Subscriptions: cyclic data-change and alarm notifications

[ale-rinaldi]

Subscriptions: cyclic data-change (and alarm) notifications

The PLC can push value updates instead of being polled. python-snap7 has a create_subscription() stub (s7/_s7commplus_client.py) but no working notification receive/parse or credit management. Implement the full subscription flow.

Goal

• create_subscription(items, cycle_ms) → create a Subscription object on the PLC and register the variables.
• receive_notification(timeout) → block for the next pushed update and return the changed values.
• Credit management so the flow doesn't stall.

Approach (mirror C# Notification + AlarmsHandler)

1. Create the subscription: CreateObject of a Subscription (Ids.ClassSubscription = 1001) with SubscriptionReferenceList (1048) = the item address array, SubscriptionCycleTime (1049), SubscriptionCreditLimit (1053). (For alarms the ref list is the alarm subsystem address 0x80010000.)
2. Receive notifications: the PLC sends unsolicited Notification PDUs (Core/Notification.cs::DeserializeFromPdu) carrying NotificationCreditTick, SequenceNumber and the changed P-values. These arrive outside the request/response cycle — the connection needs an unsolicited-PDU path (a receive queue / background reader) that coexists with send_request and the TLS-in-COTP tunnel. Note SystemEvent PDUs use ProtocolVersion 0xfe (header-only) and can report fatal protocol errors.
3. Credit: call SubscriptionSetCreditLimit() shortly before the credit tick expires to keep a constant data flow.

C# reference (thomas-v2/S7CommPlusDriver, LGPL-3.0)

• Core/Notification.cs — notification PDU parse
• Alarming/AlarmsHandler.cs — subscription create (SubscriptionReferenceList, credit), the notification loop, SubscriptionSetCreditLimit
• Core/SystemEvent.cs — the 0xfe path

Already in place

• Transport: corrected V2 wire format, TLS-in-COTP, fragment reassembly (fix(s7commplus): correct V2 symbolic reads on a real S7-1500 (wire format + async/server TLS-in-COTP) gijzelaerr/python-snap7#745/fix(s7commplus): restrict TLS groups in-code so the handshake works on OpenSSL >= 3.5 gijzelaerr/python-snap7#746/feat(s7commplus): enumerate datablocks via EXPLORE (request fix + fragment reassembly) gijzelaerr/python-snap7#749).
• IDs: CLASS_SUBSCRIPTION, SUBSCRIPTION_REFERENCE_LIST, SUBSCRIPTION_CYCLE_TIME, SUBSCRIPTION_CREDIT_LIMIT, SUBSCRIPTION_ACTIVE in protocol.py; create_subscription() stub.

Caveat

The biggest design point is the unsolicited-receive path: today _recv_s7_data/send_request assume strict request→response. Notifications break that, so a small receive loop/queue (thread or selector) feeding decrypted PDUs is needed, careful with the MemoryBIO TLS tunnel.

Validation

Subscribe to a couple of fast-changing tags on a live S7-1500 (e.g. a fast-changing analog value) and confirm periodic pushed updates match direct reads.