1 files changed, 100 insertions, 2 deletions
diff --git a/activerecord/test/cases/migration_test.rb b/activerecord/test/cases/migration_test.rb
index 10f1c7216f..c3c204cf9f 100644
--- a/activerecord/test/cases/migration_test.rb
+++ b/activerecord/test/cases/migration_test.rb
@@ -1,6 +1,7 @@
-require "cases/helper"
-require "cases/migration/helper"
+require 'cases/helper'
+require 'cases/migration/helper'
 require 'bigdecimal/util'
+require 'concurrent/atomic/count_down_latch'
 
 require 'models/person'
 require 'models/topic'
@@ -522,6 +523,79 @@ class MigrationTest < ActiveRecord::TestCase
     end
   end
 
+  if ActiveRecord::Base.connection.supports_advisory_locks?
+    def test_migrator_generates_valid_lock_id
+      migration = Class.new(ActiveRecord::Migration).new
+      migrator = ActiveRecord::Migrator.new(:up, [migration], 100)
+
+      lock_id = migrator.send(:generate_migrator_advisory_lock_id)
+
+      assert ActiveRecord::Base.connection.get_advisory_lock(lock_id),
+        "the Migrator should have generated a valid lock id, but it didn't"
+      assert ActiveRecord::Base.connection.release_advisory_lock(lock_id),
+        "the Migrator should have generated a valid lock id, but it didn't"
+    end
+
+    def test_generate_migrator_advisory_lock_id
+      # It is important we are consistent with how we generate this so that
+      # exclusive locking works across migrator versions
+      migration = Class.new(ActiveRecord::Migration).new
+      migrator = ActiveRecord::Migrator.new(:up, [migration], 100)
+
+      lock_id = migrator.send(:generate_migrator_advisory_lock_id)
+
+      current_database = ActiveRecord::Base.connection.current_database
+      salt = ActiveRecord::Migrator::MIGRATOR_SALT
+      expected_id = Zlib.crc32(current_database) * salt
+
+      assert lock_id == expected_id, "expected lock id generated by the migrator to be #{expected_id}, but it was #{lock_id} instead"
+      assert lock_id.is_a?(Fixnum), "expected lock id to be a Fixnum, but it wasn't"
+      assert lock_id.bit_length <= 63, "lock id must be a signed integer of max 63 bits magnitude"
+    end
+
+    def test_migrator_one_up_with_unavailable_lock
+      assert_no_column Person, :last_name
+
+      migration = Class.new(ActiveRecord::Migration) {
+        def version; 100 end
+        def migrate(x)
+          add_column "people", "last_name", :string
+        end
+      }.new
+
+      migrator = ActiveRecord::Migrator.new(:up, [migration], 100)
+      lock_id = migrator.send(:generate_migrator_advisory_lock_id)
+
+      with_another_process_holding_lock(lock_id) do
+        assert_raise(ActiveRecord::ConcurrentMigrationError) { migrator.migrate }
+      end
+
+      assert_no_column Person, :last_name,
+        "without an advisory lock, the Migrator should not make any changes, but it did."
+    end
+
+    def test_migrator_one_up_with_unavailable_lock_using_run
+      assert_no_column Person, :last_name
+
+      migration = Class.new(ActiveRecord::Migration) {
+        def version; 100 end
+        def migrate(x)
+          add_column "people", "last_name", :string
+        end
+      }.new
+
+      migrator = ActiveRecord::Migrator.new(:up, [migration], 100)
+      lock_id = migrator.send(:generate_migrator_advisory_lock_id)
+
+      with_another_process_holding_lock(lock_id) do
+        assert_raise(ActiveRecord::ConcurrentMigrationError) { migrator.run }
+      end
+
+      assert_no_column Person, :last_name,
+        "without an advisory lock, the Migrator should not make any changes, but it did."
+    end
+  end
+
   protected
     # This is needed to isolate class_attribute assignments like `table_name_prefix`
     # for each test case.
@@ -531,6 +605,30 @@ class MigrationTest < ActiveRecord::TestCase
         def self.base_class; self; end
       }
     end
+
+    def with_another_process_holding_lock(lock_id)
+      thread_lock = Concurrent::CountDownLatch.new
+      test_terminated = Concurrent::CountDownLatch.new
+
+      other_process = Thread.new do
+        begin
+          conn = ActiveRecord::Base.connection_pool.checkout
+          conn.get_advisory_lock(lock_id)
+          thread_lock.count_down
+          test_terminated.wait # hold the lock open until we tested everything
+        ensure
+          conn.release_advisory_lock(lock_id)
+          ActiveRecord::Base.connection_pool.checkin(conn)
+        end
+      end
+
+      thread_lock.wait # wait until the 'other process' has the lock
+
+      yield
+
+      test_terminated.count_down
+      other_process.join
+    end
 end
 
 class ReservedWordsMigrationTest < ActiveRecord::TestCase