Prevent nested transactions with *DB.Transaction

[qilx]

Make sure these boxes checked before submitting your pull request.

• Do only one thing
• No API-breaking changes
• New code/logic commented & tested

What did this pull request do?

This will prevent creating nested transactions while calling *DB.Transaction(...) multiple times.
Only fist call will create transaction. Rest will just reuse it.

[dynajoe]

Could this be used in .Begin() for those that don't always use .Transaction?

[qilx]

It is probably not useful put it into .Begin(). Because Because even so you have to call .Commit() only once at the end.
Only solution that I can think of is putting also counter into *DB, which would count number of .Begin() calls and then ignored counter-1 calls to .Commit(). But I would say that it is easier to just use .Transaction() (and more error prone).

[sjmtan]

I find this interface to be a bit confusing, but I don't use it - so take my feedback with a grain of salt. I think this changes the semantics of the function a bit - when you write the function block as a programmer, you sort of assume that at the end of the function block, it'll be committed since that's the behavior in most cases.

Only in the nested case is this not true if I'm reading it correctly, and you can imagine issues arising if the "nested" transaction has an RPC call after.

In my company's codebase, we panic on these nested transactions in tests/removing ways to have nested transactions, and while I'm not saying that it is the ideal behavior, it forces our developers to make sure that they are aware of what data is committed or not.