Add MsgLeaveGroup to group module

[aaronc]

Summary

An ability for a member to unilaterally leave a group.

Problem Definition

Normally in a social group, if one member decides to leave a group, they simply can. Nobody else needs to agree.

Also, if I am using a group to manage a single wallet with multiple keys. I would like the ability to "log out" of one device without needing the other devices to approve.

Proposal

Add MsgLeaveGroup:

message MsgLeaveGroup {
  string member_address = 1;
  uint64 group_id = 2;
}

One difficulty with implementing this is that it could make ThresholdDecisionPolicy fail if there are not enough members to meet the threshold if a member leaves. To address this, we either need to:

• add validation logic preventing members from leaving if that would break the threshold, or
• switch to a percentage decision policy or hybrid threshold/decision policy to accomodate members leaving

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[aaronc]

@blushi @frumioj thoughts?

[frumioj]

Is "logout" the same as "leave group"? Why?
Can the group admin leave the group?
Are the members of the group actual users (ie. social group case) or simply "wielders of a key" (on behalf of users)?
I see these groups as being like the members of a household, with a shared kitty for household expenses. No problem to leave as such, but what happens to their money (if invested) and their vote as to what is a valid expense?

[blushi]

Can the group admin leave the group?

A group admin is not necessarily part of the group members. But a group admin can update the group with a new admin.

[blushi]

One difficulty with implementing this is that it could make ThresholdDecisionPolicy fail if there are not enough members to meet the threshold if a member leaves. To address this, we either need to:

add validation logic preventing members from leaving if that would break the threshold, or
switch to a percentage decision policy or hybrid threshold/decision policy to accomodate members leaving

The first option would be the most straightforward to implement for now but then I agree this could come with a bad UX because that would require the group account admin to update the decision policy beforehand.
But that's also the option that would work best for any kind of decision policy because even if we switch to a percentage decision policy, we cannot assume that it will always be the case.

[blushi]

should we reopen this? cc/ @frumioj @aaronc