DEV: s/\$redis/Discourse\.redis (#8431)

This commit also adds a rubocop rule to prevent global variables.

config/application.rb

@@ -243,7 +243,7 @@ module Discourse
     require 'logster/redis_store'
     # Use redis for our cache
     config.cache_store = DiscourseRedis.new_redis_store
-    $redis = DiscourseRedis.new
+    $redis = DiscourseRedis.new # rubocop:disable Style/GlobalVars
     Logster.store = Logster::RedisStore.new(DiscourseRedis.new)
 
     # we configure rack cache on demand in an initializer

config/initializers/001-redis.rb

@@ -2,5 +2,5 @@
 
 if Rails.env.development? && ENV['DISCOURSE_FLUSH_REDIS']
   puts "Flushing redis (development mode)"
-  $redis.flushall
+  Discourse.redis.flushall
 end

config/initializers/100-sidekiq.rb

@@ -24,7 +24,7 @@ end
 
 MiniScheduler.configure do |config|
 
-  config.redis = $redis
+  config.redis = Discourse.redis
 
   config.job_exception_handler do |ex, context|
     Discourse.handle_job_exception(ex, context)

config/initializers/100-watch_for_restart.rb

@@ -29,7 +29,7 @@ Thread.new do
 
         if old_time != time
           Rails.logger.info "attempting to reload #{$$} #{$PROGRAM_NAME} in #{wait_seconds} seconds"
-          $shutdown = true
+          $shutdown = true # rubocop:disable Style/GlobalVars
           sleep wait_seconds
           Rails.logger.info "restarting #{$$}"
           Process.kill("USR2", $$)

config/unicorn.conf.rb

@@ -155,7 +155,7 @@ before_fork do |server, worker|
               sleep 10
               force_kill_rogue_sidekiq
             end
-            $redis._client.disconnect
+            Discourse.redis._client.disconnect
           end
         end
 
@@ -170,7 +170,7 @@ before_fork do |server, worker|
 
   end
 
-  $redis._client.disconnect
+  Discourse.redis._client.disconnect
 
   # Throttle the master from forking too quickly by sleeping.  Due
   # to the implementation of standard Unix signal handlers, this