Merge branch 'migration-helpers' into 'master'

Added helper methods for database migrations

These helpers can ultimately be used to write migrations that don't
require downtime.

See #15464 for more information.

See merge request !3860

3 files changed, 224 insertions, 0 deletions

diff --git a/lib/gitlab/database/migration_helpers.rb b/lib/gitlab/database/migration_helpers.rb
new file mode 100644
index 00000000000..31a87b73fbe
--- /dev/null
+++ b/lib/gitlab/database/migration_helpers.rb
@@ -0,0 +1,134 @@
+module Gitlab
+  module Database
+    module MigrationHelpers
+      # Creates a new index, concurrently when supported
+      #
+      # On PostgreSQL this method creates an index concurrently, on MySQL this
+      # creates a regular index.
+      #
+      # Example:
+      #
+      #     add_concurrent_index :users, :some_column
+      #
+      # See Rails' `add_index` for more info on the available arguments.
+      def add_concurrent_index(*args)
+        if transaction_open?
+          raise 'add_concurrent_index can not be run inside a transaction, ' \
+            'you can disable transactions by calling disable_ddl_transaction! ' \
+            'in the body of your migration class'
+        end
+
+        if Database.postgresql?
+          args << { algorithm: :concurrently }
+        end
+
+        add_index(*args)
+      end
+
+      # Updates the value of a column in batches.
+      #
+      # This method updates the table in batches of 5% of the total row count.
+      # Any data inserted while running this method (or after it has finished
+      # running) is _not_ updated automatically.
+      #
+      # This method _only_ updates rows where the column's value is set to NULL.
+      #
+      # table - The name of the table.
+      # column - The name of the column to update.
+      # value - The value for the column.
+      def update_column_in_batches(table, column, value)
+        quoted_table = quote_table_name(table)
+        quoted_column = quote_column_name(column)
+        quoted_value = quote(value)
+        processed = 0
+
+        total = exec_query("SELECT COUNT(*) AS count FROM #{quoted_table}").
+          to_hash.
+          first['count'].
+          to_i
+
+        # Update in batches of 5% with an upper limit of 5000 rows.
+        batch_size = ((total / 100.0) * 5.0).ceil
+
+        while processed < total
+          start_row = exec_query(%Q{
+            SELECT id
+            FROM #{quoted_table}
+            ORDER BY id ASC
+            LIMIT 1 OFFSET #{processed}
+          }).to_hash.first
+
+          stop_row = exec_query(%Q{
+            SELECT id
+            FROM #{quoted_table}
+            ORDER BY id ASC
+            LIMIT 1 OFFSET #{processed + batch_size}
+          }).to_hash.first
+
+          query = %Q{
+            UPDATE #{quoted_table}
+            SET #{quoted_column} = #{quoted_value}
+            WHERE id >= #{start_row['id']}
+          }
+
+          if stop_row
+            query += " AND id < #{stop_row['id']}"
+          end
+
+          execute(query)
+
+          processed += batch_size
+        end
+      end
+
+      # Adds a column with a default value without locking an entire table.
+      #
+      # This method runs the following steps:
+      #
+      # 1. Add the column with a default value of NULL.
+      # 2. Update all existing rows in batches.
+      # 3. Change the default value of the column to the specified value.
+      # 4. Update any remaining rows.
+      #
+      # These steps ensure a column can be added to a large and commonly used
+      # table without locking the entire table for the duration of the table
+      # modification.
+      #
+      # table - The name of the table to update.
+      # column - The name of the column to add.
+      # type - The column type (e.g. `:integer`).
+      # default - The default value for the column.
+      # allow_null - When set to `true` the column will allow NULL values, the
+      #              default is to not allow NULL values.
+      def add_column_with_default(table, column, type, default:, allow_null: false)
+        if transaction_open?
+          raise 'add_column_with_default can not be run inside a transaction, ' \
+            'you can disable transactions by calling disable_ddl_transaction! ' \
+            'in the body of your migration class'
+        end
+
+        transaction do
+          add_column(table, column, type, default: nil)
+
+          # Changing the default before the update ensures any newly inserted
+          # rows already use the proper default value.
+          change_column_default(table, column, default)
+        end
+
+        begin
+          transaction do
+            update_column_in_batches(table, column, default)
+          end
+        # We want to rescue _all_ exceptions here, even those that don't inherit
+        # from StandardError.
+        rescue Exception => error # rubocop: disable all
+          remove_column(table, column)
+
+          raise error
+        end
+
+        change_column_null(table, column, false) unless allow_null
+      end
+    end
+  end
+end
diff --git a/lib/templates/active_record/migration/create_table_migration.rb b/lib/templates/active_record/migration/create_table_migration.rb
new file mode 100644
index 00000000000..27acc75dcc4
--- /dev/null
+++ b/lib/templates/active_record/migration/create_table_migration.rb
@@ -0,0 +1,35 @@
+# See http://doc.gitlab.com/ce/development/migration_style_guide.html
+# for more information on how to write migrations for GitLab.
+
+class <%= migration_class_name %> < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+
+  # When using the methods "add_concurrent_index" or "add_column_with_default"
+  # you must disable the use of transactions as these methods can not run in an
+  # existing transaction. When using "add_concurrent_index" make sure that this
+  # method is the _only_ method called in the migration, any other changes
+  # should go in a separate migration. This ensures that upon failure _only_ the
+  # index creation fails and can be retried or reverted easily.
+  #
+  # To disable transactions uncomment the following line and remove these
+  # comments:
+  # disable_ddl_transaction!
+
+  def change
+    create_table :<%= table_name %> do |t|
+<% attributes.each do |attribute| -%>
+<% if attribute.password_digest? -%>
+      t.string :password_digest<%= attribute.inject_options %>
+<% else -%>
+      t.<%= attribute.type %> :<%= attribute.name %><%= attribute.inject_options %>
+<% end -%>
+<% end -%>
+<% if options[:timestamps] %>
+      t.timestamps null: false
+<% end -%>
+    end
+<% attributes_with_index.each do |attribute| -%>
+    add_index :<%= table_name %>, :<%= attribute.index_name %><%= attribute.inject_index_options %>
+<% end -%>
+  end
+end
diff --git a/lib/templates/active_record/migration/migration.rb b/lib/templates/active_record/migration/migration.rb
new file mode 100644
index 00000000000..06bdea11367
--- /dev/null
+++ b/lib/templates/active_record/migration/migration.rb
@@ -0,0 +1,55 @@
+# See http://doc.gitlab.com/ce/development/migration_style_guide.html
+# for more information on how to write migrations for GitLab.
+
+class <%= migration_class_name %> < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+
+  # When using the methods "add_concurrent_index" or "add_column_with_default"
+  # you must disable the use of transactions as these methods can not run in an
+  # existing transaction. When using "add_concurrent_index" make sure that this
+  # method is the _only_ method called in the migration, any other changes
+  # should go in a separate migration. This ensures that upon failure _only_ the
+  # index creation fails and can be retried or reverted easily.
+  #
+  # To disable transactions uncomment the following line and remove these
+  # comments:
+  # disable_ddl_transaction!
+
+<%- if migration_action == 'add' -%>
+  def change
+<% attributes.each do |attribute| -%>
+  <%- if attribute.reference? -%>
+    add_reference :<%= table_name %>, :<%= attribute.name %><%= attribute.inject_options %>
+  <%- else -%>
+    add_column :<%= table_name %>, :<%= attribute.name %>, :<%= attribute.type %><%= attribute.inject_options %>
+    <%- if attribute.has_index? -%>
+    add_index :<%= table_name %>, :<%= attribute.index_name %><%= attribute.inject_index_options %>
+    <%- end -%>
+  <%- end -%>
+<%- end -%>
+  end
+<%- elsif migration_action == 'join' -%>
+  def change
+    create_join_table :<%= join_tables.first %>, :<%= join_tables.second %> do |t|
+    <%- attributes.each do |attribute| -%>
+      <%= '# ' unless attribute.has_index? -%>t.index <%= attribute.index_name %><%= attribute.inject_index_options %>
+    <%- end -%>
+    end
+  end
+<%- else -%>
+  def change
+<% attributes.each do |attribute| -%>
+<%- if migration_action -%>
+  <%- if attribute.reference? -%>
+    remove_reference :<%= table_name %>, :<%= attribute.name %><%= attribute.inject_options %>
+  <%- else -%>
+    <%- if attribute.has_index? -%>
+    remove_index :<%= table_name %>, :<%= attribute.index_name %><%= attribute.inject_index_options %>
+    <%- end -%>
+    remove_column :<%= table_name %>, :<%= attribute.name %>, :<%= attribute.type %><%= attribute.inject_options %>
+  <%- end -%>
+<%- end -%>
+<%- end -%>
+  end
+<%- end -%>
+end