DEV: Allow DROP NOT NULL in pre-deploy migrations (#20775)

Our SafeMigrate system is designed to prevent tables/columns being dropped in pre-deploy migrations. Its regex-based detection was triggering incorrectly on `ALTER COLUMN DROP NOT NULL`.

lib/migration/safe_migrate.rb

@@ -129,7 +129,7 @@ class Migration::SafeMigrate
         in use by live applications.
       TEXT
       raise Discourse::InvalidMigration, "Attempt was made to drop a table"
-    elsif sql =~ /\A\s*alter\s+table.*(?:rename|drop)\s+/i
+    elsif sql =~ /\A\s*alter\s+table.*(?:rename|drop(?!\s+not\s+null))\s+/i
       $stdout.puts("", <<~TEXT)
         WARNING
         -------------------------------------------------------------------------------------

spec/fixtures/db/migrate/drop_not_null/20990309014015_drop_not_null.rb

@@ -0,0 +1,11 @@
+# frozen_string_literal: true
+
+class DropNotNull < ActiveRecord::Migration[5.1]
+  def up
+    change_column_null :users, :username, true
+  end
+
+  def down
+    raise "not tested"
+  end
+end

spec/lib/migration/safe_migrate_spec.rb

@@ -70,6 +70,16 @@ RSpec.describe Migration::SafeMigrate do
     expect { User.first.username }.not_to raise_error
   end
 
+  it "allows dropping NOT NULL" do
+    Migration::SafeMigrate.enable!
+
+    path = File.expand_path "#{Rails.root}/spec/fixtures/db/migrate/drop_not_null"
+
+    output = capture_stdout { migrate_up(path) }
+
+    expect(output).to include("change_column_null(:users, :username, true)")
+  end
+
   it "supports being disabled" do
     Migration::SafeMigrate.enable!
     Migration::SafeMigrate.disable!