Add component to comment with the database role

With multiple databases in Rails 6, it can be difficult to tell from
logging if your database query is being made against the leader or
follower database. This change adds a new component, `db_role`, which
can be used with the database host and name to annotate the query with
the current ActiveRecord role.

.travis.yml

@@ -22,3 +22,4 @@ gemfile:
   - gemfiles/5.0.gemfile
   - gemfiles/5.1.gemfile
   - gemfiles/5.2.gemfile
+  - gemfiles/6.0.gemfile

gemfiles/6.0.gemfile

@@ -0,0 +1,8 @@
+source "https://rubygems.org"
+
+gem "mysql2", "~> 0.4.10"
+gem "pg", "~> 0.15"
+gem "sqlite3", "~> 1.3.6"
+gem "rails", "= 6.0.3"
+
+gemspec :path => "../"

lib/marginalia/comment.rb

@@ -155,6 +155,11 @@ def self.db_host
           end
         end
 
+        def self.db_role
+          return if marginalia_adapter.pool.nil?
+          ActiveRecord::Base.current_role
+        end
+
         def self.database
           if self.connection_config.present?
             self.connection_config[:database]

test/query_comments_test.rb

@@ -17,6 +17,10 @@ def adapter_pool_available?
   Gem::Version.new(ActiveRecord::VERSION::STRING) >= Gem::Version.new('3.2.19')
 end
 
+def database_role_available?
+  Gem::Version.new(ActiveRecord::VERSION::STRING) >= Gem::Version.new('6.0.0')
+end
+
 require "minitest/autorun"
 require "mocha/minitest"
 require 'logger'
@@ -235,6 +239,14 @@ def test_db_host
       assert_match %r{/\*db_host:localhost}, @queries.first
     end
 
+    if database_role_available?
+      def test_db_role
+        Marginalia::Comment.components = [:db_role]
+        API::V1::PostsController.action(:driver_only).call(@env)
+        assert_match %r{/\*db_role:writing}, @queries.first
+      end
+    end
+
     def test_database
       Marginalia::Comment.components = [:database]
       API::V1::PostsController.action(:driver_only).call(@env)