Add a search backfill app background processing task

To run when the device is idle, charging and connected to the network.

ElementX/Sources/Application/AppCoordinator.swift

@@ -170,6 +170,7 @@ class AppCoordinator: AppCoordinatorProtocol, AuthenticationFlowCoordinatorDeleg
         observeAppLockChanges()
         
         registerBackgroundAppRefresh()
+        registerSearchBackfill()
         
         appSettings.analyticsConsentStatePublisher
             .dropFirst() // Sentry is configured during init; only reconfigure when consent state actually changes
@@ -1189,6 +1190,7 @@ class AppCoordinator: AppCoordinatorProtocol, AuthenticationFlowCoordinatorDeleg
         
         scheduleDelayedPauseServices()
         scheduleBackgroundAppRefresh()
+        scheduleSearchBackfill()
     }
     
     @objc
@@ -1325,3 +1327,63 @@ class AppCoordinator: AppCoordinatorProtocol, AuthenticationFlowCoordinatorDeleg
             }
     }
 }
+
+// MARK: Search backfill
+
+private extension AppCoordinator {
+    func registerSearchBackfill() {
+        let result = BGTaskScheduler.shared.register(forTaskWithIdentifier: appSettings.searchBackfillTaskIdentifier, using: .main) { [weak self] task in
+            guard let task = task as? BGProcessingTask else {
+                MXLog.error("Invalid search backfill configuration")
+                return
+            }
+            
+            Task {
+                await self?.handleSearchBackfill(task)
+            }
+        }
+        
+        MXLog.info("Register search backfill with result: \(result)")
+    }
+    
+    func scheduleSearchBackfill() {
+        guard let userSession, userSession.userSettings.globalSearchEnabled, #available(iOS 26.0, *) else {
+            return
+        }
+        
+        let request = BGProcessingTaskRequest(identifier: appSettings.searchBackfillTaskIdentifier)
+        request.requiresExternalPower = true
+        request.requiresNetworkConnectivity = true
+        
+        do {
+            try BGTaskScheduler.shared.submit(request)
+            MXLog.info("Successfully scheduled search backfill task")
+        } catch {
+            MXLog.error("Failed scheduling search backfill with error: \(error)")
+        }
+    }
+    
+    func handleSearchBackfill(_ task: BGProcessingTask) async {
+        MXLog.info("Started search backfill task")
+        
+        guard let clientProxy = userSession?.clientProxy else {
+            task.setTaskCompleted(success: false)
+            return
+        }
+        
+        task.expirationHandler = { @Sendable [weak self] in
+            MXLog.info("Search backfill task is about to expire.")
+            Task { @MainActor in self?.userSession?.clientProxy.stopSearchBackfill() }
+        }
+        
+        clientProxy.startSearchBackfill(strategy: .background)
+        
+        // TaskHandle can't be awaited, so poll until the sweep finishes or is stopped on expiry.
+        while clientProxy.isSearchBackfillRunning {
+            try? await Task.sleep(for: .seconds(1))
+        }
+        
+        MXLog.info("Marking search backfill task as complete.")
+        task.setTaskCompleted(success: true)
+    }
+}

ElementX/Sources/Application/Settings/AppSettings.swift

@@ -151,6 +151,8 @@ final nonisolated class AppSettings: @unchecked Sendable {
     
     /// The task identifier used for background app refresh. Also used in main target's the Info.plist
     let backgroundAppRefreshTaskIdentifier = "io.element.elementx.background.refresh"
+    /// The task identifier used for the search index backfill. Also used in main target's the Info.plist
+    let searchBackfillTaskIdentifier = "io.element.elementx.background.search-backfill"
     
     /// A URL where users can go read more about the app.
     private(set) var websiteURL: URL = "https://element.io"

ElementX/SupportingFiles/Info.plist

@@ -5,6 +5,7 @@
 	<key>BGTaskSchedulerPermittedIdentifiers</key>
 	<array>
 		<string>io.element.elementx.background.refresh</string>
+		<string>io.element.elementx.background.search-backfill</string>
 	</array>
 	<key>CFBundleDevelopmentRegion</key>
 	<string>$(DEVELOPMENT_LANGUAGE)</string>

ElementX/SupportingFiles/target.yml

@@ -100,7 +100,8 @@ targets:
           location
         ]
         BGTaskSchedulerPermittedIdentifiers: [
-          io.element.elementx.background.refresh
+          io.element.elementx.background.refresh,
+          io.element.elementx.background.search-backfill
         ]
         UTExportedTypeDeclarations:
         - UTTypeConformsTo: [public.text]