Re: [Tarantool-patches] [PATCH v4 09/16] box: split promote() into reasonable parts

[Vladislav Shpilevoy via Tarantool-patches <tarantool-patches@dev.tarantool.org>]

Thanks for working on this!

See 3 comments below.

> diff --git a/src/box/box.cc b/src/box/box.cc
> index 86370514a..445875f8f 100644
> --- a/src/box/box.cc
> +++ b/src/box/box.cc
> @@ -1527,6 +1527,147 @@ box_wait_quorum(uint32_t lead_id, int64_t target_lsn, int quorum,

<...>

> +
> +/**
> + * Check whether the greatest promote term has changed since it was last read.
> + * IOW check that a foreign PROMOTE arrived while we were sleeping.
> + */
> +static int
> +box_check_promote_term_changed(uint64_t promote_term)

1. Normally you call check functions using the pattern
"check_something_correct". Here the correct behaviour is the term
being intact. So I propose to rename it to box_check_promote_term_intact.

> +{
> +	if (txn_limbo.promote_greatest_term != promote_term) {
> +		diag_set(ClientError, ER_INTERFERING_PROMOTE,
> +			 txn_limbo.owner_id);
> +		return -1;
> +	}
> +	return 0;
> +}

<...>

> +
> +/**
> + * A helper to wait until all limbo entries are ready to be confirmed, i.e.
> + * written to WAL and have gathered a quorum of ACKs from replicas.
> + * Return lsn of the last limbo entry on success, -1 on error.
> + */
> +static int64_t
> +box_wait_limbo_acked(void)
> +{
> +	if (txn_limbo_is_empty(&txn_limbo))
> +		return txn_limbo.confirmed_lsn;
> +
> +	uint64_t promote_term = txn_limbo.promote_greatest_term;
> +	int quorum = replication_synchro_quorum;
> +	struct txn_limbo_entry *last_entry;
> +	last_entry = txn_limbo_last_synchro_entry(&txn_limbo);
> +	/* Wait for the last entries WAL write. */
> +	if (last_entry->lsn < 0) {
> +		int64_t tid = last_entry->txn->id;
> +
> +		if (wal_sync(NULL) < 0)
> +			return -1;
> +
> +		if (box_check_promote_term_changed(promote_term) < 0)

2. Why < 0? It is not a in the code guidelines, but don't we usually
use '!= 0'? '< 0' normally assumes you can get > 0, 0, and < 0 meaning
different things, like it is done in iproto occassionally.

> +			return -1;
> +		if (txn_limbo_is_empty(&txn_limbo))
> +			return txn_limbo.confirmed_lsn;
> +		if (tid != txn_limbo_last_synchro_entry(&txn_limbo)->txn->id) {
> +			diag_set(ClientError, ER_QUORUM_WAIT, quorum,
> +				 "new synchronous transactions appeared");
> +			return -1;
> +		}
> +	}

<...>

> +
> +/** Write and process a PROMOTE request. */
> +static void
> +box_issue_promote(uint32_t prev_leader_id, int64_t promote_lsn)
> +{
> +	assert(box_raft()->volatile_term == box_raft()->term);
> +	assert(promote_lsn >= 0);
> +	txn_limbo_write_promote(&txn_limbo, promote_lsn,
> +				box_raft()->term);

3. Maybe cache box_raft() value in a variable? Its usage would look shorter
then. The same in other places where it is used more than once. Up to
you.

> +	struct synchro_request req = {
> +		.type = IPROTO_PROMOTE,
> +		.replica_id = prev_leader_id,
> +		.origin_id = instance_id,
> +		.lsn = promote_lsn,
> +		.term = box_raft()->term,
> +	};
> +	txn_limbo_process(&txn_limbo, &req);
> +	assert(txn_limbo_is_empty(&txn_limbo));
> +}