feat(SystemStatus.SubwayCache): save status

[thecristen]

Just noticed this while I was inspecting memory usage on the homepage. The homepage can call subway_status() and trigger a new computation of Subway Status. This PR enhances that by not only sending the newly-fetched status to the homepage, but also saving it into the ETS table for later retrieval!

[joshlarson]

I don't think this will do anything. The ETS table is defined as protected, which means that processes other than the SubwayCache GenServer won't be able to write to it. When I tested this out by having init/1 not write to the :subway_status table, I just got an error message:

2026-07-20 11:01:50.937 [error] mbta_id=43634282 request_id=GMQHmiH5yG7feH0AAAZi state=set duration=14.232 status=500 params={} path=/ method=GET headers={}
2026-07-20 11:01:50.941 [error] mbta_id=43634282 request_id=GMQHmiH5yG7feH0AAAZi ** (ArgumentError) errors were found at the given arguments:

  * 1st argument: the table identifier refers to an ETS table with insufficient access rights

    (stdlib 7.3) :ets.insert(:subway_status, {"status", %{"Blue" => [%{branch_ids: [], status_entries: [%{status: :normal, time: :current, alerts: [], multiple: false}]}], "Green" => [%{branch_ids: [], status_entries: [%{status: :delay, time: :current, alerts: [%Alerts.Alert{id: "1023051", active_period: [{#DateTime<2026-07-20 10:40:00-04:00 EDT America/New_York>, #DateTime<2026-07-20 13:01:31-04:00 EDT America/New_Yor
k>}], banner: nil, cause: :unknown_cause, created_at: #DateTime<2026-07-20 10:40:54-04:00 EDT America/New_York>, description: nil, effect: :delay, header: "Green Line: Delays of about 15 minutes due to an earlier disabled train at Park Street. Service is proceeding.", image: nil, image_alternative_text: nil, informed_entity: %Alerts.InformedEntitySet{activities: MapSet.new([:exit, :ride, :board]), direction_id: MapSet.
new([nil]), entities: [%Alerts.InformedEntity{activities: MapSet.new([:exit, :ride, :board]), direction_id: nil, facility: nil, route: "Green-E", route_type: 0, stop: nil, trip: nil}, %Alerts.InformedEntity{activities: MapSet.new([:exit, :ride, :board]), direction_id: nil, facility: nil, route: "Green", route_type: 0, stop: nil, trip: nil}, %Alerts.InformedEntity{activities: MapSet.new([:exit, :ride, :board]), directio
n_id: nil, facility: nil, route: "Green-B", route_type: 0, stop: nil, trip: nil}, %Alerts.InformedEntity{activities: MapSet.new([:exit, :ride, :board]), direction_id: nil, facility: nil, route: "Green-C", route_type: 0, stop: nil, trip: nil}, %Alerts.InformedEntity{activities: MapSet.new([:exit, :ride, :board]), direction_id: nil, facility: nil, route: "Green-D", route_type: 0, stop: nil, trip: nil}], facility: MapSet.
new([nil]), route: MapSet.new(["Green", ...]), ...}, ...}], ...}]}, ...], ...}})
    (dotcom 0.0.1) lib/dotcom/system_status/subway_cache.ex:31: Dotcom.SystemStatus.SubwayCache.subway_status/0
    (dotcom 0.0.1) lib/dotcom_web/controllers/page_controller.ex:142: DotcomWeb.PageController.subway_status/2
    (dotcom 0.0.1) lib/dotcom_web/controllers/page_controller.ex:1: DotcomWeb.PageController.phoenix_controller_pipeline/2
    (phoenix 1.8.6) lib/phoenix/router.ex:416: Phoenix.Router.__call__/5
    (dotcom 0.0.1) deps/plug/lib/plug/error_handler.ex:80: DotcomWeb.Router.call/2
    (dotcom 0.0.1) lib/dotcom_web/endpoint.ex:1: DotcomWeb.Endpoint.plug_builder_call/2
    (dotcom 0.0.1) deps/plug/lib/plug/debugger.ex:155: DotcomWeb.Endpoint."call (overridable 3)"/2
    (dotcom 0.0.1) lib/dotcom_web/endpoint.ex:1: DotcomWeb.Endpoint."call (overridable 4)"/2
    (dotcom 0.0.1) lib/dotcom_web/endpoint.ex:1: DotcomWeb.Endpoint.call/2
    (phoenix 1.8.6) lib/phoenix/endpoint/sync_code_reload_plug.ex:22: Phoenix.Endpoint.SyncCodeReloadPlug.do_call/4
    (bandit 1.10.4) lib/bandit/pipeline.ex:131: Bandit.Pipeline.call_plug!/2
    (bandit 1.10.4) lib/bandit/pipeline.ex:42: Bandit.Pipeline.run/5
    (bandit 1.10.4) lib/bandit/http1/handler.ex:13: Bandit.HTTP1.Handler.handle_data/3
    (bandit 1.10.4) lib/bandit/delegating_handler.ex:18: Bandit.DelegatingHandler.handle_data/3
    (bandit 1.10.4) lib/bandit/delegating_handler.ex:8: Bandit.DelegatingHandler.handle_info/2
    (stdlib 7.3) gen_server.erl:2434: :gen_server.try_handle_info/3
    (stdlib 7.3) gen_server.erl:2420: :gen_server.handle_msg/3
    (stdlib 7.3) proc_lib.erl:333: :proc_lib.init_p_do_apply/3

That said... this use case is exceedingly rare - the :subway_status ETS table is only empty for the 1 second or so on app startup, and I'm not entirely sure that the DotcomWeb process will even start serving traffic before SubwayCache.init/1 has returned anyway, which means that this might be infinitely rare!

[thecristen]

Thanks for the sanity check!