test_broken_bignum: stop using fork + waitpid

lib/json/truffle_ruby/generator.rb

@@ -413,7 +413,7 @@ def generate(obj, anIO = nil)
               buf << obj.to_json(self)
             end
           when Integer
-            buf << obj.to_s
+            buf << String(obj)
           when Symbol
             if @strict
               fast_serialize_string(obj.name, buf)
@@ -652,7 +652,9 @@ def json_transform(state)
 
         module Integer
           # Returns a JSON string representation for this Integer number.
-          def to_json(*) to_s end
+          def to_json(*)
+            String(self)
+          end
         end
 
         module Float

test/json/json_generator_test.rb

@@ -493,25 +493,27 @@ def foo.to_h
     assert_equal '2', state.indent
   end
 
-  def test_broken_bignum # [ruby-core:38867]
-    pid = fork do
-      x = 1 << 64
-      x.class.class_eval do
-        def to_s
-        end
+  def test_broken_bignum # [Bug #5173]
+    bignum = 1 << 64
+    bignum_to_s = bignum.to_s
+
+    original_to_s = bignum.class.instance_method(:to_s)
+    bignum.class.class_eval do
+      def to_s
+        nil
       end
-      begin
-        JSON::Ext::Generator::State.new.generate(x)
-        exit 1
-      rescue TypeError
-        exit 0
+      alias_method :to_s, :to_s
+    end
+    case RUBY_PLATFORM
+    when "java"
+      assert_equal bignum_to_s, JSON.generate(bignum)
+    else
+      assert_raise(TypeError) do
+        JSON.generate(bignum)
       end
     end
-    _, status = Process.waitpid2(pid)
-    assert status.success?
-  rescue NotImplementedError
-    # forking to avoid modifying core class of a parent process and
-    # introducing race conditions of tests are run in parallel
+  ensure
+    bignum.class.define_method(:to_s, original_to_s) if original_to_s
   end
 
   def test_hash_likeness_set_symbol