Unable to destroy Nomination Pools

[Ank4n]

First reported for Pool 231.

When the owner tries to withdraw_unbonded, they get a Token: Frozen error.

The issue seems to have started since the last polkadot runtime upgrade.

Issue

The pool seems to have an extra consumer reference. The pool is expected to have two consumer references

• explicit consumer increment by pallet-staking.
• applying lock on ledger increments consumer.

but most of the pool including #231 has 3 references. The destruction of pool fails when it tries to transfer all the remaining amount from pool account to the owner specifically on this condition. Since it has one remaining consumer, it cannot kill the account and expects ED to remain in the pool account.

This issue affects almost all the current pools if they try to destroy.

Cause

This PR seems to have led to this issue: #1976.
Earlier, applying a lock would increase consumer by 2, and similarly decrease by 2 when removed.

With the above fix, consumer references are only increased by 1. The older accounts with an extra consumer reference though are now in a bad state and cannot be killed.

Solution

There seems to be no easy way to detect on-chain which accounts have an extra consumer reference. For pool accounts, we could expect to only have 2 consumers and potentially this can be fixed in a migration.

A better solution would be fixing for all the accounts that have this extra reference.
The issue is mentioned and tracked at #2037.

Update

We could explicitly kill account when pools are destroyed. This can be either by force consumer decrement or a separate (new) fungible api that transfers and kills account.

[kianenigma]

I suggest exploring a more short-sighted solution, even if it involved using governance, to fix all the current and prospect pools, even if a more systematic solution cannot be found. Not being able to destroy the pool implies a greater economic damage (500+ DOT) vs. an account (1, 0.01 DOT depending on the chain).