Runners: Prevent duplicate runner reservations

Previously, a race condition could occur and cause invalid runner records (i.e., two entries for the same contributor and execution environment). With a recent change in dc9d321, we validated these in Rails, but neither removed old records already present in the database nor did we add a unique constraint. This commit adds the missing parts and ensures the Ruby method retries in case of non-unique errors.

Amends dc9d321
Fixes CODEOCEAN-19S
Fixes CODEOCEAN-19T

Author: MrSerth

app/models/runner.rb

@@ -38,15 +38,18 @@ def self.management_active?
   def self.for(contributor, execution_environment)
     runner = find_by(contributor:, execution_environment:)
     if runner.nil?
-      runner = Runner.create(contributor:, execution_environment:)
       # The `strategy` is added through the before_validation hook `:request_id`.
-      raise Runner::Error::Unknown.new("Runner could not be saved: #{runner.errors.inspect}") unless runner.persisted?
+      runner = Runner.create!(contributor:, execution_environment:)
     else
       # This information is required but not persisted in the runner model.
       runner.strategy = strategy_class.new(runner.runner_id, runner.execution_environment)
     end
 
     runner
+  rescue ActiveRecord::RecordNotUnique
+    retry
+  rescue ActiveRecord::RecordInvalid => e
+    raise Runner::Error::Unknown.new("Runner could not be saved: #{e.inspect}")
   end
 
   def copy_files(files, exclusive: true)

db/migrate/20250713114125_delete_invalid_runner_reservations.rb

@@ -0,0 +1,31 @@
+# frozen_string_literal: true
+
+class DeleteInvalidRunnerReservations < ActiveRecord::Migration[8.0]
+  def change
+    # Keep the first occurrence of each unique combination of execution_environment_id, contributor_type, and contributor_id.
+    # Delete all other duplicates from the `runners` table.
+
+    up_only do
+      execute <<-SQL.squish
+        WITH numbered_runner_reservations AS (
+          SELECT id, execution_environment_id, contributor_type, contributor_id,
+            ROW_NUMBER() OVER (
+                PARTITION BY execution_environment_id, contributor_type, contributor_id
+                ORDER BY created_at
+            ) AS row_num
+          FROM runners
+        )
+        DELETE
+        FROM runners
+        WHERE id IN (
+            SELECT id
+            FROM numbered_runner_reservations
+            WHERE row_num > 1
+        );
+      SQL
+    end
+
+    remove_index :runners, %i[runner_id contributor_id contributor_type], unique: true, if_exists: true
+    add_index :runners, %i[execution_environment_id contributor_id contributor_type], unique: true
+  end
+end