MSC3848: Introduce errcodes for specific event sending failures #3848
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| # MSC3848: Introduce errcodes for specific event sending failures. | ||
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| When performing an action on the C-S API, sometimes requests will fail with | ||
| a generic `M_FORBIDDEN` error with implementations providing a more meaningful | ||
| context in the `error` field. While some client implementations have taken it | ||
| upon themselves to scan these error fields for probable causes, this isn't a | ||
| particularly safe or spec-complaint way to react to errors. | ||
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| Some examples of failures which require more standardized error information | ||
| include already-joined errors when a user tries to invite another user to a room, | ||
| or being unable to send an event into a room due to lacking the required power level. | ||
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| For this reason, this proposal suggests including new `errcode` definitions | ||
| which provide more specific information about the failure. | ||
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| ## Proposal | ||
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| New `errcode` would be introduced into the error body of a response | ||
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There was a problem hiding this comment. this sounds like we're adding a whole new property, which I don't think is the case? |
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| (https://spec.matrix.org/v1.3/client-server-api/#standard-error-response). | ||
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| `M_ALREADY_JOINED` would be fired when a membership action fails when the authenticated user | ||
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There was a problem hiding this comment. could you maybe structure this so that it's clearer which text applies to which error code? Maybe subsection headers? |
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| is already joined to the room. | ||
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| This would cover endpoints: | ||
| - [POST /_matrix/client/v3/rooms/{roomId}/invite](https://spec.matrix.org/v1.3/client-server-api/#post_matrixclientv3roomsroomidinvite) | ||
| - [POST /_matrix/client/v3/rooms/knock/{roomIdOrAlias}](https://spec.matrix.org/v1.3/client-server-api/#post_matrixclientv3knockroomidoralias) | ||
| - [PUT /_matrix/client/v3/rooms/{roomId}/state/{eventType}/{stateKey}](https://spec.matrix.org/v1.3/client-server-api/#put_matrixclientv3roomsroomidstateeventtypestatekey) | ||
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| 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}`. | ||
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| `M_INSUFFICIENT_POWER` would be when the authenticated user does not have the required power level to | ||
| perform an action in the room. | ||
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| `M_NOT_JOINED` would be when the authenticated user is not joined to a room, but attempts to perform | ||
| an action in it. | ||
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| Both errcodes would cover endpoints: | ||
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There was a problem hiding this comment. Synapse also returns not in room when There was a problem hiding this comment. I tested this on Synapse and found:
I think we should consistently define the behavior for trying to leave a room you are not in, rather than having 3 different error responses (differing messages are fine) for effectively the same state. My gut feeling is that we should probably be a no-op. There was a problem hiding this comment. |
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| - [POST /_matrix/client/v3/rooms/{roomId}/invite](https://spec.matrix.org/v1.3/client-server-api/#post_matrixclientv3roomsroomidinvite) | ||
| - [POST /_matrix/client/v3/rooms/knock/{roomIdOrAlias}](https://spec.matrix.org/v1.3/client-server-api/#post_matrixclientv3knockroomidoralias) | ||
| - [POST /_matrix/client/v3/rooms/{roomId}/unban](https://spec.matrix.org/v1.3/client-server-api/#post_matrixclientv3roomsroomidban) | ||
| - [POST /_matrix/client/v3/rooms/{roomId}/ban](https://spec.matrix.org/v1.3/client-server-api/#post_matrixclientv3roomsroomidban) | ||
| - [POST /_matrix/client/v3/rooms/{roomId}/kick](https://spec.matrix.org/v1.3/client-server-api/#post_matrixclientv3roomsroomidkick) | ||
| - [PUT /_matrix/client/v3/rooms/{roomId}/state/{eventType}/{stateKey}](https://spec.matrix.org/v1.3/client-server-api/#put_matrixclientv3roomsroomidstateeventtypestatekey) | ||
| - [PUT /_matrix/client/v3/rooms/{roomId}/send/{eventType}/{txnId}](https://spec.matrix.org/v1.3/client-server-api/#put_matrixclientv3roomsroomidsendeventtypetxnid) | ||
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| ## Potential issues | ||
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| 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. For this reason, the unstable implementation | ||
| will continue to provide the old errcode in the body of the error while providing the | ||
| new proposed errcode under its own field. This gives clients a chance to adapt to the | ||
| new errcode / ensure their behaviours with unexpected errcodes are acceptable. | ||
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| ## Alternatives | ||
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There was a problem hiding this comment. Another alternative is making The drawbacks are that this is still a breaking change today, and potentially over-engineering the problem. If we expect errcodes to change infrequently, it might be better to continue to version bump the spec. The other problem is maintaining a potentially long tail of errcodes for older and older generations of the spec. |
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| 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. There is precedence already in the spec for specific | ||
| error codes for specific failures, so there is little need to subclass. | ||
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| ## Security considerations | ||
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| None. | ||
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| ## Unstable prefix | ||
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| While this MSC is not considered stable for implementation, implementations should use `org.matrix.msc3848.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. | ||
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| The errcodes will be have the prefix `M_` replaced with `ORG.MATRIX.MSC3848.`. | ||
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| ## Dependencies | ||
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| None. | ||
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I'm concerned that this MSC is trying to move faster than safety allows for. While implementations can (and should) move faster than the spec, and the spec should not arbitrarily delay process, this is the sort of MSC which needs time to settle and think about the problem at hand.
Specifically, I'm concerned that this MSC hasn't taken into consideration backwards compatibility of the ecosystem: we have a very long tail of old clients which will not have had a chance to upgrade, and may never will. To help move the spec along faster than the eternal tide of old clients, we'd effectively have to consider this a breaking change.
Breaking changes to endpoints are relatively easy, but also a bit messy in implementation: we'd bump the endpoint version and declare that the error codes are only valid on the new endpoint version. Thus, clients opt-in to using the error codes. We would also deprecate the now-old endpoints to force clients to consider using the new endpoints. Eventually, we'd remove the old ones.
The complexity for implementations is that clients would need to version check the server (or possibly rely on 404-like semantics), and servers would need to specifically handle their error codes per endpoint version (it would not be legal to return the new error codes on the old endpoint version).
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The intention isn't to move particularly fast (and doesn't have an urgent project backing it), but while there is spare bandwidth going it felt wise to keep moving on an MSC before it gets forgotten about. That said, happy to have this sit for a bit and have implementations play with it so we can get a wider understanding of whether the MSC works.
I can happily include a passage about requiring v4 (or vNext) endpoints for this errcode to be returned, otherwise servers will continue to respond with
M_FORBIDDEN.Wrt implementations, would it be better for us to continue to have an experimental errcode key returned (like the proposal currently suggests) or use a unstable version prefix like
/_matrix/client/org.matrix.msc3848/room/{rId}/invitewhich would probably match the reality of what clients have to do?There was a problem hiding this comment.
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The MSC would likely claim v4, and if for some reason something else beats it to it then we might consider bundling the two MSCs into one version, or ask that this MSC use v5 or whatever. For now though, it's much simpler to assume v4 will be the thing.
I'd encourage both implementations, I think. Not at the same time mind you, but it can be worth seeing which of the approaches works "better" for clients and what their concerns are. We might also explore alternative error code formats, as suggested in the SCT office room (array of error codes?).
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+1 - I don't think we can, in good conscience, change the error codes on existing endpoints. It's frustrating, because this is very much unspecced behaviour that servers should be within their rights to change, but our continual failure to spec useful error codes means that the existing error codes become part of the de-facto spec.
I'd be more relaxed about replacing
M_UNKNOWN. Anyone attaching semantic meaning to that deserves to keep both pieces when it breaks.So yeah, new endpoint versions here, please, unless you can justify any particular change as being ok.
(OTOH: I very much applaud efforts to make the error codes more useful.)