Add explicit migration breakpoints (#33089)

.github/workflows/test-migrations.yml

@@ -12,6 +12,7 @@ on:
       - '**/*.rb'
       - '.github/workflows/test-migrations.yml'
       - 'lib/tasks/tests.rake'
+      - 'lib/tasks/db.rake'
 
   pull_request:
     paths:
@@ -90,6 +91,11 @@ jobs:
           bin/rails db:drop
           bin/rails db:create
           SKIP_POST_DEPLOYMENT_MIGRATIONS=true bin/rails tests:migrations:prepare_database
+
+          # Migrate up to v4.2.0 breakpoint
+          bin/rails db:migrate VERSION=20230907150100
+
+          # Migrate the rest
           SKIP_POST_DEPLOYMENT_MIGRATIONS=true bin/rails db:migrate
           bin/rails db:migrate
           bin/rails tests:migrations:check_database

lib/tasks/db.rake

@@ -43,8 +43,14 @@ namespace :db do
   end
 
   task pre_migration_check: :environment do
-    version = ActiveRecord::Base.connection.database_version
-    abort 'This version of Mastodon requires PostgreSQL 12.0 or newer. Please update PostgreSQL before updating Mastodon.' if version < 120_000
+    pg_version = ActiveRecord::Base.connection.database_version
+    abort 'This version of Mastodon requires PostgreSQL 12.0 or newer. Please update PostgreSQL before updating Mastodon.' if pg_version < 120_000
+
+    schema_version = ActiveRecord::Migrator.current_version
+    abort <<~MESSAGE if ENV['SKIP_POST_DEPLOYMENT_MIGRATIONS'] && schema_version < 2023_09_07_150100
+      Zero-downtime migrations from Mastodon versions earlier than 4.2.0 are not supported.
+      Please update to Mastodon 4.2.x first or upgrade by stopping all services and running migrations without `SKIP_POST_DEPLOYMENT_MIGRATIONS`.
+    MESSAGE
   end
 
   Rake::Task['db:migrate'].enhance(['db:pre_migration_check'])