Deprecate different default for log_level in production

This is a more conservative approach to 2602a49. Also changed the comment to be
more inline with everything else in the file (describing what the config value
is doing and why). People should just read the docs for alternatives.

railties/lib/rails/application/bootstrap.rb

@@ -1,5 +1,6 @@
 require "active_support/notifications"
 require "active_support/dependencies"
+require "active_support/deprecation"
 require "active_support/descendants_tracker"
 
 module Rails
@@ -54,6 +55,18 @@ INFO
           logger
         end
 
+        if Rails.env.production? && !config.has_explicit_log_level?
+          ActiveSupport::Deprecation.warn \
+           "You did not specify a `log_level` in `production.rb`. Currently, " \
+           "the default value for `log_leve` is `:info` for the production " \
+           "environment and `:debug` in all other environments. In Rails 5 " \
+           "the default value will be unified to `:debug` across all " \
+           "environments. To preserve the current setting, add the following " \
+           "line to your `production.rb`:\n" \
+           "\n" \
+           "   config.log_level = :info\n\n"
+        end
+
         Rails.logger.level = ActiveSupport::Logger.const_get(config.log_level.to_s.upcase)
       end
 

railties/lib/rails/application/configuration.rb

@@ -15,7 +15,6 @@ module Rails
                     :time_zone, :reload_classes_only_on_change,
                     :beginning_of_week, :filter_redirect, :x
 
-      attr_writer :log_level
       attr_reader :encoding
 
       def initialize(*)
@@ -34,6 +33,7 @@ module Rails
         @session_options               = {}
         @time_zone                     = "UTC"
         @beginning_of_week             = :monday
+        @has_explicit_log_level        = false
         @log_level                     = nil
         @middleware                    = app_middleware
         @generators                    = app_generators
@@ -117,8 +117,17 @@ module Rails
         raise e, "Cannot load `Rails.application.database_configuration`:\n#{e.message}", e.backtrace
       end
 
+      def has_explicit_log_level? # :nodoc:
+        @has_explicit_log_level
+      end
+
+      def log_level=(level)
+        @has_explicit_log_level = !!(level)
+        @log_level = level
+      end
+
       def log_level
-        @log_level ||= Rails.env.production? ? :info : :debug
+        @log_level ||= (Rails.env.production? ? :info : :debug)
       end
 
       def colorize_logging

railties/lib/rails/generators/rails/app/templates/config/environments/production.rb.tt

@@ -45,7 +45,8 @@ Rails.application.configure do
   # Force all access to the app over SSL, use Strict-Transport-Security, and use secure cookies.
   # config.force_ssl = true
 
-  # Set to :info to decrease the log volume.
+  # Use the lowest log_level to ensure availability of diagnostic information
+  # when problems arise.
   config.log_level = :debug
 
   # Prepend all log lines with the following tags.