connection leak when using r2dbc-pool with r2dbc-mysql

[1528110566]

Environment
Windows 11
JDK 1.8.0-381
Spring-parent 2.7.13
r2dbc-mysql 0.9.3

Describe the bug
I've been stucked with an bug recently, and I search anywhere on GitHub but nothing.
Here is an issue that very close to answer. pgjdbc/r2dbc-postgresql#661
So I wrote an issue under this repo.

It's quit simple. Update something in database transaction, throw an exception if the updated rows is not 1(always true because it's not exists). 2 update statements are triggled by Flux parallel.

Query query = Query.query(Criteria.where("id").is(3));
Update update = Update.update("name", "3");
List<Integer> list = new ArrayList<>();
list.add(0);
list.add(1);
for (int i = 0; i < 1000; i++) {
    Flux.fromIterable(list)
            .flatMap(item -> {
                return transactionalOperator.transactional(template.update(query, update, User.class)
                        .doOnNext(res -> {
                            if (res != 1) {
                                throw new RuntimeException("test");
                            }
                        }));
            })
            .subscribe();
}

The funny thing is, when I change .doOnNext to another block(attention on )), it's works as expected.

Query query = Query.query(Criteria.where("id").is(3));
Update update = Update.update("name", "3");
List<Integer> list = new ArrayList<>();
list.add(0);
list.add(1);
for (int i = 0; i < 1000; i++) {
    Flux.fromIterable(list)
            .flatMap(item -> {
                return transactionalOperator.transactional(template.update(query, update, User.class))
                        .doOnNext(res -> {
                            if (res != 1) {
                                throw new RuntimeException("test");
                            }
                        });
            })
            .subscribe();
}

So, I believe something must not compatible with cancel in reactor and spring-tx.
Another thing confusing me is, why the number of r2dbc_pool_acquired_connections always 9.

To Reproduce
https://github.com/1528110566/r2dbc-connection-leak-demo
This is a demo that can reproduce this bug.

1. call http://localhost:8080/test
2. wait background threads are finished
3. call http://localhost:8080/actuator/prometheus
4. search r2dbc_pool_acquired_connectionsand r2dbc_pool_idle_connections. We'll find the number of connection in the pool is not as expected. This can be proved with debugging on https://github.com/r2dbc/r2dbc-pool/blob/bf6540eee26aa55bba4cb6ab0c6a3c622524679c/src/main/java/io/r2dbc/pool/ConnectionPool.java#L106. We'will find the number of connection is actually smaller than expected.

[jchrys]

Thanks for describing the issue in detail. Let me look into it

[jchrys]

@1528110566
This doesn't seem to be caused by r2dbc-mysql. Could you kindly raise a ticket with r2dbc-pool?

[1528110566]

Sure, I will.

[1528110566]

Expecting a good news...
r2dbc/r2dbc-pool#219