ActiveStorage file in a spec : ActiveRecord::LockWaitTimeout - ruby-on-rails

Question

Yesterday I had a stub problem, which turns out to be an ActiveStorage problem. I work on a system that stores cached data with ActiveStorage. We are using Rails 5.2.3 with the multiverse gem.
I want to load a specific file in the setup of a request spec.
So I create a user object :
user = create :user, name: 'bob'
using that factory :
FactoryBot.define do
factory :user
name { 'Robert' }
email { 'robert#email.com' }
end
end
and then try to attach a file to it like so :
user.cached_data.attach(
io: File.open(Rails.root.join('spec', 'support', 'sample_data.json'),
filename: 'sample_data.json',
content_type: 'application/json'
)
My spec is stuck there, it don't even launch, and I get that error :
ActiveRecord::LockWaitTimeout:
Mysql2::Error::TimeoutError: Lock wait timeout exceeded; try restarting transaction: INSERT INTO `active_storage_attachments` (`name`, `record_type`, `record_id`, `blob_id`, `created_at`) VALUES ('cached_data', 'User', 102, 60, '2019-08-22 16:24:07')
One of my coworkers told me that if I remove the config.use_transactional_fixtures = true in my rails_helper, that would remove my transaction problem. But I want to keep that line, which seems like a healthy standard to me.
EDIT 26/08/19
I used the MySQL console to launch the SHOW ENGINE INNODB STATUS; command during the lock. From the output I got that list of transactions :
------------
TRANSACTIONS
------------
Trx id counter 295543
Purge done for trx's n:o < 295541 undo n:o < 0 state: running but idle
History list length 66
LIST OF TRANSACTIONS FOR EACH SESSION:
---TRANSACTION 295531, not started
MySQL thread id 26, OS thread handle 0x700008449000, query id 214 localhost db_user
---TRANSACTION 0, not started
MySQL thread id 20, OS thread handle 0x7000083fc000, query id 251 localhost root init
SHOW ENGINE INNODB STATUS
---TRANSACTION 295427, not started
MySQL thread id 1, OS thread handle 0x7000083af000, query id 0 Waiting for requests
---TRANSACTION 295542, ACTIVE 40 sec
2 lock struct(s), heap size 360, 1 row lock(s), undo log entries 1
MySQL thread id 29, OS thread handle 0x700008530000, query id 242 localhost db_user
Trx #rec lock waits 0 #table lock waits 0
Trx total rec lock wait time 0 SEC
Trx total table lock wait time 0 SEC
---TRANSACTION 295541, ACTIVE 40 sec inserting
mysql tables in use 1, locked 1
LOCK WAIT 3 lock struct(s), heap size 1184, 1 row lock(s), undo log entries 1
MySQL thread id 28, OS thread handle 0x7000084e3000, query id 247 localhost db_user update
INSERT INTO `active_storage_attachments` (`name`, `record_type`, `record_id`, `blob_id`, `created_at`) VALUES ('cached_data', 'User', 99, 55, '2019-08-26 10:08:49')
Trx read view will not see trx with id >= 295542, sees < 295532
Trx #rec lock waits 1 #table lock waits 0
Trx total rec lock wait time 0 SEC
Trx total table lock wait time 0 SEC
------- TRX HAS BEEN WAITING 40 SEC FOR THIS LOCK TO BE GRANTED:
RECORD LOCKS space id 5557 page no 3 n bits 96 index `PRIMARY` of table `test`.`active_storage_blobs` trx table locks 2 total table locks 2 trx id 295541 lock mode S locks rec but not gap waiting lock hold time 40 wait time before grant 0
------------------
---TRANSACTION 295532, ACTIVE 40 sec
2 lock struct(s), heap size 360, 0 row lock(s), undo log entries 3
MySQL thread id 27, OS thread handle 0x700008496000, query id 243 localhost db_user
Trx #rec lock waits 0 #table lock waits 0
Trx total rec lock wait time 0 SEC
Trx total table lock wait time 0 SEC