faraday_behind_oauth2.rb

require 'oauth2'
require 'json'

# Assume I've got the client_id, client_key and tenant_id from somewhere

site_url     = 'https://login.microsoftonline.com'
token_url    = tenant_id + "/oauth2/token"
auth_url     = File.join(site_url, tenant_id, 'oauth2', 'token')
resource_url = 'https://management.azure.com/'

# Ok, no problem
client = OAuth2::Client.new(
  client_id,
  client_key,
  :site => site_url,
  :authorize_url => auth_url,
  :token_url => token_url
)

# My custom error handler for Faraday, which OAuth2 uses underneath the hood
class MyErrorHandler < Faraday::Response::Middleware
  def on_complete(env)
    case env.status
      when 400..600
        raise "OOPS: #{env.status}"
      else
        env.body
    end
  end
end

# Ok, seems to work
client.connection do |c|
  c.use MyErrorHandler
  c.adapter Faraday.default_adapter
end

# Yep, still working
credentials = client.client_credentials
token = credentials.get_token(:resource => resource_url)

# Deliberately use a bogus api-version string
url = 'https://management.azure.com/subscriptions?api-version=2019-09-01'

headers = {
  :accept => 'application/json',
  :content_type => 'application/json'
}

# An "InvalidApiVersionParameter" is raised. However, oauth2 raises an OAuth2::Error
# instead of using my custom error handler, which is never hooked into.
token.request(:get, url, :headers => headers)