Initial draft

proposals/XXXX-errcode-specific-failure.md

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+# MSC0000: Introduce errcodes for specific membership change failures.
+
+When performing an action on the C-S API, sometimes it will fail if the user is already joined to the room. In
+these cases, homeservers will throw a `M_FORBIDDEN` stating that the action wasn't successful. However, it's difficult
+to distinguish this kind of failure from insufficient permission errors (or other kinds of errors). This would be
+useful, as the caller can then react to the error e.g. refresh it's membership cache if it tries to invite a user
+that is already joined.
+
+## Proposal
+
+New `errcode` would be introduced into the error body of a response (https://spec.matrix.org/latest/client-server-api/#standard-error-response). 
+
+`M_ALREADY_JOINED` would be fired when a membership action fails when the user is already joined to the room.
+This would cover endpoints:
+  - [POST /_matrix/client/v3/rooms/{roomId}/invite](https://spec.matrix.org/latest/client-server-api/#post_matrixclientv3roomsroomidinvite)
+  - [POST /_matrix/client/v3/rooms/knock/{roomIdOrAlias}](https://spec.matrix.org/latest/client-server-api/#post_matrixclientv3knockroomidoralias)
+  - [PUT  /_matrix/client/v3/rooms/{roomId}/state/{eventType}/{stateKey}](https://spec.matrix.org/latest/client-server-api/#put_matrixclientv3roomsroomidstateeventtypestatekey)
+Note that it would not cover endpoints where trying to join a room when the user is already joined would no-op, like `POST /_matrix/client/v3/join/{roomIdOrAlias}`.
+
+`M_INSUFFICENT_POWER` would be when your user does not have the specific required power level to
+perform an action in the room.
+  - [POST /_matrix/client/v3/rooms/{roomId}/invite](https://spec.matrix.org/latest/client-server-api/#post_matrixclientv3roomsroomidinvite)
+  - [POST /_matrix/client/v3/rooms/knock/{roomIdOrAlias}](https://spec.matrix.org/latest/client-server-api/#post_matrixclientv3knockroomidoralias)
+  - [POST /_matrix/client/v3/rooms/{roomId}/unban](https://spec.matrix.org/latest/client-server-api/#post_matrixclientv3roomsroomidban)
+  - [POST /_matrix/client/v3/rooms/{roomId}/ban](https://spec.matrix.org/latest/client-server-api/#post_matrixclientv3roomsroomidban)
+  - [POST /_matrix/client/v3/rooms/{roomId}/kick](https://spec.matrix.org/latest/client-server-api/#post_matrixclientv3roomsroomidkick)
+  - [PUT  /_matrix/client/v3/rooms/{roomId}/state/{eventType}/{stateKey}](https://spec.matrix.org/latest/client-server-api/#put_matrixclientv3roomsroomidstateeventtypestatekey)
+
+
+## Potential issues
+
+Changing long-established error codes in Matrix will be fraught with risk, as many clients will need updating
+to support the new error types. Failure to do so might lead to unexpected behaviours or confusing error messages.
+However, the alternative is keeping the non-specific error codes and having the 
+
+## Alternatives
+
+We could introduce a second field to the error body for more specific errors, but this would likely make
+error handling in clients much more complicated.
+
+## Security considerations
+
+None.
+
+## Unstable prefix
+
+While this MSC is not considered stable for implementation, implementations should use `org.matrix.unstable.errcode`
+as a prefix to the fields on the error body. `M_FORBIDDEN` will still be emitted as a `errcode` while the
+MSC is unstable, and will be replaced when the spec stabilizes.
+
+## Dependencies
+
+None.