RespUserDevices intermediate decoding (#342)

This should fix cases where Synapse users can upload absolute nonsense
into their device keys and then that causes the entire devices response
to fail to unmarshal.

This has happened both in the `"devices"` and `"stream_id"` keys that I
can see.

federationtypes.go

@@ -227,6 +227,34 @@ type RespUserDevices struct {
 	SelfSigningKey *CrossSigningKey `json:"self_signing_key"`
 }
 
+// UnmarshalJSON is used here because people on Synapses can apparently upload
+// nonsense into their device keys in types that don't match the expected and
+// that can cause the entire response to fail to unmarshal. This simply skips
+// anything that fails to unmarshal and returns the rest.
+func (r *RespUserDevices) UnmarshalJSON(data []byte) error {
+	intermediate := struct {
+		UserID         json.RawMessage   `json:"user_id"`
+		StreamID       json.RawMessage   `json:"stream_id"`
+		Devices        []json.RawMessage `json:"devices"`
+		MasterKey      json.RawMessage   `json:"master_key"`
+		SelfSigningKey json.RawMessage   `json:"self_signing_key"`
+	}{}
+	if err := json.Unmarshal(data, &intermediate); err != nil {
+		return err
+	}
+	_ = json.Unmarshal(intermediate.UserID, &r.UserID)
+	_ = json.Unmarshal(intermediate.StreamID, &r.StreamID)
+	_ = json.Unmarshal(intermediate.MasterKey, &r.MasterKey)
+	_ = json.Unmarshal(intermediate.SelfSigningKey, &r.SelfSigningKey)
+	for _, deviceJSON := range intermediate.Devices {
+		var device RespUserDevice
+		if err := json.Unmarshal(deviceJSON, &device); err == nil {
+			r.Devices = append(r.Devices, device)
+		}
+	}
+	return nil
+}
+
 // RespUserDevice are embedded in RespUserDevices
 // https://matrix.org/docs/spec/server_server/latest#get-matrix-federation-v1-user-devices-userid
 type RespUserDevice struct {