Element Web makes request to `/_matrix/client/unstable/org.matrix.msc3575/sync` on startup, causing lost to-device-messages

[richvdh]

Introduced by matrix-org/matrix-react-sdk#12307.

Calling /_matrix/client/unstable/org.matrix.msc3575/sync causes the SS proxy to start polling synapse with our access token, which means that it will receive (some of) our to-device messages instead of us, which means we will get decryption errors.

[t3chguy]

Calling /_matrix/client/unstable/org.matrix.msc3575/sync causes the SS proxy to start polling synapse with our access token, which means that it will receive (some of) our to-device messages instead of us

This sounds like a deployment issue. Nothing in MSC3575 or MSC3885 says /_matrix/client/unstable/org.matrix.msc3575/sync will prevent to_device message delivery via normal sync so using the proxy in place of the native endpoint causes this mishap.

[turt2live]

Calling /_matrix/client/unstable/org.matrix.msc3575/sync causes the SS proxy to start polling synapse with our access token, which means that it will receive (some of) our to-device messages instead of us

This sounds like a deployment issue. Nothing in MSC3575 or MSC3885 says /_matrix/client/unstable/org.matrix.msc3575/sync will prevent to_device message delivery via normal sync so using the proxy in place of the native endpoint causes this mishap.

Using POST can also have unintended consequences generally, like high resource utilization while the server calculates an initial response (in this case). If we don't expect things to happen, we shouldn't be using verbs which make things happen.

It just happens that a byproduct of the proxy is that it starts syncing for you, which was requested by the caller using an action-capable verb.