Consider adding support for system notices

[eloquence]

It would be good to consider mechanisms by which we can display system notices to the client, for example:

• A database integrity issue was detected. Please contact your admin.
• Your SecureDrop is running v3 onion services. An upgrade to v3000 is required.

Issues to consider:

• There may be more than one such notice at a time.
• Notices will need to eventually disappear when some condition is met.

A simple mechanism might be a feed that is downloaded as part of a sync -- if the notification is present in the feed, it will continue to be shown; if it is not, it will be deleted.

Any such mechanism is likely to require changes to the API.

[zenmonkeykstop]

Also relevant to potential system integrity checks.

There's some outstanding scope questions:

• do we want to push notices from server to client?
• do we want to push notices from FPF to clients?
• do we use actual system notifications or roll-your-own in the client or updater?

[eloquence]

Milestoning to alpha for rewrite, but needs a fair bit of scoping -- we'll need to decide one way or another what kind of messages we expect to display in sd-app and how to manage them.

[jskinne3]

This will require the Server API to send notices via Sync

[cfm]

The v2 Journalist API can be extended for this in a backwards-compatible way, by defining a new API_MINOR_VERSION that corresponds to adding:¹

@dataclass
class Index:
    # ...
    # Non-source metadata (always returned):
    # ...
    notices: dict[NoticeUUID, Version] = field(default_factory=dict)

@dataclass
class BatchResponse:
    # ...
    # Non-source metadata:
    # ...
    notices: dict[NoticeUUID, Record | None] = field(default_factory=dict)

We'll need to decide what a notice actually is, first in the server's database and then in the JSON representation that gets saved by the client. But otherwise we get the v2 API's synchronization for free.

Footnotes

1. https://github.com/freedomofpress/securedrop/blob/beaf725eef760774bc9f87009467b8f23bf68628/securedrop/journalist_app/api2/types.py#L57-L64
   https://github.com/freedomofpress/securedrop/blob/beaf725eef760774bc9f87009467b8f23bf68628/securedrop/journalist_app/api2/types.py#L267-L278 ↩