Implement database flavor detection

Try to detect the PostgreSQL flavor used be executing flavor specific
statements.

https://gitlab.com/gitlab-org/database-team/team-tasks/-/issues/218

lib/gitlab/database/reflection.rb

@@ -105,6 +105,35 @@ module Gitlab
         row['system_identifier']
       end
 
+      def flavor
+        {
+          # Based on https://aws.amazon.com/premiumsupport/knowledge-center/aurora-version-number/
+          'Amazon Aurora PostgreSQL' => { statement: 'SELECT AURORA_VERSION()', error: /PG::UndefinedFunction/ },
+          # Based on https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/CHAP_PostgreSQL.html#PostgreSQL.Concepts.General.FeatureSupport.Extensions,
+          # this is also available for both Aurora and RDS, so we need to check for the former first.
+          'PostgreSQL on Amazon RDS' => { statement: 'SHOW rds.extensions', error: /PG::UndefinedObject/ },
+          # Based on https://cloud.google.com/sql/docs/postgres/flags#postgres-c this should be specific
+          # to Cloud SQL for PostgreSQL
+          'Cloud SQL for PostgreSQL' =>  { statement: 'SHOW cloudsql.iam_authentication', error: /PG::UndefinedObject/ },
+          # Based on
+          #   - https://docs.microsoft.com/en-us/azure/postgresql/flexible-server/concepts-extensions
+          #   - https://docs.microsoft.com/en-us/azure/postgresql/concepts-extensions
+          # this should be available only for Azure Database for PostgreSQL - Flexible Server.
+          'Azure Database for PostgreSQL - Flexible Server' => { statement: 'SHOW azure.extensions', error: /PG::UndefinedObject/ },
+          # Based on
+          #   - https://docs.microsoft.com/en-us/azure/postgresql/flexible-server/concepts-servers
+          #   - https://docs.microsoft.com/en-us/azure/postgresql/concepts-servers#managing-your-server
+          # this database is present on both Flexible and Single server, so we should check the former first.
+          'Azure Database for PostgreSQL - Single Server' => { statement: "SELECT datname FROM pg_database WHERE datname = 'azure_maintenance'" }
+        }.each do |flavor, conditions|
+          return flavor if connection.execute(conditions[:statement]).to_a.present?
+        rescue ActiveRecord::StatementInvalid => e
+          raise if conditions[:error] && !e.message.match?(conditions[:error])
+        end
+
+        nil
+      end
+
       private
 
       def connection

spec/lib/gitlab/database/reflection_spec.rb

@@ -259,6 +259,66 @@ RSpec.describe Gitlab::Database::Reflection do
     end
   end
 
+  describe '#flavor', :delete do
+    let(:result) { [double] }
+    let(:connection) { database.model.connection }
+
+    def stub_statements(statements)
+      statements = Array.wrap(statements)
+      execute = connection.method(:execute)
+
+      allow(connection).to receive(:execute) do |arg|
+        if statements.include?(arg)
+          result
+        else
+          execute.call(arg)
+        end
+      end
+    end
+
+    it 're-raises exceptions not matching expected messages' do
+      expect(database.model.connection)
+        .to receive(:execute)
+        .and_raise(ActiveRecord::StatementInvalid, 'Something else')
+
+      expect { database.flavor }.to raise_error ActiveRecord::StatementInvalid, /Something else/
+    end
+
+    it 'recognizes Amazon Aurora PostgreSQL' do
+      stub_statements(['SHOW rds.extensions', 'SELECT AURORA_VERSION()'])
+
+      expect(database.flavor).to eq('Amazon Aurora PostgreSQL')
+    end
+
+    it 'recognizes PostgreSQL on Amazon RDS' do
+      stub_statements('SHOW rds.extensions')
+
+      expect(database.flavor).to eq('PostgreSQL on Amazon RDS')
+    end
+
+    it 'recognizes CloudSQL for PostgreSQL' do
+      stub_statements('SHOW cloudsql.iam_authentication')
+
+      expect(database.flavor).to eq('Cloud SQL for PostgreSQL')
+    end
+
+    it 'recognizes Azure Database for PostgreSQL - Flexible Server' do
+      stub_statements(["SELECT datname FROM pg_database WHERE datname = 'azure_maintenance'", 'SHOW azure.extensions'])
+
+      expect(database.flavor).to eq('Azure Database for PostgreSQL - Flexible Server')
+    end
+
+    it 'recognizes Azure Database for PostgreSQL - Single Server' do
+      stub_statements("SELECT datname FROM pg_database WHERE datname = 'azure_maintenance'")
+
+      expect(database.flavor).to eq('Azure Database for PostgreSQL - Single Server')
+    end
+
+    it 'returns nil if can not recognize the flavor' do
+      expect(database.flavor).to be_nil
+    end
+  end
+
   describe '#config' do
     it 'returns a HashWithIndifferentAccess' do
       expect(database.config)