FIX: Include resolved locale in anonymous cache key (#10289)

This only applies when set_locale_from_accept_language_header is enabled

lib/http_language_parser.rb

@@ -0,0 +1,13 @@
+# frozen_string_literal: true
+
+module HttpLanguageParser
+  def self.parse(header)
+    # Rails I18n uses underscores between the locale and the region; the request
+    # headers use hyphens.
+    require 'http_accept_language' unless defined? HttpAcceptLanguage
+    available_locales = I18n.available_locales.map { |locale| locale.to_s.tr('_', '-') }
+    parser = HttpAcceptLanguage::Parser.new(header)
+    matched = parser.language_region_compatible_from(available_locales)&.tr('-', '_')
+    matched || SiteSetting.default_locale
+  end
+end

lib/middleware/anonymous_cache.rb

@@ -3,6 +3,7 @@
 require_dependency "mobile_detection"
 require_dependency "crawler_detection"
 require_dependency "guardian"
+require_dependency "http_language_parser"
 
 module Middleware
   class AnonymousCache
@@ -13,7 +14,8 @@ module Middleware
         c: 'key_is_crawler?',
         b: 'key_has_brotli?',
         t: 'key_cache_theme_ids',
-        ca: 'key_compress_anon'
+        ca: 'key_compress_anon',
+        l: 'key_locale'
       }
     end
 
@@ -89,6 +91,14 @@ module Middleware
         @has_brotli == :true
       end
 
+      def key_locale
+        if SiteSetting.set_locale_from_accept_language_header
+          HttpLanguageParser.parse(@env["HTTP_ACCEPT_LANGUAGE"])
+        else
+          "" # No need to key, it is the same for all anon users
+        end
+      end
+
       def is_crawler?
         @is_crawler ||=
           begin