Forbid JSON::ResumableParser#<< while the parser is in use

ext/json/ext/parser/parser.c

@@ -2274,6 +2274,8 @@ static VALUE cResumableParser_initialize(int argc, VALUE *argv, VALUE self)
     return self;
 }
 
+static JSON_ResumableParser *ResumableParser_acquire(VALUE self, bool lock);
+
 /*
  * call-seq: self << string -> self
  *
@@ -2282,13 +2284,14 @@ static VALUE cResumableParser_initialize(int argc, VALUE *argv, VALUE self)
 static VALUE cResumableParser_feed(VALUE self, VALUE str)
 {
     rb_check_frozen(self);
+
+    JSON_ResumableParser *parser = ResumableParser_acquire(self, false);
+
     str = convert_encoding(str);
     if (!RSTRING_LEN(str)) {
         return self;
     }
 
-    JSON_ResumableParser *parser = cResumableParser_get(self);
-
     size_t offset = parser->state.cursor - parser->state.start;
     const size_t remaining = parser->state.end - parser->state.cursor;
 

test/json/resumable_parser_test.rb

@@ -243,6 +243,26 @@ def test_reentrency_prevented_in_partial_value
     refute_predicate parser, :value?
   end
 
+  def test_feed_during_callback_prevented
+    parser = nil
+    callback = ->(o) do
+      parser << '99' if o == 1 # feeding while a parse is running must be rejected
+      o
+    end
+    parser = new_parser(on_load: callback)
+    parser << '[1, 2, 3]'
+    error = assert_raise ArgumentError do
+      parser.parse
+    end
+    assert_equal "ResumableParser can't be used recursively", error.message
+
+    # the lock is released, so the parser stays usable
+    parser = new_parser
+    parser << '[1, 2, 3]'
+    assert parser.parse
+    assert_equal [1, 2, 3], parser.value
+  end
+
   def test_exception_unlock_parser
     called = false
     parser = nil