M Add sharding key YAML config to p_ci_pipeline_variables (1 table)

What we need to do

The p_ci_pipeline_variables table has a pipeline_id column so we could look into the project_id from a Ci::Pipeline to identify the correct sharding key here and backfill the data accordingly.

How to do it

• Read https://docs.gitlab.com/ee/development/cells/#define-a-desired_sharding_key-to-automatically-backfill-a-sharding_key
• Modify https://gitlab.com/gitlab-org/gitlab/-/blob/master/db/docs/ci_pipeline_variables.yml
• Create all necessary migrations as we did in daed28a6
• Mark the migration as finalize
• Validate the constraint
• Set sharding key in dictionary

# db/post_migrate/...
class FinalizeBackfillPCiPipelineVariablesProjectId < Gitlab::Database::Migration[2.2]
  milestone '17.6'
  MIGRATION = 'BackfillPCiPipelineVariablesProjectId'
  disable_ddl_transaction!

  restrict_gitlab_migration gitlab_schema: :gitlab_ci

  def up
    ensure_batched_background_migration_is_finished(
      job_class_name: MIGRATION,
      table_name: :p_ci_pipeline_variables,
      column_name: :id,
      job_arguments: [
        :project_id,
        :p_ci_pipelines,
        :project_id,
        :pipeline_id,
        :partition_id
      ],
      finalize: true
    )
  end

  def down
    # no-op
  end
end

# db/post_migrate/...
# frozen_string_literal: true
class ValidatePCiPipelineVariablesProjectIdNullConstraint < Gitlab::Database::Migration[2.2]
  milestone '17.6'

  disable_ddl_transaction!

  TABLE_NAME = :p_ci_pipeline_variables
  COLUMN_NAME = :project_id
  CONSTRAINT_NAME = :check_6e932dbabf

  def up
    add_not_null_constraint(TABLE_NAME, COLUMN_NAME, constraint_name: CONSTRAINT_NAME)
  end

  def down
    remove_not_null_constraint(TABLE_NAME, COLUMN_NAME, constraint_name: CONSTRAINT_NAME)

    add_not_null_constraint(TABLE_NAME, COLUMN_NAME, constraint_name: CONSTRAINT_NAME, validate: false)
  end
end