Could we consider the possibility to assign triggers on a statement itself?
In this case you shouldn't implement all that machinery with savepoints.

On Thursday, July 18, 2019 6:45:56 PM MSK Vladimir Davydov wrote:
> Rollback triggers expect to see the database state after reverting all
> changes done by the transaction. At the same time they may use tuples
> referenced by reverted statements.
> 
> As a result, using txn_rollback_to_svp(), which both reverts changes and
> releases tuples, on yield in a DDL transaction may result in a crash.
> The crash can only be reproduced using SQL, because Lua implicitly
> references all tuples anyway.
> 
> Thanks to @Gerold103 for reporting this issue and carrying out
> preliminary investigation.
> 
> Closes #4365
> ---
> https://github.com/tarantool/tarantool/issues/4365
> https://github.com/tarantool/tarantool/commits/dv/gh-4365-fix-tx-ddl-crash-o
> n-yield
> 
>  src/box/txn.c                 | 22 +++++++++++++++++++++-
>  test/box/transaction.result   |  9 +++++++++
>  test/box/transaction.test.lua |  6 ++++++
>  3 files changed, 36 insertions(+), 1 deletion(-)
> 
> diff --git a/src/box/txn.c b/src/box/txn.c
> index 73eaee73..6d844af2 100644
> --- a/src/box/txn.c
> +++ b/src/box/txn.c
> @@ -761,10 +761,30 @@ txn_on_yield(struct trigger *trigger, void *event)
>  	(void) event;
>  	struct txn *txn = in_txn();
>  	assert(txn != NULL && !txn->can_yield);
> -	txn_rollback_to_svp(txn, NULL);
> +	/*
> +	 * Rollback triggers expect to see the database state
> +	 * after reverting all changes done by the transaction.
> +	 * At the same time they may use tuples referenced by
> +	 * reverted statements. So we rollback statements before
> +	 * running the triggers, but postpone tuple release until
> +	 * after the triggers have been invoked.
> +	 */
> +	struct txn_stmt *stmt;
> +	stailq_foreach_entry(stmt, &txn->stmts, next) {
> +		if (stmt->space != NULL)
> +			engine_rollback_statement(txn->engine, txn, stmt);
> +		stmt->space = NULL;
> +		stmt->row = NULL;
> +	}
>  	if (txn->has_triggers) {
>  		txn_run_triggers(txn, &txn->on_rollback);
>  		txn->has_triggers = false;
>  	}
> +	stailq_foreach_entry(stmt, &txn->stmts, next)
> +		txn_stmt_unref_tuples(stmt);
> +	stailq_create(&txn->stmts);
> +	txn->n_new_rows = 0;
> +	txn->n_local_rows = 0;
> +	txn->n_applier_rows = 0;
>  	txn->is_aborted_by_yield = true;
>  }
> diff --git a/test/box/transaction.result b/test/box/transaction.result
> index 9a48f2f7..771ad96e 100644
> --- a/test/box/transaction.result
> +++ b/test/box/transaction.result
> @@ -778,3 +778,12 @@ box.begin() drop() box.rollback()
>  box.begin() drop() box.commit()
>  ---
>  ...
> +--
> +-- gh-4365: DDL revered by yield triggers crash.
> +--
> +box.begin() box.execute([[CREATE TABLE test(id INTEGER PRIMARY KEY
> AUTOINCREMENT)]]) fiber.yield() +---
> +...
> +box.rollback()
> +---
> +...
> diff --git a/test/box/transaction.test.lua b/test/box/transaction.test.lua
> index 0792cc3c..d8a6e60a 100644
> --- a/test/box/transaction.test.lua
> +++ b/test/box/transaction.test.lua
> @@ -415,3 +415,9 @@ box.begin() create() box.rollback()
>  box.begin() create() box.commit()
>  box.begin() drop() box.rollback()
>  box.begin() drop() box.commit()
> +
> +--
> +-- gh-4365: DDL revered by yield triggers crash.
> +--
> +box.begin() box.execute([[CREATE TABLE test(id INTEGER PRIMARY KEY
> AUTOINCREMENT)]]) fiber.yield() +box.rollback()