feat(core): implement transaction lifecycle hooks

[rhyek]

Current lifecycle hooks cover entity and unit of work events, but
transaction-related ones are missing. Add beforeTransactionStart,
afterTransactionStart, beforeTransactionCommit, afterTransactionCommit,
beforeTransactionRollback, and afterTransactionRollback

Closes #1175

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Although #1175 mentions implementing "after" entity hooks post commit, they are not included in this PR as they are not needed. Refer to the UserSubscriber example class in test file GH1175.test.ts to see how to implement an ad-hoc afterCommitCreate hook using a combination of afterTransactionStart, afterCreate, and afterTransactionCommit.

Currently, all this PR is missing right now is documentation. Please let me know if you need me to do that as well, but I was thinking maybe @B4nan could lend a hand with that as he might have a better idea regarding form, style, and content.

[rhyek]

Btw, these hooks work correctly with nested transactions (AbstractSqlConnection). They are not run for nested transactions, only root ones. Testing is very exhaustive and covers many cases including those.

[B4nan]

Looking good, thanks! Will have few code style hints, might change it myself as I want to fiddle around with this locally anyway to understand it better.

Also few lines in MongoConnection are not covered (probably just branches caused by the optional chaining - if so, let's just force ignore those via istanbul ignore comments).

That TransactionEventBroadcaster thing looks a bit unneeded, but will need to dig deeper to better understand the code, might be missig something now.

One thing that looks stinky is that you are using isRootTransaction only in the SQL driver, not in mongo one.

[rhyek]

@B4nan glad to help.

Also few lines in MongoConnection are not covered (probably just branches caused by the optional chaining - if so, let's just force ignore those via istanbul ignore comments).

I will check this today.

That TransactionEventBroadcaster thing looks a bit unneeded, but will need to dig deeper to better understand the code, might be missig something now.

The idea here is that what the broadcaster is is agnostic to the connection and we could provide any extra context we want from the call site in the future. For example, we could choose to provide the unit of work when doing implicit transactions. I actually had planned to define the EventBroadcaster interface in the Connection core package. I will do that today. (Edit: TransactionEventBroadcaster is already a class defined in the core package).

One thing that looks stinky is that you are using isRootTransaction only in the SQL driver, not in mongo one.

Currently mongo doesn't allow for child transactions (save points), but they might in the future and in that case checking to see if it is a root transaction or not sounds like a general concern. Let me know your thoughts. I can easily move that to AbstractSqlConnection.

[rhyek]

Pushed some of the changes mentioned, added tests for mongo, and fixed coverage errors.

[rhyek]

Thank you, @B4nan.

[mcasarrubios]

Hi @rhyek, I was struggling with afterTransactionCommit hook because I didn't understand why uow param was undefined.
Watching the GH1175.test.ts file I realize that it only is defined in implicit transactions.

Is there a reason why uowis undefined in explicit transactions? I would love that it was defined as in afterFlush event. The thing is that I don't know if it is a constraint of mikro-orm or if it could be done.

I've created the question in SO too: https://stackoverflow.com/questions/76851480/in-mikro-orm-is-it-possible-to-track-unit-of-work-changes-in-the-aftertransacti

Thanks in advance.