fix variable lookup parse timing out with missing closing bracket

lib/liquid.rb

@@ -41,7 +41,7 @@ module Liquid
   AnyStartingTag              = /#{TagStart}|#{VariableStart}/o
   PartialTemplateParser       = /#{TagStart}.*?#{TagEnd}|#{VariableStart}.*?#{VariableIncompleteEnd}/om
   TemplateParser              = /(#{PartialTemplateParser}|#{AnyStartingTag})/om
-  VariableParser              = /\[(?:[^\[\]]+|\g<0>)*\]|#{VariableSegment}+\??/o
+  VariableParser              = /\[(?>[^\[\]]+|\g<0>)*\]|#{VariableSegment}+\??/o
 
   RAISE_EXCEPTION_LAMBDA = ->(_e) { raise }
 

test/integration/variable_test.rb

@@ -1,6 +1,7 @@
 # frozen_string_literal: true
 
 require 'test_helper'
+require 'timeout'
 
 class VariableTest < Minitest::Test
   include Liquid
@@ -169,4 +170,38 @@ def test_double_nested_variable_lookup
       }
     )
   end
+
+  def test_variable_lookup_should_not_hang_with_invalid_syntax
+    Timeout.timeout(1) do
+      assert_template_result(
+        'bar',
+        "{{['foo'}}",
+        {
+          'foo' => 'bar',
+        },
+        error_mode: :lax,
+      )
+    end
+
+    very_long_key = "1234567890" * 100
+
+    template_list = [
+      "{{['#{very_long_key}']}}", # valid
+      "{{['#{very_long_key}'}}", # missing closing bracket
+      "{{[['#{very_long_key}']}}", # extra open bracket
+    ]
+
+    template_list.each do |template|
+      Timeout.timeout(1) do
+        assert_template_result(
+          'bar',
+          template,
+          {
+            very_long_key => 'bar',
+          },
+          error_mode: :lax,
+        )
+      end
+    end
+  end
 end

test/unit/regexp_unit_test.rb

@@ -1,6 +1,7 @@
 # frozen_string_literal: true
 
 require 'test_helper'
+require 'timeout'
 
 class RegexpUnitTest < Minitest::Test
   include Liquid
@@ -37,10 +38,22 @@ def test_quoted_words_in_the_middle
 
   def test_variable_parser
     assert_equal(['var'],                               'var'.scan(VariableParser))
+    assert_equal(['[var]'],                             '[var]'.scan(VariableParser))
     assert_equal(['var', 'method'],                     'var.method'.scan(VariableParser))
     assert_equal(['var', '[method]'],                   'var[method]'.scan(VariableParser))
     assert_equal(['var', '[method]', '[0]'],            'var[method][0]'.scan(VariableParser))
     assert_equal(['var', '["method"]', '[0]'],          'var["method"][0]'.scan(VariableParser))
     assert_equal(['var', '[method]', '[0]', 'method'],  'var[method][0].method'.scan(VariableParser))
   end
+
+  def test_variable_parser_with_large_input
+    Timeout.timeout(1) { assert_equal(['[var]'], '[var]'.scan(VariableParser)) }
+
+    very_long_string = "foo" * 1000
+
+    # valid dynamic lookup
+    Timeout.timeout(1) { assert_equal(["[#{very_long_string}]"], "[#{very_long_string}]".scan(VariableParser)) }
+    # invalid dynamic lookup with missing closing bracket
+    Timeout.timeout(1) { assert_equal([very_long_string], "[#{very_long_string}".scan(VariableParser)) }
+  end
 end # RegexpTest