Fix process-global memory leak in folded_declaration_cache

In long-lived processes (e.g., Sidekiq workers rendering emails via
Premailer), the process-global `@folded_declaration_cache` on
`CssParser::Parser` accumulates entries from every CSS parse operation
and is never cleared, causing unbounded memory growth.

The class-level `@folded_declaration_cache` (and its `attr_reader`) on
`CssParser::Parser` was defined as a class instance variable that
persists for the lifetime of the process. While `reset!` (called in
`initialize`) creates a per-instance `@folded_declaration_cache`, the
class-level variable and accessor remained, leaking the global
reference.

Similarly, `CssParser.merge` set a module-level
`@folded_declaration_cache = {}` on every call, which also persisted
at the module level.

This commit:
- Removes the class-level `@folded_declaration_cache` and its
  `class << self; attr_reader` from `CssParser::Parser`
- Removes the module-level `@folded_declaration_cache` assignment
  in `CssParser.merge`
- The per-instance cache (set in `reset!`) is already properly scoped
  and gets garbage collected with the `Parser` instance
- Adds tests verifying the cache is per-instance and not class-level

Amp-Thread-ID: https://ampcode.com/threads/T-019c8b1f-829d-70cd-83c5-3409a635e616
Co-authored-by: Amp <amp@ampcode.com>

Author: sokolikp

lib/css_parser.rb

@@ -52,8 +52,6 @@ module CssParser
   # TODO: declaration_hashes should be able to contain a RuleSet
   #       this should be a Class method
   def self.merge(*rule_sets)
-    @folded_declaration_cache = {}
-
     # in case called like CssParser.merge([rule_set, rule_set])
     rule_sets.flatten! if rule_sets[0].is_a?(Array)
 

lib/css_parser/parser.rb

@@ -31,12 +31,6 @@ class Parser
     # Array of CSS files that have been loaded.
     attr_reader   :loaded_uris
 
-    #--
-    # Class variable? see http://www.oreillynet.com/ruby/blog/2007/01/nubygems_dont_use_class_variab_1.html
-    #++
-    @folded_declaration_cache = {}
-    class << self; attr_reader :folded_declaration_cache; end
-
     def initialize(options = {})
       @options = {
         absolute_paths: false,

test/test_css_parser_folded_declaration_cache.rb

@@ -0,0 +1,42 @@
+# frozen_string_literal: true
+
+require_relative 'test_helper'
+
+class CssParserFoldedDeclarationCacheTest < Minitest::Test
+  include CssParser
+
+  def test_folded_declaration_cache_is_per_instance
+    cp1 = Parser.new
+    cp2 = Parser.new
+
+    cp1.add_block!('p { color: red; }')
+    cp2.add_block!('p { color: blue; }')
+
+    cache1 = cp1.send(:instance_variable_get, :@folded_declaration_cache)
+    cache2 = cp2.send(:instance_variable_get, :@folded_declaration_cache)
+
+    refute_same cache1, cache2
+  end
+
+  def test_folded_declaration_cache_does_not_persist_across_instances
+    cp1 = Parser.new
+    cp1.add_block!('p { color: red; }')
+
+    cp2 = Parser.new
+    cache = cp2.send(:instance_variable_get, :@folded_declaration_cache)
+
+    assert_empty cache
+  end
+
+  def test_no_class_level_folded_declaration_cache
+    refute Parser.respond_to?(:folded_declaration_cache),
+           'Parser should not have a class-level folded_declaration_cache accessor'
+  end
+
+  def test_cache_is_reset_on_new_instance
+    cp = Parser.new
+    cache = cp.send(:instance_variable_get, :@folded_declaration_cache)
+    assert_instance_of Hash, cache
+    assert_empty cache
+  end
+end