Drop existing trigger before creating new one

- When renaming a column concurrently, drop any existing trigger before
attempting to create a new one.

When running migration specs multiple times (as it happens during
local development), the down method of previous migrations are called.
If any of the called methods contains a call to
rename_column_concurrently, a trigger will be created and not removed.
So, the next time a migration spec is run, if the same down method is
executed again, it will cause an error when attempting to create the
trigger (since it already exists). Dropping the trigger if it already
exists will prevent this problem.

lib/gitlab/database/migration_helpers.rb

@@ -746,6 +746,11 @@ module Gitlab
         VOLATILE
         EOF
 
+        execute <<-EOF.strip_heredoc
+        DROP TRIGGER IF EXISTS #{trigger}
+        ON #{table}
+        EOF
+
         execute <<-EOF.strip_heredoc
         CREATE TRIGGER #{trigger}
         BEFORE INSERT OR UPDATE

spec/lib/gitlab/database/migration_helpers_spec.rb

@@ -618,11 +618,19 @@ describe Gitlab::Database::MigrationHelpers do
       expect(model).to receive(:execute)
         .with(/CREATE OR REPLACE FUNCTION foo()/m)
 
+      expect(model).to receive(:execute)
+        .with(/DROP TRIGGER IF EXISTS foo/m)
+
       expect(model).to receive(:execute)
         .with(/CREATE TRIGGER foo/m)
 
       model.install_rename_triggers_for_postgresql('foo', :users, :old, :new)
     end
+
+    it 'does not fail if trigger already exists' do
+      model.install_rename_triggers_for_postgresql('foo', :users, :old, :new)
+      model.install_rename_triggers_for_postgresql('foo', :users, :old, :new)
+    end
   end
 
   describe '#remove_rename_triggers_for_postgresql' do