Fix JSON::ResumableParser stalling on a NUL byte

ext/json/ext/parser/parser.c

@@ -1642,7 +1642,7 @@ ALWAYS_INLINE(static) bool json_parse_any(JSON_ParserState *state, JSON_ParserCo
                     state->cursor++;
                     value = json_decode_array(state, config, 0);
                     break;
-                } else if (resumable && next == 0) {
+                } else if (resumable && eos(state)) {
                     state->cursor = value_start;
                     return false;
                 }
@@ -1691,8 +1691,14 @@ ALWAYS_INLINE(static) bool json_parse_any(JSON_ParserState *state, JSON_ParserCo
             }
 
             case 0:
-                return false;
-
+                // peek() returns 0 both at end-of-stream and for a literal NUL byte in the
+                // buffer. Only a genuine EOS means "feed me more"; a NUL byte that is not at
+                // EOS is just an invalid character.
+                if (eos(state)) {
+                    return false;
+                } else {
+                    raise_syntax_error("unexpected NULL byte: %s", state);
+                }
             default:
                 raise_syntax_error("unexpected character: %s", state);
         }
@@ -1807,7 +1813,7 @@ ALWAYS_INLINE(static) bool json_parse_any(JSON_ParserState *state, JSON_ParserCo
                 case JSON_PHASE_OBJECT_KEY:     JSON_UNREACHABLE_RETURN(false);
                 case JSON_PHASE_OBJECT_COLON:   goto JSON_PHASE_OBJECT_COLON;
             }
-        } else if (resumable && next_char == 0) {
+        } else if (resumable && eos(state)) {
             return false;
         } else {
             raise_syntax_error("expected ',' or ']' after array value", state);
@@ -1858,7 +1864,7 @@ ALWAYS_INLINE(static) bool json_parse_any(JSON_ParserState *state, JSON_ParserCo
                 case JSON_PHASE_OBJECT_KEY:     JSON_UNREACHABLE_RETURN(false);
                 case JSON_PHASE_OBJECT_COLON:   goto JSON_PHASE_OBJECT_COLON;
             }
-        } else if (resumable && next_char == 0) {
+        } else if (resumable && eos(state)) {
             return false;
         } else {
             raise_syntax_error("expected ',' or '}' after object value, got: %s", state);

test/json/resumable_parser_test.rb

@@ -133,6 +133,30 @@ def test_parse_byte_by_byte_numbers
     assert_resumed_parsing('123 ')
   end
 
+  def test_nul_byte_is_a_syntax_error
+    # A NUL byte in a structural position must raise, not stall forever waiting for more input
+    # (peek() returns 0 both at EOS and for a literal NUL byte).
+    assert_parse_error "\x00"           # document value
+    assert_parse_error "[\x00]"         # first array element
+    assert_parse_error "[1\x00]"        # after an array element (',' or ']' expected)
+    assert_parse_error "[1,\x00]"       # array element after ','
+    assert_parse_error "{\x00}"         # object key
+    assert_parse_error "{\"a\":1\x00}"  # after an object value (',' or '}' expected)
+    assert_parse_error "{\"a\":1,\x00}" # object key after ','
+  end
+
+  def test_incomplete_input_at_structural_positions_resumes
+    # Counterpart of test_nul_byte_is_a_syntax_error: a genuine EOS at the same positions must
+    # stay incomplete (return false), not raise -- this is what distinguishes EOS from a NUL.
+    assert_incomplete "["
+    assert_incomplete "[1"
+    assert_incomplete "[1,"
+    assert_incomplete "{"
+    assert_incomplete "{\"a\""
+    assert_incomplete "{\"a\":1"
+    assert_incomplete "{\"a\":1,"
+  end
+
   def test_rest
     @parser << '[1, 2, 3, "unterminated string'
     refute @parser.parse
@@ -316,6 +340,20 @@ def test_buffer_shrink
 
   private
 
+  def assert_parse_error(json)
+    parser = new_parser
+    parser << json
+    assert_raise(JSON::ParserError, "expected a parse error for #{json.inspect}") do
+      parser.parse
+    end
+  end
+
+  def assert_incomplete(json)
+    parser = new_parser
+    parser << json
+    refute(parser.parse, "expected #{json.inspect} not to produce a value")
+  end
+
   def assert_partial_value(expected, json)
     parser = new_parser
     parser << json