Fix adding defaults for concurrent column renames

By adding the default value _after_ adding the column we avoid updating
all rows in a table, saving a lot of time and unnecessary work in the
process.

lib/gitlab/database/migration_helpers.rb

@@ -283,11 +283,15 @@ module Gitlab
 
         add_column(table, new, new_type,
                    limit: old_col.limit,
-                   default: old_col.default,
                    null: old_col.null,
                    precision: old_col.precision,
                    scale: old_col.scale)
 
+        # We set the default value _after_ adding the column so we don't end up
+        # updating any existing data with the default value. This isn't
+        # necessary since we copy over old values further down.
+        change_column_default(table, new, old_col.default) if old_col.default
+
         trigger_name = rename_trigger_name(table, old, new)
         quoted_table = quote_table_name(table)
         quoted_old = quote_column_name(old)

spec/lib/gitlab/database/migration_helpers_spec.rb

@@ -382,11 +382,13 @@ describe Gitlab::Database::MigrationHelpers, lib: true do
           expect(model).to receive(:add_column).
             with(:users, :new, :integer,
                  limit: old_column.limit,
-                 default: old_column.default,
                  null: old_column.null,
                  precision: old_column.precision,
                  scale: old_column.scale)
 
+          expect(model).to receive(:change_column_default).
+            with(:users, :new, old_column.default)
+
           expect(model).to receive(:update_column_in_batches)
 
           expect(model).to receive(:copy_indexes).with(:users, :old, :new)
@@ -406,11 +408,13 @@ describe Gitlab::Database::MigrationHelpers, lib: true do
           expect(model).to receive(:add_column).
             with(:users, :new, :integer,
                  limit: old_column.limit,
-                 default: old_column.default,
                  null: old_column.null,
                  precision: old_column.precision,
                  scale: old_column.scale)
 
+          expect(model).to receive(:change_column_default).
+            with(:users, :new, old_column.default)
+
           expect(model).to receive(:update_column_in_batches)
 
           expect(model).to receive(:copy_indexes).with(:users, :old, :new)