feat: add option to delay starting processes until all migrations are run

config/initializers/_wait_for_migrations.rb

@@ -0,0 +1,5 @@
+# frozen_string_literal: true
+
+require "migration_waiter"
+
+MigrationWaiter.wait_if_appropriate

lib/migration_waiter.rb

@@ -0,0 +1,60 @@
+# frozen_string_literal: true
+
+# This initializer will wait for all pending migrations to be applied before
+# continuing to start the application. This is useful when running the application
+# in a cluster where migrations are run in a separate job which runs at the same
+# time as the other processes.
+
+class MigrationWaiter
+
+  ATTEMPTS = ENV.fetch("MIGRATION_WAITER_ATTEMPTS", 120)
+  SLEEP_TIME = ENV.fetch("MIGRATION_WAITER_SLEEP_TIME", 2)
+
+  class << self
+
+    def wait
+      attempts_remaining = ATTEMPTS
+      loop do
+        pending_migrations = ActiveRecord::Base.connection.migration_context.open.pending_migrations.size
+        if pending_migrations.zero?
+          Postal.logger.info "no pending migrations, continuing"
+          return
+        end
+
+        attempts_remaining -= 1
+
+        if attempts_remaining.zero?
+          Postal.logger.info "#{pending_migrations} migration(s) are still pending after #{ATTEMPTS} attempts, exiting"
+          Process.exit(1)
+        else
+          Postal.logger.info "waiting for #{pending_migrations} migration(s) to be applied (#{attempts_remaining} remaining)"
+          sleep SLEEP_TIME
+        end
+      end
+    end
+
+    def wait_if_appropriate
+      # Don't wait if not configured
+      return unless ENV.fetch("MIGRATION_WAITER_ENABLED", "false") == "true"
+
+      # Don't wait in the console, rake tasks or rails commands
+      return if console? || rake_task? || rails_command?
+
+      wait
+    end
+
+    def console?
+      Rails.const_defined?("Console")
+    end
+
+    def rake_task?
+      Rake.application.top_level_tasks.any?
+    end
+
+    def rails_command?
+      caller.any? { |c| c =~ /rails\/commands/ }
+    end
+
+  end
+
+end