Create Unified Join Rules MSC.

Signed-off-by: Kevin Cox <kevincox@kevincox.ca>

Fixes: https://github.com/matrix-org/matrix-doc/issues/3297

proposals/0000-unified-join-rules.md

@@ -0,0 +1,197 @@
+# MSC0000: Unified Join Rules
+
+[MSC2403 Knock](./2403-knock.md) and [MSC3083 Restricting room membership based on membership in other rooms](./3083-restricted-rooms.md) both update the join rules of a room to allow a new feature. The former defines `join_rule: knock` which allows anyone to knock to enter a room and the latter defines `join_rule: restricted` to restrict who can join a room based on a set of rules. Unfortunately these features can not be used together as a room can only have one `join_rule`.
+
+This MSC aims to solve the proliferation of `join_rule`s and allow these features to interoperate.
+
+As an added benefit this MSC is backwards-compatible. The `allow_knock` field can be added to an existing `join_rules: restricted` room and people can start knocking as it is supported by their servers and clients.
+
+## Proposal
+
+### `allow_knock`
+
+`join_rule: restricted` will be updated with the `allow_knock` key. This functions identically to the `allow` key except that it controls who can knock, instead of controlling who can join. Please see [MSC3083 Restricting room membership based on membership in other rooms](./3083-restricted-rooms.md) for how the rules are evaluated and [MSC2403 Knock](./2403-knock.md) for what it means to knock.
+
+The `allow_knock` key may be absent in which case it is treated as if it was set to `[]` (the empty list). In this case no one is allowed to knock.
+
+### `m.any`
+
+The `m.any` allow type will be defined. This type allows any user to perform the relevant action. The may be used in both the `allow` and `allow_knock` fields.
+
+### Example: Anyone can knock
+
+This shows an example of a room where you can join if you are a member of `#users:example.org` otherwise you can only knock.
+
+```json5
+{
+	"type": "m.room.join_rules"
+	"state_key": ""
+	"content": {
+		"join_rule": "restricted"
+		"allow": [{
+			"type": "m.room_membership"
+			"room_id": "#users:example.org"
+		}]
+		"allow_knock": [{
+			"type": "m.any"
+		}],
+	},
+}
+```
+
+### Example: Restricted knock
+
+This shows an example of a room where anyone in the `#users:example.org` room (or space) can knock, but anyone in `#mods:example.org` can join directly.
+
+```json5
+{
+	"type": "m.room.join_rules"
+	"state_key": ""
+	"content": {
+		"join_rule": "restricted"
+		"allow": [{
+			"type": "m.room_membership"
+			"room_id": "#mods:example.org"
+		}]
+		"allow_knock": [{
+			"type": "m.room_membership"
+			"room_id": "#users:example.org"
+		}]
+	}
+}
+```
+
+## Potential issues
+
+### Useless allow_knock entires.
+
+It is possible that entires in `allow_knock` are redundant because they are also included in `allow` so could simply join directly. While this is unsightly it is non-harmful and will not affect users or servers.
+
+```json5
+{
+	"type": "m.room.join_rules"
+	"state_key": ""
+	"content": {
+		"join_rule": "restricted"
+		"allow": [{
+			"type": "m.any"
+		}]
+		// This is irrelevant because anyone can directly join.
+		"allow_knock": [{
+			"type": "m.room_membership"
+			"room_id": "#users:example.org"
+		}]
+	}
+}
+```
+
+```json5
+{
+	"type": "m.room.join_rules"
+	"state_key": ""
+	"content": {
+		"join_rule": "restricted"
+		"allow": [{
+			"type": "m.room_membership",
+			"room_id": "#users:example.org"
+		}]
+		// This is irrelevant because everyone in this room can join directly.
+		"allow_knock": [{
+			"type": "m.room_membership"
+			"room_id": "#users:example.org"
+		}]
+	}
+}
+```
+
+```json5
+{
+	"type": "m.room.join_rules"
+	"state_key": ""
+	"content": {
+		"join_rule": "restricted"
+		"allow": [{
+			"type": "m.any"
+		}, {
+			// This is irrelevant because of the m.any rule.
+			"type": "m.room_membership",
+			"room_id": "#users:example.org"
+		}]
+	}
+}
+```
+
+Clients may consider helping users to clean up unnecessary elements from the `allow` and `allow_knock` lists.
+
+### Multiple ways to specify the same rules.
+
+After this MSC is implemented it will be possible to specify all other (current) `join_rule` types in terms of `join_rule: restricted`.
+
+This is considered a feature. Once this MSC is widely supported it is expected that rooms are created using `join_rule: restricted` preferentially for simplicity. The other types can be considered deprecated. In order to simplify the protocol a future room version may consider dropping support for anything but `restricted`.
+
+The following is equivalent to `join_rule: public` (as far as join rules are concerned).
+
+```json5
+{
+	"type": "m.room.join_rules"
+	"state_key": ""
+	"content": {
+		"join_rule": "restricted"
+		"allow": [{
+			"type": "m.any"
+		}]
+	}
+}
+```
+
+The following is equivalent to `join_rule: invite`.
+
+```json5
+{
+	"type": "m.room.join_rules"
+	"state_key": ""
+	"content": {
+		"join_rule": "restricted"
+		"allow": []
+	}
+}
+```
+
+The following is equivalent to `join_rule: knock`.
+
+```json5
+{
+	"type": "m.room.join_rules"
+	"state_key": ""
+	"content": {
+		"join_rule": "restricted"
+		"allow": []
+		"allow_knock": [{
+			"type": "m.any"
+		}]
+	}
+}
+```
+
+## Alternatives
+
+### Introduce a new mechanism for knock.
+
+Knock is arguably not a join rule, it is a rule for who may request an invite. It could be moved to a separate mechanism to avoid the conflict with `join_rule: restricted`.
+
+This MSC is a better alternative because:
+
+- Knock does affect how people may join a room so it makes sense to handle it in the same location as other ways to join.
+- Allowing `m.room_membership` restricted knocking is a useful feature that appears "for free" with this unification.
+
+## Security considerations
+
+Other than the general risk of adding complexity to `join_rules` there are no further security implications.
+
+It is worth noting the [Security considerations of `m.room_membership`](./3083-restricted-rooms.md#security-considerations) as this MSC does not improve them and the same concerns apply (although with less consequences) to `allow_knock`.
+
+## Unstable prefix
+
+Until this specification is approved please the follow changes should be used for implementation:
+- Use `ca.kevincox.allow_knock.v1` instead of the `allow_knock` attribute.
+- use `ca.kevincox.any.v1` instead of `type: m.any`.