StringIO, "new", strio_s_new, -1);
     rb_define_singleton_method(StringIO, "open", strio_s_open, -1);
     rb_define_method(StringIO, "initialize", strio_initialize, -1);

test/stringio/test_stringio.rb

@@ -237,9 +237,8 @@ class TestStringIO < Test::Unit::TestCase
 
   def test_write_integer_overflow
     f = StringIO.new
+    f.pos = StringIO::MAX_LENGTH
     assert_raise(ArgumentError) {
-      # JRuby errors when setting pos to an out-of-range value
-      f.pos = RbConfig::LIMITS["LONG_MAX"]
       f.write("pos + len overflows")
     }
   end
@@ -900,8 +899,9 @@ class TestStringIO < Test::Unit::TestCase
   end
 
   def test_overflow
-    return if RbConfig::SIZEOF["void*"] > RbConfig::SIZEOF["long"]
-    limit = RbConfig::LIMITS["INTPTR_MAX"] - 0x10
+    intptr_max = RbConfig::LIMITS["INTPTR_MAX"]
+    return if intptr_max > StringIO::MAX_LENGTH
+    limit = intptr_max - 0x10
     assert_separately(%w[-rstringio], "#{<<-"begin;"}\n#{<<-"end;"}")
     begin;
       limit = #{limit}