Add constraints to Runner model and table

The runners table lacked a few important uniqueness constraints and indices, too. Specifically, the runner_id is expected to be unique (at least per Runner::Strategy). Since CodeOcean currently supports only one Runner::Strategy concurrently, we set the constraint (and disallow NULL values).

Furthermore, we link Runner properly to ExecutionEnvironments. Since the table primarily serves as a "cache" for previous runner assignments, we can easily drop those runners for non-present ExecutionEnvironments.

Author: MrSerth

app/models/execution_environment.rb

@@ -15,6 +15,7 @@ class ExecutionEnvironment < ApplicationRecord
   belongs_to :file_type
   has_many :error_templates
   has_many :testrun_execution_environments, dependent: :destroy
+  has_many :runners, dependent: :destroy
 
   scope :with_exercises, -> { where('id IN (SELECT execution_environment_id FROM exercises)') }
 

app/models/runner.rb

@@ -11,8 +11,8 @@ class Runner < ApplicationRecord
   RESERVATION_BUFFER = 1.second
 
   before_validation :request_id
-  validates :runner_id, presence: true
-
+  validates :runner_id, presence: true, uniqueness: true
+  validates :execution_environment_id, uniqueness: {scope: %i[contributor_id contributor_type]}
   attr_accessor :strategy
 
   def self.strategy_class

db/migrate/20250502133509_add_constraints_to_runner.rb

@@ -0,0 +1,28 @@
+# frozen_string_literal: true
+
+class AddConstraintsToRunner < ActiveRecord::Migration[8.0]
+  class ExecutionEnvironment < ApplicationRecord
+    has_many :runners
+  end
+
+  class Runner < ApplicationRecord
+    belongs_to :execution_environment
+  end
+
+  def change
+    change_column_null :runners, :runner_id, false
+    change_column_null :runners, :execution_environment_id, false
+    change_column_null :runners, :contributor_id, false
+    change_column_null :runners, :contributor_type, false
+
+    add_index :runners, :runner_id, unique: true
+
+    up_only do
+      # We cannot add a foreign key to a table that has rows that violate the constraint.
+      Runner.where.not(execution_environment_id: ExecutionEnvironment.select(:id)).delete_all
+    end
+    add_foreign_key :runners, :execution_environments
+
+    add_index :runners, %i[runner_id contributor_id contributor_type], unique: true
+  end
+end