[Tarantool-patches] [PATCH v3 07/12] box: introduce `box.ctl.demote`

[Vladislav Shpilevoy]

Thanks for the fixes!

>> 2. On top of this commit and on top of the branch too I tried to
>> promote a candidate and got a strange error in the logs, although
>> the promotion was successful:
>>
>> -- 
>> -- Instance 1
>> -- 
>>
>> -- Step 1
>> box.cfg{
>>      listen = 3313,
>>      election_mode = 'candidate',
>>      replication_synchro_quorum = 2,
>>      replication = {'localhost:3314', 'localhost:3313'}
>> }
>> box.schema.user.grant('guest', 'super')
>>
>>
>> -- 
>> -- Instance 2
>> -- 
>>
>> -- Step 2
>> box.cfg{
>>      listen = 3314,
>>      election_mode = 'voter',
>>      replication_synchro_quorum = 2,
>>      replication = {'localhost:3314', 'localhost:3313'},
>>      read_only = true,
>> }
>>
>> -- Step 3
>> box.cfg{read_only = false, election_mode = 'candidate'}
>>
>> -- Step 4
>> box.ctl.promote()
>>
>> main/112/raft_worker box.cc:1538 E> ER_UNSUPPORTED: box.ctl.promote does not support simultaneous invocations
>> ---
>> ...
> 
> That's because once a candidate becomes the leader, it tries to issue `box.ctl.promote()`, and fails,
> since we're already in `box.ctl.promote()` call.
> I'm not sure how to handle that properly. This doesn't break anything though.

Still, the error message looks really not good. There is an option -
make box_promote() for candidate node just call raft_promote() and set
box_in_promote = false. Then wait for the term outcome. Will it work?
You would need to rebase your patchset on master branch then.

It might be a little easier to do if you apply the diff below. (Warning:
I didn't test it.) The motivation is that one of the main reasons why I
wanted box_promote() simplified was because of the strange meaning of some
flags. In particular, try_wait flag somewhy triggered elections before the
waiting which is super not obvious why. How does 'wait' come to 'elections'?

In the diff I tried to remove these flags entirely. And now you have a
single place in the code of box_promote(), where ELECTION_MODE_CANDIDATE
stuff is handled. Here you could try the proposal I gave above.

====================
diff --git a/src/box/box.cc b/src/box/box.cc
index f68fffcab..e7765b657 100644
--- a/src/box/box.cc
+++ b/src/box/box.cc
@@ -1698,34 +1698,6 @@ box_issue_demote(uint32_t prev_leader_id, int64_t promote_lsn)
 	assert(txn_limbo_is_empty(&txn_limbo));
 }
 
-/**
- * Check whether this instance may run a promote() and set promote parameters
- * according to its election mode.
- */
-static int
-box_check_promote_election_mode(bool *try_wait, bool *run_elections)
-{
-	switch (box_election_mode) {
-	case ELECTION_MODE_OFF:
-		if (try_wait != NULL)
-			*try_wait = true;
-		break;
-	case ELECTION_MODE_VOTER:
-		assert(box_raft()->state == RAFT_STATE_FOLLOWER);
-		diag_set(ClientError, ER_UNSUPPORTED, "election_mode='voter'",
-			 "manual elections");
-		return -1;
-	case ELECTION_MODE_MANUAL:
-	case ELECTION_MODE_CANDIDATE:
-		if (run_elections != NULL)
-			*run_elections = box_raft()->state != RAFT_STATE_LEADER;
-		break;
-	default:
-		unreachable();
-	}
-	return 0;
-}
-
 /* A guard to block multiple simultaneous promote()/demote() invocations. */
 static bool box_in_promote = false;
 
@@ -1757,27 +1729,35 @@ box_promote(void)
 	if (is_leader)
 		return 0;
 
-	bool run_elections = false;
-	bool try_wait = false;
-
-	if (box_check_promote_election_mode(&try_wait, &run_elections) < 0)
-		return -1;
-
-	int64_t wait_lsn = -1;
-
-	if (run_elections && box_run_elections() < 0)
-		return -1;
-	if (try_wait) {
-		if (box_try_wait_confirm(2 * replication_synchro_timeout) < 0)
+	switch (box_election_mode) {
+	case ELECTION_MODE_OFF:
+		if (box_try_wait_confirm(2 * replication_synchro_timeout) != 0)
+			return -1;
+		if (box_trigger_elections() != 0)
 			return -1;
-		if (box_trigger_elections() < 0)
+		break;
+	case ELECTION_MODE_VOTER:
+		assert(box_raft()->state == RAFT_STATE_FOLLOWER);
+		diag_set(ClientError, ER_UNSUPPORTED, "election_mode='voter'",
+			 "manual elections");
+		return -1;
+	case ELECTION_MODE_MANUAL:
+	case ELECTION_MODE_CANDIDATE:
+		if (box_raft()->state != RAFT_STATE_LEADER &&
+		    box_run_elections() != 0)
 			return -1;
+		break;
+	default:
+		unreachable();
 	}
-	if ((wait_lsn = box_wait_limbo_acked()) < 0)
+	if (box_check_promote_election_mode(&try_wait, &run_elections) < 0)
 		return -1;
 
-	box_issue_promote(txn_limbo.owner_id, wait_lsn);
+	int64_t wait_lsn = box_wait_limbo_acked();
+	if (wait_lsn < 0)
+		return -1;
 
+	box_issue_promote(txn_limbo.owner_id, wait_lsn);
 	return 0;
 }
 
@@ -1804,29 +1784,16 @@ box_demote(void)
 		is_leader = is_leader && box_raft()->state == RAFT_STATE_LEADER;
 	if (!is_leader)
 		return 0;
-
-	bool try_wait = false;
-
-	if (box_check_promote_election_mode(&try_wait, NULL) < 0)
-		return -1;
-
-	int64_t wait_lsn = -1;
-
 	if (box_trigger_elections() < 0)
 		return -1;
-
 	if (box_election_mode != ELECTION_MODE_OFF)
 		return 0;
-
-	if (try_wait &&
-	    box_try_wait_confirm(2 * replication_synchro_timeout) < 0)
+	if (box_try_wait_confirm(2 * replication_synchro_timeout) != 0)
 		return -1;
-
-	if ((wait_lsn = box_wait_limbo_acked()) < 0)
+	int64_t wait_lsn = box_wait_limbo_acked();
+	if (wait_lsn < 0)
 		return -1;
-
 	box_issue_demote(txn_limbo.owner_id, wait_lsn);
-
 	return 0;
 }