Fix JSON::ResumableParser#parse leaking the in_use lock on an empty buffer

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: 2 people

ext/json/ext/parser/parser.c

@@ -2377,6 +2377,7 @@ static VALUE cResumableParser_parse(VALUE self)
 {
     JSON_ResumableParser *parser = ResumableParser_acquire(self, true);
     if (!parser->buffer) {
+        parser->in_use = false;
         return Qfalse;
     }
 

test/json/resumable_parser_test.rb

@@ -48,6 +48,21 @@ def test_clear
     refute_predicate @parser, :value?
   end
 
+  def test_parse_with_empty_buffer_keeps_parser_usable
+    # parse before any feed must not leak the in_use lock
+    refute @parser.parse
+    @parser << '[1, 2, 3]'
+    assert @parser.parse
+    assert_equal [1, 2, 3], @parser.value
+
+    # same after a clear with no following feed
+    @parser.clear
+    refute @parser.parse
+    @parser << '[4]'
+    assert @parser.parse
+    assert_equal [4], @parser.value
+  end
+
   def test_parse_document_direct
     @parser << '[true]'
     assert_equal true, @parser.parse