Ability to check if underlying database is read only

lib/gitlab/database.rb

@@ -42,6 +42,21 @@ module Gitlab
       !self.read_only?
     end
 
+    # check whether the underlying database is in read-only mode
+    def self.db_read_only?
+      if postgresql?
+        ActiveRecord::Base.connection.execute('SELECT pg_is_in_recovery()')
+          .first
+          .fetch('pg_is_in_recovery') == 't'
+      else
+        false
+      end
+    end
+
+    def self.db_read_write?
+      !self.db_read_only?
+    end
+
     def self.version
       @version ||= database_version.match(/\A(?:PostgreSQL |)([^\s]+).*\z/)[1]
     end

spec/lib/gitlab/database_spec.rb

@@ -357,6 +357,35 @@ describe Gitlab::Database do
     end
   end
 
+  describe '.db_read_only?' do
+    context 'when using PostgreSQL' do
+      before do
+        allow(ActiveRecord::Base.connection).to receive(:execute).and_call_original
+        expect(described_class).to receive(:postgresql?).and_return(true)
+      end
+
+      it 'detects a read only database' do
+        allow(ActiveRecord::Base.connection).to receive(:execute).with('SELECT pg_is_in_recovery()').and_return([{ "pg_is_in_recovery" => "t" }])
+
+        expect(described_class.db_read_only?).to be_truthy
+      end
+
+      it 'detects a read write database' do
+        allow(ActiveRecord::Base.connection).to receive(:execute).with('SELECT pg_is_in_recovery()').and_return([{ "pg_is_in_recovery" => "f" }])
+
+        expect(described_class.db_read_only?).to be_falsey
+      end
+    end
+
+    context 'when using MySQL' do
+      before do
+        expect(described_class).to receive(:postgresql?).and_return(false)
+      end
+
+      it { expect(described_class.db_read_only?).to be_falsey }
+    end
+  end
+
   describe '#sanitize_timestamp' do
     let(:max_timestamp) { Time.at((1 << 31) - 1) }