Reallow setting secret_key_base to nil when local

Previously, secret_key_base was allowed to be set to `nil` in local
environments (or with SECRET_KEY_BASE_DUMMY) because validation would
only happen on usage and not on the setter. This was recently
[changed][1] to make it easier to identify exactly where a
secret_key_base was being set to an invalid value.

However, this broke some applications which unconditionally set
secret_key_base to some external value in dev/test. Before the change,
the set value could be `nil` and fall back to the generated local secret
on usage.

This commit restores that behavior so that applications can continue to
set secret_key_base unconditionally, since the nil value will end up
getting replaced by the generated local secret anyways.

[1]: c2901eb

Author: skipkayhil

railties/lib/rails/application/configuration.rb

@@ -512,7 +512,7 @@ def colorize_logging=(val)
 
       def secret_key_base
         @secret_key_base || begin
-          self.secret_key_base = if Rails.env.local? || ENV["SECRET_KEY_BASE_DUMMY"]
+          self.secret_key_base = if generate_local_secret?
             generate_local_secret
           else
             ENV["SECRET_KEY_BASE"] || Rails.application.credentials.secret_key_base
@@ -521,7 +521,9 @@ def secret_key_base
       end
 
       def secret_key_base=(new_secret_key_base)
-        if new_secret_key_base.is_a?(String) && new_secret_key_base.present?
+        if new_secret_key_base.nil? && generate_local_secret?
+          @secret_key_base = generate_local_secret
+        elsif new_secret_key_base.is_a?(String) && new_secret_key_base.present?
           @secret_key_base = new_secret_key_base
         elsif new_secret_key_base
           raise ArgumentError, "`secret_key_base` for #{Rails.env} environment must be a type of String`"
@@ -647,6 +649,10 @@ def generate_local_secret
 
           File.binread(key_file)
         end
+
+        def generate_local_secret?
+          Rails.env.local? || ENV["SECRET_KEY_BASE_DUMMY"]
+        end
     end
   end
 end

railties/test/application/configuration_test.rb

@@ -790,7 +790,7 @@ def index
 
     test "application will generate secret_key_base in tmp file if blank in development" do
       app_file "config/initializers/secret_token.rb", <<-RUBY
-        Rails.application.credentials.secret_key_base = nil
+        Rails.application.config.secret_key_base = nil
       RUBY
 
       # For test that works even if tmp dir does not exist.
@@ -804,7 +804,7 @@ def index
 
     test "application will generate secret_key_base in tmp file if blank in test" do
       app_file "config/initializers/secret_token.rb", <<-RUBY
-        Rails.application.credentials.secret_key_base = nil
+        Rails.application.config.secret_key_base = nil
       RUBY
 
       # For test that works even if tmp dir does not exist.