[discovery.mdns] Devices may apply a grace period for removal from the Inbox (#2635)

Signed-off-by: Andrew Fiddian-Green <software@whitebear.ch>

bundles/org.openhab.core.config.discovery.mdns/src/main/java/org/openhab/core/config/discovery/mdns/MDNSDiscoveryParticipant.java

@@ -66,4 +66,18 @@ public interface MDNSDiscoveryParticipant {
      */
     @Nullable
     ThingUID getThingUID(ServiceInfo service);
+
+    /**
+     * Some openHAB bindings handle devices that can sometimes be a bit late in updating their mDNS announcements, which
+     * means that such devices are repeatedly removed from, and (re)added to, the Inbox.
+     *
+     * To prevent this, a binding that implements an MDNSDiscoveryParticipant may OPTIONALLY implement this method to
+     * specify an additional delay period (grace period) to wait before the device is removed from the Inbox.
+     *
+     * @param serviceInfo the mDNS ServiceInfo describing the device on the network.
+     * @return the additional grace period delay in seconds before the device will be removed from the Inbox.
+     */
+    default long getRemovalGracePeriodSeconds(ServiceInfo serviceInfo) {
+        return 0;
+    }
 }

bundles/org.openhab.core.config.discovery.mdns/src/main/java/org/openhab/core/config/discovery/mdns/internal/MDNSDiscoveryService.java

@@ -16,7 +16,9 @@ import java.time.Duration;
 import java.util.HashSet;
 import java.util.Map;
 import java.util.Set;
+import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.CopyOnWriteArraySet;
+import java.util.concurrent.ScheduledFuture;
 import java.util.concurrent.TimeUnit;
 
 import javax.jmdns.ServiceEvent;
@@ -64,6 +66,11 @@ public class MDNSDiscoveryService extends AbstractDiscoveryService implements Se
 
     private final MDNSClient mdnsClient;
 
+    /**
+     * Map of scheduled tasks to remove devices from the Inbox.
+     */
+    private Map<String, ScheduledFuture<?>> deviceRemovalTasks = new ConcurrentHashMap<>();
+
     @Activate
     public MDNSDiscoveryService(final @Nullable Map<String, Object> configProperties,
             final @Reference MDNSClient mdnsClient, final @Reference TranslationProvider i18nProvider,
@@ -200,7 +207,14 @@ public class MDNSDiscoveryService extends AbstractDiscoveryService implements Se
                 try {
                     ThingUID thingUID = participant.getThingUID(serviceEvent.getInfo());
                     if (thingUID != null) {
-                        thingRemoved(thingUID);
+                        ServiceInfo serviceInfo = serviceEvent.getInfo();
+                        long gracePeriod = participant.getRemovalGracePeriodSeconds(serviceInfo);
+                        if (gracePeriod <= 0) {
+                            thingRemoved(thingUID);
+                        } else {
+                            cancelRemovalTask(serviceInfo);
+                            scheduleRemovalTask(thingUID, serviceInfo, gracePeriod);
+                        }
                     }
                 } catch (Exception e) {
                     logger.error("Participant '{}' threw an exception", participant.getClass().getName(), e);
@@ -221,6 +235,7 @@ public class MDNSDiscoveryService extends AbstractDiscoveryService implements Se
                     try {
                         DiscoveryResult result = participant.createResult(serviceEvent.getInfo());
                         if (result != null) {
+                            cancelRemovalTask(serviceEvent.getInfo());
                             final DiscoveryResult resultNew = getLocalizedDiscoveryResult(result,
                                     FrameworkUtil.getBundle(participant.getClass()));
                             thingDiscovered(resultNew);
@@ -232,4 +247,30 @@ public class MDNSDiscoveryService extends AbstractDiscoveryService implements Se
             }
         }
     }
+
+    /**
+     * If the device has been scheduled to be removed, cancel its respective removal task.
+     *
+     * @param serviceInfo the mDNS ServiceInfo describing the device on the network.
+     */
+    private void cancelRemovalTask(ServiceInfo serviceInfo) {
+        ScheduledFuture<?> deviceRemovalTask = deviceRemovalTasks.remove(serviceInfo.getQualifiedName());
+        if (deviceRemovalTask != null) {
+            deviceRemovalTask.cancel(false);
+        }
+    }
+
+    /**
+     * Schedule a task that will remove the device from the Inbox after the given grace period has expired.
+     *
+     * @param thingUID the UID of the Thing to be removed.
+     * @param serviceInfo the mDNS ServiceInfo describing the device on the network.
+     * @param gracePeriod the scheduled delay in seconds.
+     */
+    private void scheduleRemovalTask(ThingUID thingUID, ServiceInfo serviceInfo, long gracePeriod) {
+        deviceRemovalTasks.put(serviceInfo.getQualifiedName(), scheduler.schedule(() -> {
+            thingRemoved(thingUID);
+            cancelRemovalTask(serviceInfo);
+        }, gracePeriod, TimeUnit.SECONDS));
+    }
 }