[Darwin] MTRDevice should trigger resubscription on connectivity changes

src/darwin/Framework/CHIP/MTRDevice.mm

@@ -28,6 +28,7 @@
 #import "MTRCommandTimedCheck.h"
 #import "MTRConversion.h"
 #import "MTRDefines_Internal.h"
+#import "MTRDeviceConnectivityMonitor.h"
 #import "MTRDeviceControllerOverXPC.h"
 #import "MTRDeviceController_Internal.h"
 #import "MTRDevice_Internal.h"
@@ -355,6 +356,7 @@ @implementation MTRDevice {
     // _setupSubscription or via the auto-resubscribe behavior of the
     // ReadClient).  Nil if we have had no such failures.
     NSDate * _Nullable _lastSubscriptionFailureTime;
+    MTRDeviceConnectivityMonitor * _connectivityMonitor;
 }
 
 - (instancetype)initWithNodeID:(NSNumber *)nodeID controller:(MTRDeviceController *)controller
@@ -669,6 +671,8 @@ - (void)invalidate
     // subscription.  In that case, _internalDeviceState will update when the
     // subscription is actually terminated.
 
+    [self _stopConnectivityMonitoring];
+
     os_unfair_lock_unlock(&self->_lock);
 }
 
@@ -861,6 +865,9 @@ - (void)_handleSubscriptionEstablished
 
     [self _changeState:MTRDeviceStateReachable];
 
+    // No need to monitor connectivity after subscription establishment
+    [self _stopConnectivityMonitoring];
+
     os_unfair_lock_unlock(&self->_lock);
 
     os_unfair_lock_lock(&self->_timeSyncLock);
@@ -894,6 +901,9 @@ - (void)_handleResubscriptionNeeded
     // former case we recently had a subscription and do not want to be forcing
     // retries immediately.
     _lastSubscriptionFailureTime = [NSDate now];
+
+    // Set up connectivity monitoring in case network routability changes for the positive, to accellerate resubscription
+    [self _setupConnectivityMonitoring];
 }
 
 - (void)_handleSubscriptionReset:(NSNumber * _Nullable)retryDelay
@@ -1241,6 +1251,44 @@ - (void)_createDataVersionFilterListFromDictionary:(NSDictionary<MTRClusterPath
     *count = maxDataVersionFilterSize;
 }
 
+- (void)_setupConnectivityMonitoring
+{
+    // Dispatch to own queue first to avoid deadlock with syncGetCompressedFabricID
+    dispatch_async(self.queue, ^{
+        // Get the required info before setting up the connectivity monitor
+        NSNumber * compressedFabricID = [self->_deviceController syncGetCompressedFabricID];
+        if (!compressedFabricID) {
+            MTR_LOG_INFO("%@ could not get compressed fabricID", self);
+            return;
+        }
+
+        // Now lock for _connectivityMonitor
+        std::lock_guard lock(self->_lock);
+        if (self->_connectivityMonitor) {
+            // already monitoring
+            return;
+        }
+
+        self->_connectivityMonitor = [[MTRDeviceConnectivityMonitor alloc] initWithCompressedFabricID:compressedFabricID nodeID:self.nodeID];
+        [self->_connectivityMonitor startMonitoringWithHandler:^{
+            [self->_deviceController asyncDispatchToMatterQueue:^{
+                [self _triggerResubscribeWithReason:"read-through skipped while not subscribed" nodeLikelyReachable:YES];
+            }
+                                                   errorHandler:nil];
+        } queue:self.queue];
+    });
+}
+
+- (void)_stopConnectivityMonitoring
+{
+    os_unfair_lock_assert_owner(&_lock);
+
+    if (_connectivityMonitor) {
+        [_connectivityMonitor stopMonitoring];
+        _connectivityMonitor = nil;
+    }
+}
+
 // assume lock is held
 - (void)_setupSubscription
 {
@@ -1462,6 +1510,9 @@ - (void)_setupSubscription
                    callback->AdoptClusterStateCache(std::move(clusterStateCache));
                    callback.release();
                }];
+
+    // Set up connectivity monitoring in case network becomes routable after any part of the subscription process goes into backoff retries.
+    [self _setupConnectivityMonitoring];
 }
 
 #ifdef DEBUG

src/darwin/Framework/CHIP/MTRDeviceConnectivityMonitor.h

@@ -0,0 +1,43 @@
+/**
+ *    Copyright (c) 2023 Project CHIP Authors
+ *
+ *    Licensed under the Apache License, Version 2.0 (the "License");
+ *    you may not use this file except in compliance with the License.
+ *    You may obtain a copy of the License at
+ *
+ *        http://www.apache.org/licenses/LICENSE-2.0
+ *
+ *    Unless required by applicable law or agreed to in writing, software
+ *    distributed under the License is distributed on an "AS IS" BASIS,
+ *    WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ *    See the License for the specific language governing permissions and
+ *    limitations under the License.
+ */
+
+#import <Foundation/Foundation.h>
+
+#import "MTRDefines_Internal.h"
+
+NS_ASSUME_NONNULL_BEGIN
+
+typedef void (^MTRDeviceConnectivityMonitorHandler)(void);
+
+/**
+ * Class that a matter dns-sd instance name, and monitors connectivity to the device.
+ */
+MTR_TESTABLE
+@interface MTRDeviceConnectivityMonitor : NSObject
+- (instancetype)initWithCompressedFabricID:(NSNumber *)compressedFabricID nodeID:(NSNumber *)nodeID;
+
+/**
+ * Any time a path becomes satisfied or route becomes viable, the registered handler will be called.
+ */
+- (void)startMonitoringWithHandler:(MTRDeviceConnectivityMonitorHandler)handler queue:(dispatch_queue_t)queue;
+
+/**
+ * Stops the monitoring. After this method returns no more calls to the handler will be made.
+ */
+- (void)stopMonitoring;
+@end
+
+NS_ASSUME_NONNULL_END