"Failed loading configuration file" error with Psych 4

[Masafumi Koba]

Hello, I encounter the following error with Psych 4 and Rails 6.1.4:

$ bin/rails r 'puts Rails.version'
[Scout] [06/30/21 17:26:59 +0900 F200420NT01.local (380)] INFO : Failed loading configuration file (/tmp/repro/config/scout_apm.yml): Unknown alias: defaults. ScoutAPM will continue starting with configuration from ENV and defaults
6.1.4

Here is a part of Gemfile:

gem 'scout_apm', '4.1.1'
gem 'psych', '4.0.1'

Here is config/scout_apm.yml:

common: &defaults
  name: YOUR_APPLICATION_NAME
  key: YOUR_APPLICATION_KEY
  monitor: true

test:
  monitor: false

development:
  <<: *defaults

production:
  <<: *defaults

When using Psych 3, this error never occurs:

-gem 'psych', '4.0.1'
+gem 'psych', '< 4'

I think this is a similar problem with rails/rails#42257 (Psych 4 incompatibility).

[Dave Anderson]

Resolved by #435 - pending release

[Masafumi Koba]

@dlanderson Thank you!