OnUnbalanced triggers even when zero

[emielsebastiaan]

In a recent runtime upgrade (Kusama) the treasury.Deposit-event was added.
This event is triggered quite frequently: https://polkascan.io/pre/kusama-cc2/event?module=treasury&event=Deposit&page=1

Most instances of the event have a zero value; Examples:

• https://polkascan.io/pre/kusama-cc2/event/582836-3
• https://polkascan.io/pre/kusama-cc2/event/582808-3

It looks like only the events that are triggered at the end of a session have a non-zero value; Examples:

1. End of session: 257

• https://polkascan.io/pre/kusama-cc2/event/582742-4 (~0.18 KSM)
• https://polkascan.io/pre/kusama-cc2/event/582742-6 (~260.882 KSM)

2. End of session: 256

• https://polkascan.io/pre/kusama-cc2/event/580733-3 (~32.103 KSM)
• https://polkascan.io/pre/kusama-cc2/event/580733-5 (~260.838 KSM)

I have the following questions/remarks:

1. Why are some of those events triggered mid-session?
2. Why do those mid-session events have a zero value?
3. Shouldn't these mid-session events simply be omitted (bloat/no added value)?
4. Why are there 2 such events at the end of the session?
5. If these 2 events have distinct meaning, shouldn't it be two distinct event types?
6. If they do not have a distinct meaning, why not combine these two events into one event with a combined value?

[gavofyork]

1. There are many reasons why treasury acts as a sink for funds. Transaction fees, slashes and unused inflation are some of them but there are several others. Many happen mid-block.

2. It's possible when a free transaction gets executed (a heartbeat might fall into this), or a zero-slash is made. The way it's written, it just acts as an indiscriminate sink and we're not currently doing anything to avoid creating or recording "no-ops".

3. Yes

4. Once for the slash, once for the non-rewarded inflation.

5. They're the same meaning from the point of view of the treasury. You need to look at the previous event if you want the context.

6. They are two discrete events. We can't coalesce them any more than we can coalesce two separate transfers between the same two accounts.

[gavofyork]

Reopening to track point 3.