add test for sub-partitioning

spec/safe_statements_spec.rb

@@ -3523,7 +3523,7 @@ def up
               end
             end
 
-            it "waits to acquire a lock if the table is partitioned and child tables are blocked" do
+            it "waits to acquire a lock if the table is partitioned and child table is blocked" do
               stub_const("PgHaMigrations::LOCK_TIMEOUT_SECONDS", 1)
 
               ActiveRecord::Base.connection.drop_table(table_name)
@@ -3551,6 +3551,40 @@ def up
               end
             end
 
+            it "waits to acquire a lock if the table is partitioned and child sub-partition is blocked" do
+              stub_const("PgHaMigrations::LOCK_TIMEOUT_SECONDS", 1)
+
+              ActiveRecord::Base.connection.drop_table(table_name)
+              create_range_partitioned_table(table_name, migration_klass)
+              create_range_partitioned_table("#{table_name}_sub", migration_klass, with_partman: true)
+              ActiveRecord::Base.connection.execute(<<~SQL)
+                ALTER TABLE bogus_table
+                ATTACH PARTITION bogus_table_sub
+                FOR VALUES FROM ('2020-01-01') TO ('2020-02-01')
+              SQL
+
+              begin
+                thread = Thread.new do
+                  migration.safely_acquire_lock_for_table(:bogus_table_sub_default) { sleep 3 }
+                end
+
+                sleep 1.1
+
+                expect(PgHaMigrations::BlockingDatabaseTransactions).to receive(:find_blocking_transactions)
+                  .at_least(3)
+                  .times
+                  .and_call_original
+
+                migration.suppress_messages do
+                  migration.safely_acquire_lock_for_table(table_name) do
+                    expect(locks_for_table(table_name, connection: alternate_connection)).not_to be_empty
+                  end
+                end
+              ensure
+                thread.join
+              end
+            end
+
             it "fails lock acquisition quickly if Postgres doesn't grant an exclusive lock but then retries" do
               stub_const("PgHaMigrations::LOCK_TIMEOUT_SECONDS", 1)