Detect our custom polling with LofarDevice

Closes L2SS-1740

tangostationcontrol/tangostationcontrol/common/events/subscriptions.py

@@ -102,6 +102,25 @@ class EventSubscriptions:
                     f"Callback {callback} for device {device} attribute {attribute_name} threw an exception: {exception_to_str(ex)}"
                 )
 
+    def poll_period(self, proxy: DeviceProxy, attr_name: str) -> int | None:
+        """Return the polling period for the given attribute, in milliseconds,
+        or None if the attribute is not polled at all."""
+
+        # LofarDevice polls a set of attributes not covered by Tango's poll loop
+        try:
+            if is_attribute_polled_by_lofardevice(attr_name):
+                return DEFAULT_METRICS_POLLING_PERIOD_MS
+        except AttributeError:
+            # Not a LofarDevice
+            pass
+
+        # Check if polled by Tango
+        if proxy.is_attribute_polled(attr_name):
+            return proxy.get_attribute_poll_period(attr_name)
+
+        # No polling detected
+        return None
+
     def subscribe_change_event(
         self,
         proxy: DeviceProxy,
@@ -120,6 +139,24 @@ class EventSubscriptions:
         the callback.
         """
 
+        # make sure th attribute is polled often enough
+        current_period = poll_period(proxy, attr_name)
+
+        if current_period is None or current_period > period:
+            if current_period is None:
+                logger.info(f"Enabling polling for {proxy.name()}/{attr_name}.")
+            else:
+                logger.info(
+                    "Adjusting tango controlled polling period from %d to %d",
+                    current_period,
+                    period,
+                )
+
+            # NB: This requires either abs_change or rel_change to be set
+            #     for numerical attributes.
+            proxy.poll_attribute(attr_name, period)
+
+        # add or extend subscription to CHANGE_EVENT
         if sub := self._get_subscription(proxy.name(), attr_name):
             # extend existing subscription
             logger.debug(
@@ -127,17 +164,6 @@ class EventSubscriptions:
             )
             sub.callbacks.append(callback)
 
-            # Check if polled by tango and adjust polling period if necessary
-            if proxy.is_attribute_polled(attr_name):
-                current_period = proxy.get_attribute_poll_period(attr_name)
-                if current_period > period:
-                    logger.info(
-                        "Adjusting tango controlled polling period from %d to %d",
-                        current_period,
-                        period,
-                    )
-                    proxy.poll_attribute(attr_name, period)
-
             # manually trigger first call, mimicking Tango doing so when subscribing
             try:
                 value = proxy.read_attribute(attr_name).value
@@ -157,13 +183,6 @@ class EventSubscriptions:
                 callbacks=[callback],
             )
 
-            # make sure the attribute is polled, otherwise we wont receive event
-            if not proxy.is_attribute_polled_by_lofardevice(attr_name):
-                # LOFARDevice does not poll this attribute, but maybe Tango does
-                if not proxy.is_attribute_polled(attr_name):
-                    logger.info(f"Enabling polling for {proxy.name()}/{attr_name}.")
-                    proxy.poll_attribute(attr_name, period)
-
             # subscribe
             logger.debug(
                 f"Subscribing callback {callback} to CHANGE_EVENT on device {proxy} attribute {attr_name}"