Improve task cancellation management in SubscriptionManager

The widespread usage of the SubscriptionManager caused new crashes to
occur when swapping apps.

This was caused due to an access to Ndb memory after Ndb has been closed
from the app background signal.

The issue was fixed with improved task management logic and ensuring all
subscription tasks are finished before closing Ndb.

Signed-off-by: Daniel D’Aquino <daniel@daquino.me>

damus/ContentView.swift

@@ -512,6 +512,7 @@ struct ContentView: View {
             case .background:
                 print("txn: 📙 DAMUS BACKGROUNDED")
                 Task { @MainActor in
+                    await damus_state.nostrNetwork.close()  // Close ndb streaming tasks before closing ndb to avoid memory errors
                     damus_state.ndb.close()
                 }
                 break

damus/Core/Networking/NostrNetworkManager/NostrNetworkManager.swift

@@ -234,7 +234,8 @@ class NostrNetworkManager {
     
     // MARK: - App lifecycle functions
     
-    func close() {
+    func close() async {
+        await self.reader.cancelAllTasks()
         pool.close()
     }
 }

damus/Core/Networking/NostrNetworkManager/SubscriptionManager.swift

@@ -4,6 +4,7 @@
 //
 //  Created by Daniel D’Aquino on 2025-03-25.
 //
+import Foundation
 
 extension NostrNetworkManager {
     /// Reads or fetches information from RelayPool and NostrDB, and provides an easier and unified higher-level interface.
@@ -14,10 +15,12 @@ extension NostrNetworkManager {
     class SubscriptionManager {
         private let pool: RelayPool
         private var ndb: Ndb
+        private var taskManager: TaskManager
         
         init(pool: RelayPool, ndb: Ndb) {
             self.pool = pool
             self.ndb = ndb
+            self.taskManager = TaskManager()
         }
         
         // MARK: - Reading data from Nostr
@@ -35,6 +38,7 @@ extension NostrNetworkManager {
                 let ndbStreamTask = Task {
                     do {
                         for await item in try self.ndb.subscribe(filters: try filters.map({ try NdbFilter(from: $0) })) {
+                            try Task.checkCancellation()
                             switch item {
                             case .eose:
                                 continuation.yield(.eose)
@@ -48,24 +52,71 @@ extension NostrNetworkManager {
                                     }
                                     lend(unownedNote)
                                 }
+                                try Task.checkCancellation()
                                 continuation.yield(.event(borrow: lender))
                             }
                         }
                     }
                     catch {
-                        Log.error("NDB streaming error: %s", for: .ndb, error.localizedDescription)
+                        Log.error("NDB streaming error: %s", for: .subscription_manager, error.localizedDescription)
                     }
+                    continuation.finish()
                 }
                 let streamTask = Task {
-                    for await _ in self.pool.subscribe(filters: filters, to: desiredRelays) {
-                        // NO-OP. Notes will be automatically ingested by NostrDB
-                        // TODO: Improve efficiency of subscriptions?
+                    do {
+                        for await _ in self.pool.subscribe(filters: filters, to: desiredRelays) {
+                            // NO-OP. Notes will be automatically ingested by NostrDB
+                            // TODO: Improve efficiency of subscriptions?
+                            try Task.checkCancellation()
+                        }
+                    }
+                    catch {
+                        Log.error("Network streaming error: %s", for: .subscription_manager, error.localizedDescription)
+                    }
+                    continuation.finish()
+                }
+                
+                Task {
+                    let ndbStreamTaskId = await self.taskManager.add(task: ndbStreamTask)
+                    let streamTaskId = await self.taskManager.add(task: streamTask)
+                    
+                    continuation.onTermination = { @Sendable _ in
+                        Task {
+                            await self.taskManager.cancelAndCleanUp(taskId: ndbStreamTaskId)
+                            await self.taskManager.cancelAndCleanUp(taskId: streamTaskId)
+                        }
                     }
                 }
-                continuation.onTermination = { @Sendable _ in
-                    streamTask.cancel() // Close the RelayPool stream when caller stops streaming
-                    ndbStreamTask.cancel()
+            }
+        }
+        
+        func cancelAllTasks() async {
+            await self.taskManager.cancelAllTasks()
+        }
+        
+        actor TaskManager {
+            private var tasks: [UUID: Task<Void, Never>] = [:]
+            
+            func add(task: Task<Void, Never>) -> UUID {
+                let taskId = UUID()
+                self.tasks[taskId] = task
+                return taskId
+            }
+            
+            func cancelAndCleanUp(taskId: UUID) async {
+                self.tasks[taskId]?.cancel()
+                await self.tasks[taskId]?.value
+                self.tasks[taskId] = nil
+                return
+            }
+            
+            func cancelAllTasks() async {
+                Log.info("Cancelling all SubscriptionManager tasks", for: .subscription_manager)
+                for (taskId, _) in self.tasks {
+                    Log.info("Cancelling SubscriptionManager task %s", for: .subscription_manager, taskId.uuidString)
+                    await cancelAndCleanUp(taskId: taskId)
                 }
+                Log.info("Cancelled all SubscriptionManager tasks", for: .subscription_manager)
             }
         }
     }

damus/Core/Storage/DamusState.swift

@@ -164,8 +164,10 @@ class DamusState: HeadlessDamusState {
             try await self.push_notification_client.revoke_token()
         }
         wallet.disconnect()
-        nostrNetwork.close()
-        ndb.close()
+        Task {
+            await nostrNetwork.close()  // Close ndb streaming tasks before closing ndb to avoid memory errors
+            ndb.close()
+        }
     }
 
     static var empty: DamusState {

damus/Shared/Utilities/Log.swift

@@ -14,6 +14,7 @@ enum LogCategory: String {
     case render
     case storage
     case networking
+    case subscription_manager
     case timeline
     /// Logs related to Nostr Wallet Connect components
     case nwc