debian-mirror-gitlab/doc/development/migration_style_guide.md

# Migration Style Guide

When writing migrations for GitLab, you have to take into account that
these will be run by hundreds of thousands of organizations of all sizes, some with
many years of data in their database.

In addition, having to take a server offline for an upgrade small or big is a
big burden for most organizations. For this reason, it is important that your
migrations are written carefully, can be applied online, and adhere to the style
guide below.

Migrations are **not** allowed to require GitLab installations to be taken
offline unless _absolutely necessary_.

When downtime is necessary the migration has to be approved by:

1. The VP of Engineering
1. A Backend Maintainer
1. A Database Maintainer

An up-to-date list of people holding these titles can be found at
<https://about.gitlab.com/company/team/>.

When writing your migrations, also consider that databases might have stale data
or inconsistencies and guard for that. Try to make as few assumptions as
possible about the state of the database.

Please don't depend on GitLab-specific code since it can change in future
versions. If needed copy-paste GitLab code into the migration to make it forward
compatible.

For GitLab.com, please take into consideration that regular migrations (under `db/migrate`)
are run before [Canary is deployed](https://about.gitlab.com/handbook/engineering/infrastructure/library/canary/#configuration-and-deployment),
and post-deployment migrations (`db/post_migrate`) are run after the deployment to production has finished.

## Schema Changes

Changes to the schema should be committed to `db/structure.sql`. This
file is automatically generated by Rails, so you normally should not
edit this file by hand. If your migration is adding a column to a
table, that column will be added at the bottom. Please do not reorder
columns manually for existing tables as this will cause confusing to
other people using `db/structure.sql` generated by Rails.

When your local database in your GDK is diverging from the schema from
`master` it might be hard to cleanly commit the schema changes to
Git. In that case you can use the `scripts/regenerate-schema` script to
regenerate a clean `db/structure.sql` for the migrations you're
adding. This script will apply all migrations found in `db/migrate`
or `db/post_migrate`, so if there are any migrations you don't want to
commit to the schema, rename or remove them. If your branch is not
targeting `master` you can set the `TARGET` environment variable.

```shell
# Regenerate schema against `master`
scripts/regenerate-schema

# Regenerate schema against `12-9-stable-ee`
TARGET=12-9-stable-ee scripts/regenerate-schema
```

## What Requires Downtime?

The document ["What Requires Downtime?"](what_requires_downtime.md) specifies
various database operations, such as

- [dropping and renaming columns](what_requires_downtime.md#dropping-columns)
- [changing column constraints and types](what_requires_downtime.md#changing-column-constraints)
- [adding and dropping indexes, tables, and foreign keys](what_requires_downtime.md#adding-indexes)

and whether they require downtime and how to work around that whenever possible.

## Downtime Tagging

Every migration must specify if it requires downtime or not, and if it should
require downtime it must also specify a reason for this. This is required even
if 99% of the migrations won't require downtime as this makes it easier to find
the migrations that _do_ require downtime.

To tag a migration, add the following two constants to the migration class'
body:

- `DOWNTIME`: a boolean that when set to `true` indicates the migration requires
  downtime.
- `DOWNTIME_REASON`: a String containing the reason for the migration requiring
  downtime. This constant **must** be set when `DOWNTIME` is set to `true`.

For example:

```ruby
class MyMigration < ActiveRecord::Migration[6.0]
  DOWNTIME = true
  DOWNTIME_REASON = 'This migration requires downtime because ...'

  def change
    ...
  end
end
```

It is an error (that is, CI will fail) if the `DOWNTIME` constant is missing
from a migration class.

## Reversibility

Your migration **must be** reversible. This is very important, as it should
be possible to downgrade in case of a vulnerability or bugs.

In your migration, add a comment describing how the reversibility of the
migration was tested.

Some migrations cannot be reversed. For example, some data migrations can't be
reversed because we lose information about the state of the database before the migration.
You should still create a `down` method with a comment, explaining why
the changes performed by the `up` method can't be reversed, so that the
migration itself can be reversed, even if the changes performed during the migration
can't be reversed:

```ruby
def down
  # no-op

  # comment explaining why changes performed by `up` cannot be reversed.
end
```

## Atomicity

By default, migrations are single transaction. That is, a transaction is opened
at the beginning of the migration, and committed after all steps are processed.

Running migrations in a single transaction makes sure that if one of the steps fails,
none of the steps will be executed, leaving the database in valid state.
Therefore, either:

- Put all migrations in one single-transaction migration.
- If necessary, put most actions in one migration and create a separate migration
  for the steps that cannot be done in a single transaction.

For example, if you create an empty table and need to build an index for it,
it is recommended to use a regular single-transaction migration and the default
rails schema statement: [`add_index`](https://api.rubyonrails.org/v5.2/classes/ActiveRecord/ConnectionAdapters/SchemaStatements.html#method-i-add_index).
This is a blocking operation, but it won't cause problems because the table is not yet used,
and therefore it does not have any records yet.

## Heavy operations in a single transaction

When using a single-transaction migration, a transaction will hold on a database connection
for the duration of the migration, so you must make sure the actions in the migration
do not take too much time: In general, queries executed in a migration need to fit comfortably
within `15s` on GitLab.com.

In case you need to insert, update, or delete a significant amount of data, you:

- Must disable the single transaction with `disable_ddl_transaction!`.
- Should consider doing it in a [Background Migration](background_migrations.md).

## Retry mechanism when acquiring database locks

When changing the database schema, we use helper methods to invoke DDL (Data Definition
Language) statements. In some cases, these DDL statements require a specific database lock.

Example:

```ruby
def change
  remove_column :users, :full_name, :string
end
```

Executing this migration requires an exclusive lock on the `users` table. When the table
is concurrently accessed and modified by other processes, acquiring the lock may take
a while. The lock request is waiting in a queue and it may also block other queries
on the `users` table once it has been enqueued.

More information about PostgresSQL locks: [Explicit Locking](https://www.postgresql.org/docs/current/explicit-locking.html)

For stability reasons, GitLab.com has a specific [`statement_timeout`](../user/gitlab_com/index.md#postgresql)
set. When the migration is invoked, any database query will have
a fixed time to execute. In a worst-case scenario, the request will sit in the
lock queue, blocking other queries for the duration of the configured statement timeout,
then failing with `canceling statement due to statement timeout` error.

This problem could cause failed application upgrade processes and even application
stability issues, since the table may be inaccessible for a short period of time.

To increase the reliability and stability of database migrations, the GitLab codebase
offers a helper method to retry the operations with different `lock_timeout` settings
and wait time between the attempts. Multiple smaller attempts to acquire the necessary
lock allow the database to process other statements.

### Examples

**Removing a column:**

```ruby
include Gitlab::Database::MigrationHelpers

def up
  with_lock_retries do
    remove_column :users, :full_name
  end
end

def down
  with_lock_retries do
    add_column :users, :full_name, :string
  end
end
```

**Removing a foreign key:**

```ruby
include Gitlab::Database::MigrationHelpers

def up
  with_lock_retries do
    remove_foreign_key :issues, :projects
  end
end

def down
  with_lock_retries do
    add_foreign_key :issues, :projects
  end
end
```

**Changing default value for a column:**

```ruby
include Gitlab::Database::MigrationHelpers

def up
  with_lock_retries do
    change_column_default :merge_requests, :lock_version, from: nil, to: 0
  end
end

def down
  with_lock_retries do
    change_column_default :merge_requests, :lock_version, from: 0, to: nil
  end
end
```

**Creating a new table with a foreign key:**

We can simply wrap the `create_table` method with `with_lock_retries`:

```ruby
def up
  with_lock_retries do
    create_table :issues do |t|
      t.references :project, index: true, null: false, foreign_key: { on_delete: :cascade }
      t.string :title, limit: 255
    end
  end
end

def down
  drop_table :issues
end
```

**Creating a new table when we have two foreign keys:**

For this, we'll need three migrations:

1. Creating the table without foreign keys (with the indices).
1. Add foreign key to the first table.
1. Add foreign key to the second table.

Creating the table:

```ruby
def up
  create_table :imports do |t|
    t.bigint :project_id, null: false
    t.bigint :user_id, null: false
    t.string :jid, limit: 255
  end

  add_index :imports, :project_id
  add_index :imports, :user_id
end

def down
  drop_table :imports
end
```

Adding foreign key to `projects`:

```ruby
include Gitlab::Database::MigrationHelpers

def up
  with_lock_retries do
    add_foreign_key :imports, :projects, column: :project_id, on_delete: :cascade
  end
end

def down
  with_lock_retries do
    remove_foreign_key :imports, column: :project_id
  end
end
```

Adding foreign key to `users`:

```ruby
include Gitlab::Database::MigrationHelpers

def up
  with_lock_retries do
    add_foreign_key :imports, :users, column: :user_id, on_delete: :cascade
  end
end

def down
  with_lock_retries do
    remove_foreign_key :imports, column: :user_id
  end
end
```

**Usage with `disable_ddl_transaction!`**

Generally the `with_lock_retries` helper should work with `disabled_ddl_transaction!`. A custom RuboCop rule ensures that only allowed methods can be placed within the lock retries block.

```ruby
disable_ddl_transaction!

def up
  with_lock_retries do
    add_column :users, :name, :text
  end

  add_text_limit :users, :name, 255 # Includes constraint validation (full table scan)
end
```

The RuboCop rule generally allows standard Rails migration methods, listed below. This example will cause a Rubocop offense:

```ruby
disabled_ddl_transaction!

def up
  with_lock_retries do
    add_concurrent_index :users, :name
  end
end
```

### When to use the helper method

The `with_lock_retries` helper method can be used when you normally use
standard Rails migration helper methods. Calling more than one migration
helper is not a problem if they're executed on the same table.

Using the `with_lock_retries` helper method is advised when a database
migration involves one of the high-traffic tables:

- `users`
- `projects`
- `namespaces`
- `issues`
- `merge_requests`
- `ci_pipelines`
- `ci_builds`
- `notes`

Example changes:

- `add_foreign_key` / `remove_foreign_key`
- `add_column` / `remove_column`
- `change_column_default`
- `create_table` / `drop_table`

**Note:** `with_lock_retries` method **cannot** be used within the `change` method, you must manually define the `up` and `down` methods to make the migration reversible.

### How the helper method works

1. Iterate 50 times.
1. For each iteration, set a pre-configured `lock_timeout`.
1. Try to execute the given block. (`remove_column`).
1. If `LockWaitTimeout` error is raised, sleep for the pre-configured `sleep_time`
and retry the block.
1. If no error is raised, the current iteration has successfully executed the block.

For more information check the [`Gitlab::Database::WithLockRetries`](https://gitlab.com/gitlab-org/gitlab/-/blob/master/lib/gitlab/database/with_lock_retries.rb) class. The `with_lock_retries` helper method is implemented in the [`Gitlab::Database::MigrationHelpers`](https://gitlab.com/gitlab-org/gitlab/-/blob/master/lib/gitlab/database/migration_helpers.rb) module.

In a worst-case scenario, the method:

- Executes the block for a maximum of 50 times over 40 minutes.
  - Most of the time is spent in a pre-configured sleep period after each iteration.
- After the 50th retry, the block will be executed without `lock_timeout`, just
like a standard migration invocation.
- If a lock cannot be acquired, the migration will fail with `statement timeout` error.

The migration might fail if there is a very long running transaction (40+ minutes)
accessing the `users` table.

## Multi-Threading

Sometimes a migration might need to use multiple Ruby threads to speed up a
migration. For this to work your migration needs to include the module
`Gitlab::Database::MultiThreadedMigration`:

```ruby
class MyMigration < ActiveRecord::Migration[6.0]
  include Gitlab::Database::MigrationHelpers
  include Gitlab::Database::MultiThreadedMigration
end
```

You can then use the method `with_multiple_threads` to perform work in separate
threads. For example:

```ruby
class MyMigration < ActiveRecord::Migration[6.0]
  include Gitlab::Database::MigrationHelpers
  include Gitlab::Database::MultiThreadedMigration

  def up
    with_multiple_threads(4) do
      disable_statement_timeout

      # ...
    end
  end
end
```

Here the call to `disable_statement_timeout` will use the connection local to
the `with_multiple_threads` block, instead of re-using the global connection
pool. This ensures each thread has its own connection object, and won't time
out when trying to obtain one.

**NOTE:** PostgreSQL has a maximum amount of connections that it allows. This
limit can vary from installation to installation. As a result, it's recommended
you do not use more than 32 threads in a single migration. Usually, 4-8 threads
should be more than enough.

## Removing indexes

If the table is not empty when removing an index, make sure to use the method
`remove_concurrent_index` instead of the regular `remove_index` method.
The `remove_concurrent_index` method drops indexes concurrently, so no locking is required,
and there is no need for downtime. To use this method, you must disable single-transaction mode
by calling the method `disable_ddl_transaction!` in the body of your migration
class like so:

```ruby
class MyMigration < ActiveRecord::Migration[6.0]
  include Gitlab::Database::MigrationHelpers
  disable_ddl_transaction!

  def up
    remove_concurrent_index :table_name, :column_name
  end
end
```

Note that it is not necessary to check if the index exists prior to
removing it.

For a small table (such as an empty one or one with less than `1,000` records),
it is recommended to use `remove_index` in a single-transaction migration,
combining it with other operations that don't require `disable_ddl_transaction!`.

## Adding indexes

Before adding an index, consider if this one is necessary. There are situations in which an index
might not be required, like:

- The table is small (less than `1,000` records) and it's not expected to exponentially grow in size.
- Any existing indexes filter out enough rows.
- The reduction in query timings after the index is added is not significant.

Additionally, wide indexes are not required to match all filter criteria of queries, we just need
to cover enough columns so that the index lookup has a small enough selectivity. Please review our
[Adding Database indexes](adding_database_indexes.md) guide for more details.

When adding an index to a non-empty table make sure to use the method
`add_concurrent_index` instead of the regular `add_index` method.
The `add_concurrent_index` method automatically creates concurrent indexes
when using PostgreSQL, removing the need for downtime.

To use this method, you must disable single-transactions mode
by calling the method `disable_ddl_transaction!` in the body of your migration
class like so:

```ruby
class MyMigration < ActiveRecord::Migration[6.0]
  include Gitlab::Database::MigrationHelpers

  DOWNTIME = false

  disable_ddl_transaction!

  def up
    add_concurrent_index :table, :column
  end

  def down
    remove_concurrent_index :table, :column
  end
end
```

If you need to add a unique index, please keep in mind there is the possibility
of existing duplicates being present in the database. This means that should
always _first_ add a migration that removes any duplicates, before adding the
unique index.

For a small table (such as an empty one or one with less than `1,000` records),
it is recommended to use `add_index` in a single-transaction migration, combining it with other
operations that don't require `disable_ddl_transaction!`.

## Adding foreign-key constraints

When adding a foreign-key constraint to either an existing or a new column also
remember to add an index on the column.

This is **required** for all foreign-keys, e.g., to support efficient cascading
deleting: when a lot of rows in a table get deleted, the referenced records need
to be deleted too. The database has to look for corresponding records in the
referenced table. Without an index, this will result in a sequential scan on the
table, which can take a long time.

Here's an example where we add a new column with a foreign key
constraint. Note it includes `index: true` to create an index for it.

```ruby
class Migration < ActiveRecord::Migration[6.0]

  def change
    add_reference :model, :other_model, index: true, foreign_key: { on_delete: :cascade }
  end
end
```

When adding a foreign-key constraint to an existing column in a non-empty table,
we have to employ `add_concurrent_foreign_key` and `add_concurrent_index`
instead of `add_reference`.

For an empty table (such as a fresh one), it is recommended to use
`add_reference` in a single-transaction migration, combining it with other
operations that don't require `disable_ddl_transaction!`.

You can read more about adding [foreign key constraints to an existing column](database/add_foreign_key_to_existing_column.md).

## `NOT NULL` constraints

> [Introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/38358) in GitLab 13.0.

See the style guide on [`NOT NULL` constraints](database/not_null_constraints.md) for more information.

## Adding Columns With Default Values

With PostgreSQL 11 being the minimum version since GitLab 13.0, adding columns with default values has become much easier and
the standard `add_column` helper should be used in all cases.

Before PostgreSQL 11, adding a column with a default was problematic as it would
have caused a full table rewrite. The corresponding helper `add_column_with_default`
has been deprecated and will be removed in a later release.

NOTE: **Note:**
If a backport adding a column with a default value is needed for %12.9 or earlier versions,
it should use `add_column_with_default` helper. If a [large table](https://gitlab.com/gitlab-org/gitlab/-/blob/master/rubocop/rubocop-migrations.yml#L3)
is involved, backporting to %12.9 is contraindicated.

## Changing the column default

One might think that changing a default column with `change_column_default` is an
expensive and disruptive operation for larger tables, but in reality it's not.

Take the following migration as an example:

```ruby
class DefaultRequestAccessGroups < ActiveRecord::Migration[5.2]
  DOWNTIME = false

  def change
    change_column_default(:namespaces, :request_access_enabled, from: false, to: true)
  end
end
```

Migration above changes the default column value of one of our largest
tables: `namespaces`. This can be translated to:

```sql
ALTER TABLE namespaces
ALTER COLUMN request_access_enabled
DEFAULT false
```

In this particular case, the default value exists and we're just changing the metadata for
`request_access_enabled` column, which does not imply a rewrite of all the existing records
in the `namespaces` table. Only when creating a new column with a default, all the records are going be rewritten.

NOTE: **Note:**
A faster [ALTER TABLE ADD COLUMN with a non-null default](https://www.depesz.com/2018/04/04/waiting-for-postgresql-11-fast-alter-table-add-column-with-a-non-null-default/)
was introduced on PostgresSQL 11.0, removing the need of rewriting the table when a new column with a default value is added.

For the reasons mentioned above, it's safe to use `change_column_default` in a single-transaction migration
without requiring `disable_ddl_transaction!`.

## Updating an existing column

To update an existing column to a particular value, you can use
`update_column_in_batches`. This will split the updates into batches, so we
don't update too many rows at in a single statement.

This updates the column `foo` in the `projects` table to 10, where `some_column`
is `'hello'`:

```ruby
update_column_in_batches(:projects, :foo, 10) do |table, query|
  query.where(table[:some_column].eq('hello'))
end
```

If a computed update is needed, the value can be wrapped in `Arel.sql`, so Arel
treats it as an SQL literal. It's also a required deprecation for [Rails 6](https://gitlab.com/gitlab-org/gitlab/-/issues/28497).

The below example is the same as the one above, but
the value is set to the product of the `bar` and `baz` columns:

```ruby
update_value = Arel.sql('bar * baz')

update_column_in_batches(:projects, :foo, update_value) do |table, query|
  query.where(table[:some_column].eq('hello'))
end
```

Like `add_column_with_default`, there is a RuboCop cop to detect usage of this
on large tables. In the case of `update_column_in_batches`, it may be acceptable
to run on a large table, as long as it is only updating a small subset of the
rows in the table, but do not ignore that without validating on the GitLab.com
staging environment - or asking someone else to do so for you - beforehand.

## Dropping a database table

Dropping a database table is uncommon, and the `drop_table` method
provided by Rails is generally considered safe. Before dropping the table,
please consider the following:

If your table has foreign keys on a high-traffic table (like `projects`), then
the `DROP TABLE` statement might fail with **statement timeout** error. Determining
what tables are high traffic can be difficult. Self-managed instances might
use different features of GitLab with different usage patterns, thus making
assumptions based on GitLab.com is not enough.

Table **has no records** (feature was never in use) and **no foreign
keys**:

- Simply use the `drop_table` method in your migration.

```ruby
def change
  drop_table :my_table
end
```

Table **has records** but **no foreign keys**:

- First release: Remove the application code related to the table, such as models,
controllers and services.
- Second release: Use the `drop_table` method in your migration.

```ruby
def up
  drop_table :my_table
end

def down
  # create_table ...
end
```

Table **has foreign keys**:

- First release: Remove the application code related to the table, such as models,
controllers, and services.
- Second release: Remove the foreign keys using the `with_lock_retries`
helper method. Use `drop_table` in another migration file.

**Migrations for the second release:**

Removing the foreign key on the `projects` table:

```ruby
# first migration file

def up
  with_lock_retries do
    remove_foreign_key :my_table, :projects
  end
end

def down
  with_lock_retries do
    add_foreign_key :my_table, :projects
  end
end
```

Dropping the table:

```ruby
# second migration file

def up
  drop_table :my_table
end

def down
  # create_table ...
end
```

## Integer column type

By default, an integer column can hold up to a 4-byte (32-bit) number. That is
a max value of 2,147,483,647. Be aware of this when creating a column that will
hold file sizes in byte units. If you are tracking file size in bytes, this
restricts the maximum file size to just over 2GB.

To allow an integer column to hold up to an 8-byte (64-bit) number, explicitly
set the limit to 8-bytes. This will allow the column to hold a value up to
`9,223,372,036,854,775,807`.

Rails migration example:

```ruby
add_column(:projects, :foo, :integer, default: 10, limit: 8)
```

## Strings and the Text data type

> [Introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/30453) in GitLab 13.0.

See the [text data type](database/strings_and_the_text_data_type.md) style guide for more information.

## Timestamp column type

By default, Rails uses the `timestamp` data type that stores timestamp data
without timezone information. The `timestamp` data type is used by calling
either the `add_timestamps` or the `timestamps` method.

Also, Rails converts the `:datetime` data type to the `timestamp` one.

Example:

```ruby
# timestamps
create_table :users do |t|
  t.timestamps
end

# add_timestamps
def up
  add_timestamps :users
end

# :datetime
def up
  add_column :users, :last_sign_in, :datetime
end
```

Instead of using these methods, one should use the following methods to store
timestamps with timezones:

- `add_timestamps_with_timezone`
- `timestamps_with_timezone`
- `datetime_with_timezone`

This ensures all timestamps have a time zone specified. This, in turn, means
existing timestamps won't suddenly use a different timezone when the system's
timezone changes. It also makes it very clear which timezone was used in the
first place.

## Storing JSON in database

The Rails 5 natively supports `JSONB` (binary JSON) column type.
Example migration adding this column:

```ruby
class AddOptionsToBuildMetadata < ActiveRecord::Migration[5.0]
  DOWNTIME = false

  def change
    add_column :ci_builds_metadata, :config_options, :jsonb
  end
end
```

You have to use a serializer to provide a translation layer:

```ruby
class BuildMetadata
  serialize :config_options, Serializers::JSON # rubocop:disable Cop/ActiveRecordSerialize
end
```

When using a `JSONB` column, use the [JsonSchemaValidator](https://gitlab.com/gitlab-org/gitlab/-/blob/master/app/validators/json_schema_validator.rb) to keep control of the data being inserted over time.

```ruby
class BuildMetadata
  validates :config_options, json_schema: { filename: 'build_metadata_config_option' }
end
```

## Testing

See the [Testing Rails migrations](testing_guide/testing_migrations_guide.md) style guide.

## Data migration

Please prefer Arel and plain SQL over usual ActiveRecord syntax. In case of
using plain SQL, you need to quote all input manually with `quote_string` helper.

Example with Arel:

```ruby
users = Arel::Table.new(:users)
users.group(users[:user_id]).having(users[:id].count.gt(5))

#update other tables with these results
```

Example with plain SQL and `quote_string` helper:

```ruby
select_all("SELECT name, COUNT(id) as cnt FROM tags GROUP BY name HAVING COUNT(id) > 1").each do |tag|
  tag_name = quote_string(tag["name"])
  duplicate_ids = select_all("SELECT id FROM tags WHERE name = '#{tag_name}'").map{|tag| tag["id"]}
  origin_tag_id = duplicate_ids.first
  duplicate_ids.delete origin_tag_id

  execute("UPDATE taggings SET tag_id = #{origin_tag_id} WHERE tag_id IN(#{duplicate_ids.join(",")})")
  execute("DELETE FROM tags WHERE id IN(#{duplicate_ids.join(",")})")
end
```

If you need more complex logic, you can define and use models local to a
migration. For example:

```ruby
class MyMigration < ActiveRecord::Migration[6.0]
  class Project < ActiveRecord::Base
    self.table_name = 'projects'
  end

  def up
    # Reset the column information of all the models that update the database
    # to ensure the Active Record's knowledge of the table structure is current
    Project.reset_column_information

    # ... ...
  end
end
```

When doing so be sure to explicitly set the model's table name, so it's not
derived from the class name or namespace.

Finally, make sure that `reset_column_information` is run in the `up` method of
the migration for all local Models that update the database.

The reason for that is that all migration classes are loaded at the beginning
(when `db:migrate` starts), so they can get out of sync with the table schema
they map to in case another migration updates that schema. That makes the data
migration fail when trying to insert or make updates to the underlying table,
as the new columns are reported as `unknown attribute` by `ActiveRecord`.

### Renaming reserved paths

When a new route for projects is introduced, it could conflict with any
existing records. The path for these records should be renamed, and the
related data should be moved on disk.

Since we had to do this a few times already, there are now some helpers to help
with this.

To use this you can include `Gitlab::Database::RenameReservedPathsMigration::V1`
in your migration. This will provide 3 methods which you can pass one or more
paths that need to be rejected.

**`rename_root_paths`**: This will rename the path of all _namespaces_ with the
given name that don't have a `parent_id`.

**`rename_child_paths`**: This will rename the path of all _namespaces_ with the
given name that have a `parent_id`.

**`rename_wildcard_paths`**: This will rename the path of all _projects_, and all
_namespaces_ that have a `project_id`.

The `path` column for these rows will be renamed to their previous value followed
by an integer. For example: `users` would turn into `users0`