feat(events): add flushAndWait on Apple (tier 2)

LaunchDarkly/GeneratedCode/mocks.generated.swift

@@ -221,6 +221,15 @@ final class EventReportingMock: EventReporting {
         flushReceivedCompletion = completion
         try! flushCallback?()
     }
+
+    var flushReportingOutcomeCallCount = 0
+    var flushReportingOutcomeCallback: (() throws -> Void)?
+    var flushReportingOutcomeReceivedCompletion: (FlushOutcomeClosure)?
+    func flushReportingOutcome(completion: @escaping FlushOutcomeClosure) {
+        flushReportingOutcomeCallCount += 1
+        flushReportingOutcomeReceivedCompletion = completion
+        try! flushReportingOutcomeCallback?()
+    }
 }
 
 // MARK: - FeatureFlagCachingMock
@@ -437,7 +446,7 @@ final class ThrottlingMock: Throttling {
 
     var runThrottledCallCount = 0
     var runThrottledCallback: (() throws -> Void)?
-    var runThrottledReceivedRunClosure: RunClosure?
+    var runThrottledReceivedRunClosure: (RunClosure)?
     func runThrottled(_ runClosure: @escaping RunClosure) {
         runThrottledCallCount += 1
         runThrottledReceivedRunClosure = runClosure

LaunchDarkly/LaunchDarkly/LDClient.swift

@@ -40,11 +40,11 @@ public class LDClient {
 
     // MARK: - State Controls and Indicators
 
-    private static var instances: [String: LDClient]?
-    private static let instancesQueue = DispatchQueue(label: "com.launchdarkly.LDClient.instancesQueue")
+    static var instances: [String: LDClient]?
+    static let instancesQueue = DispatchQueue(label: "com.launchdarkly.LDClient.instancesQueue")
 
     // If the SDK is provided a timeout value that exceeds this value, a warning will be logged.
-    private static let longTimeoutInterval: TimeInterval = 15
+    static let longTimeoutInterval: TimeInterval = 15
 
     /**
      Reports the online/offline state of the LDClient.
@@ -263,6 +263,23 @@ public class LDClient {
 
     @objc private func didEnterBackground() {
         os_log("%s", log: config.logger, type: .debug, typeName(and: #function))
+        // A backgrounded process is suspended once idle, so deliver inside an activity assertion. Only where suspension
+        // is real: macOS posts this notification whenever the application loses focus, and never suspends it for that.
+        #if os(iOS) || os(tvOS)
+        backgroundDelivery.run { [weak self] finished in
+            guard let self = self
+            else {
+                finished()
+                return
+            }
+            self.eventReporter.flushReportingOutcome { delivered in
+                if !delivered {
+                    os_log("%s events did not reach LaunchDarkly before suspension", log: self.config.logger, type: .debug, self.typeName(and: #function))
+                }
+                finished()
+            }
+        }
+        #endif
         Thread.performOnMain {
             runMode = .background
         }
@@ -275,6 +292,9 @@ public class LDClient {
         }
     }
 
+    /// One assertion per client, however many times the application is backgrounded while a delivery is running.
+    private let backgroundDelivery: BackgroundActivity
+
     let config: LDConfig
     /// Identifies this client's environment to a hook, without handing it the mobile key that identifies the
     /// environment to LaunchDarkly. Hashed once here rather than per evaluation, because a deduping hook asks for it on
@@ -786,23 +806,6 @@ public class LDClient {
         plugin.register(client: self, metadata: environmentMetadata)
     }
 
-    /**
-     Tells the SDK to immediately send any currently queued events to LaunchDarkly.
-
-     There should not normally be a need to call this function. While online, the LDClient automatically reports events
-     on an interval defined by `LDConfig.eventFlushInterval`. Note that this function does not block until events are
-     sent, it only triggers a background task to send events immediately.
-     */
-    public func flush() {
-        LDClient.instancesQueue.sync(flags: .barrier) {
-            LDClient.instances?.forEach { $1.internalFlush() }
-        }
-    }
-
-    private func internalFlush() {
-        eventReporter.flush(completion: nil)
-    }
-
     /// Static, and over a logger rather than a client, so that the event reporter can be given it while the client
     /// that owns the reporter is still being built.
     private static func logEventSyncComplete(_ result: SynchronizingError?, _ logger: OSLog) {
@@ -1027,6 +1030,7 @@ public class LDClient {
         throttler = self.serviceFactory.makeThrottler(environmentReporter: environmentReporter)
 
         config = configuration
+        backgroundDelivery = BackgroundActivity(reason: "LaunchDarkly event delivery", logger: configuration.logger)
         context = startContext ?? LDContext()
 
         if config.autoEnvAttributes {
@@ -1089,6 +1093,16 @@ public class LDClient {
 
 extension LDClient: TypeIdentifying { }
 
+private extension LDClient {
+    func onEventSyncComplete(result: SynchronizingError?) {
+        if let synchronizingError = result {
+            os_log("%s result: %s", log: config.logger, type: .debug, typeName(and: #function), String(describing: synchronizingError))
+        } else {
+            os_log("%s result: success", log: config.logger, type: .debug, typeName(and: #function))
+        }
+    }
+}
+
 #if DEBUG
 extension LDClient {
     func setRunMode(_ runMode: LDClientRunMode) {

LaunchDarkly/LaunchDarkly/LDClientFlush.swift

@@ -0,0 +1,156 @@
+import Foundation
+import OSLog
+
+// MARK: - Flush
+extension LDClient {
+    /**
+     Tells the SDK to immediately send any currently queued events to LaunchDarkly.
+
+     There should not normally be a need to call this function. While online, the LDClient automatically reports events
+     on an interval defined by `LDConfig.eventFlushInterval`. Note that this function does not block until events are
+     sent, it only triggers a background task to send events immediately.
+
+     Because it does not wait, it cannot report how the delivery went. Use `flush(completion:)` where that matters
+     but blocking does not suit, or `flushAndWait` where blocking is acceptable.
+     */
+    public func flush() {
+        LDClient.instancesQueue.sync(flags: .barrier) {
+            LDClient.instances?.forEach { $1.internalFlush() }
+        }
+    }
+
+    /**
+     Sends any currently queued events to LaunchDarkly and reports how the delivery went.
+
+     Like `flushAndWait`, but without blocking: the completion runs once every environment has answered. There is no
+     timeout, because a delivery always ends, by its own request timeouts if nothing else. This is the one to reach
+     for from a Swift concurrency task, where blocking is not safe.
+
+     - parameter completion: Given `true` if LaunchDarkly accepted the events, or there were none to send, and
+       `false` if the client is offline or closed, or any of the events were lost. See `flushAndWait` for what being
+       lost covers. Runs once, on a background queue that is neither the caller's thread nor the one recorded events
+       wait on, so it is free to record or flush again.
+     */
+    public func flush(completion: @escaping (Bool) -> Void) {
+        let clients = instancesIncludingThisClient()
+        guard !clients.isEmpty
+        else {
+            os_log("%s called on a closed client", log: config.logger, type: .debug, self.typeName(and: #function))
+            completion(false)
+            return
+        }
+
+        let lock = UnfairLock()
+        /// Only to be used while holding `lock`: each environment answers from whichever thread its delivery ended on.
+        var delivered = true
+        var outstanding = clients.count
+
+        for client in clients {
+            client.eventReporter.flushReportingOutcome { result in
+                lock.lock()
+                delivered = delivered && result
+                outstanding -= 1
+                // Nil until the last environment answers, so the completion runs once. A reporter answering twice
+                // takes the count past zero rather than back to it, which cannot match again.
+                let outcome = outstanding == 0 ? delivered : nil
+                lock.unlock()
+
+                if let outcome {
+                    completion(outcome)
+                }
+            }
+        }
+    }
+
+    /// Every environment's client, or none where this client is no longer one of them.
+    private func instancesIncludingThisClient() -> [LDClient] {
+        let clients = LDClient.instancesQueue.sync { LDClient.instances.map { Array($0.values) } } ?? []
+        return clients.contains { $0 === self } ? clients : []
+    }
+
+    /**
+     Sends any currently queued events to LaunchDarkly and waits up to `timeout` seconds for the result.
+
+     The timeout bounds the wait, not the delivery: an in-flight request is left to finish. Multiple environments
+     deliver at the same time and share the one budget, so the call takes no longer for several of them than for one.
+
+     A delivery that definitively failed is reported as a failure. Some other LaunchDarkly SDKs report that as a
+     success, on the grounds that the attempt is over, so an expectation carried from another platform may not hold
+     here.
+
+     This blocks the thread it is called on until the answer is in or the budget runs out. Where the answer is not
+     needed, `flush` queues the same delivery and returns at once; where it is needed but blocking does not suit,
+     `flush(completion:)` reports it without waiting.
+
+     Do not call this from a Swift concurrency task. Blocking there holds a thread of the cooperative pool, which
+     cannot grow to replace it, and enough of them will stall every other task in the process. Use
+     `flush(completion:)` from a task, wrapped in a continuation where you want to await it.
+
+     This is not a crash-time mechanism. It is safe to call from the main thread, but keep the budget well below
+     15 seconds there.
+
+     - parameter timeout: How long to wait, in seconds. Zero or less does not wait, and so reports `false`. There is
+       no upper limit: a delivery always ends, by its own request timeouts if nothing else, so a very long timeout
+       waits for that.
+     - returns: `true` if LaunchDarkly accepted the events, or there were none to send. `false` if the timeout expired
+       first, the client is offline or closed, or any of the events were lost: refused by LaunchDarkly, still failing
+       after one retry, or unable to be serialized. Lost events are not kept for a later flush, so calling this again
+       does not resend them. A refusal also stops event delivery until the client is next set online, and until then
+       this returns `false` even with nothing to send. A `false` because the timeout expired does not mean the events
+       were not sent: the delivery is left running, and may still arrive.
+     */
+    @discardableResult
+    public func flushAndWait(timeout: TimeInterval) -> Bool {
+        if timeout > LDClient.longTimeoutInterval {
+            os_log("%s LDClient.flushAndWait was called with a timeout greater than %f seconds. We recommend a timeout of less than %f seconds.", log: config.logger, type: .info, self.typeName(and: #function), LDClient.longTimeoutInterval, LDClient.longTimeoutInterval)
+        }
+
+        let clients = instancesIncludingThisClient()
+        guard !clients.isEmpty
+        else {
+            // Closed, or replaced by a later start, or caught while a start is still filling the instances in. None of
+            // them can deliver this client's events, and saying otherwise would tell the caller they were safe.
+            os_log("%s called on a closed client", log: config.logger, type: .debug, self.typeName(and: #function))
+            return false
+        }
+        let deadline = DispatchTime.now() + max(0, timeout)
+        // Counts answers rather than tracking each delivery, so that a reporter answering twice costs a spare permit
+        // no one reads. A DispatchGroup would fit as well, but its `leave` traps without a matching `enter`, which
+        // turns a broken reporter into a crash in the application.
+        let answered = DispatchSemaphore(value: 0)
+        let lock = UnfairLock()
+        /// Only to be used while holding `lock`: each environment answers from whichever thread its delivery ended on.
+        var delivered = true
+
+        // Every environment is started before any of them is waited on. Each has its own event reporter, so waiting on
+        // one before starting the next would spend the caller's budget on deliveries that could have been running all
+        // along, and leave the last environment with none of it.
+        for client in clients {
+            client.eventReporter.flushReportingOutcome { result in
+                lock.lock()
+                delivered = delivered && result
+                lock.unlock()
+                answered.signal()
+            }
+        }
+
+        // Every wait is against the one deadline rather than a fresh copy of it, so the timeout the caller asked for
+        // is the time this call can take however many environments there are. A delivery still running when the wait
+        // gives up is left to finish rather than canceled: its events are already in flight, so abandoning it would
+        // only make losing them certain.
+        for _ in clients {
+            guard answered.wait(timeout: deadline) == .success
+            else { return false }
+        }
+
+        // The waits count answers rather than environments, so a reporter answering twice can end them while another
+        // environment's answer is still being written.
+        lock.lock()
+        defer { lock.unlock() }
+        return delivered
+    }
+
+    func internalFlush() {
+        eventReporter.flush(completion: nil)
+    }
+}

LaunchDarkly/LaunchDarkly/Networking/DarklyService.swift

@@ -189,7 +189,12 @@ final class DarklyService: DarklyServiceProvider {
     // MARK: Publish Events
 
     func publishEventData(_ eventData: Data, _ payloadId: String, completion: ServiceCompletionHandler?) {
-        guard hasMobileKey(#function) else { return }
+        guard hasMobileKey(#function)
+        else {
+            // Answered rather than dropped: the reporter holds every later delivery until this one answers.
+            completion?((nil, nil, URLError(.userAuthenticationRequired), nil))
+            return
+        }
         let url = config.eventsUrl.appendingPathComponent(EventRequestPath.bulk)
         let headers = [HTTPHeaders.HeaderKey.eventPayloadIDHeader: payloadId].merging(httpHeaders.eventRequestHeaders) { $1 }
         doPublish(url: url, headers: headers, body: eventData, completion: completion)

LaunchDarkly/LaunchDarkly/ObjectiveC/ObjcLDClient.swift

@@ -534,11 +534,58 @@ public final class ObjcLDClient: NSObject {
      There should not normally be a need to call this function. While online, the LDClient automatically reports events
      on an interval defined by `LDConfig.eventFlushInterval`. Note that this function does not block until events are
      sent, it only triggers a background task to send events immediately.
+
+     Because it does not wait, it cannot report how the delivery went. Use `flushWithCompletion:` where that matters
+     but blocking does not suit, or `flushAndWait` where blocking is acceptable.
      */
     @objc public func flush() {
         ldClient.flush()
     }
 
+    /**
+     Sends any currently queued events to LaunchDarkly and reports how the delivery went.
+
+     Like `flushAndWait`, but without blocking: the completion runs once every environment has answered. There is no
+     timeout, because a delivery always ends, by its own request timeouts if nothing else.
+
+     - parameter completion: Given `YES` if LaunchDarkly accepted the events, or there were none to send, and `NO` if
+       the client is offline or closed, or any of the events were lost. See `flushAndWait` for what being lost
+       covers. Runs once, on a background queue that is not the caller's thread.
+     */
+    @objc public func flush(completion: @escaping (Bool) -> Void) {
+        ldClient.flush(completion: completion)
+    }
+
+    /**
+     Sends any currently queued events to LaunchDarkly and waits up to `timeout` seconds for the result.
+
+     The timeout bounds the wait, not the delivery: an in-flight request is left to finish. Multiple environments
+     deliver at the same time and share the one budget, so the call takes no longer for several of them than for one.
+
+     A delivery that definitively failed is reported as a failure. Some other LaunchDarkly SDKs report that as a
+     success, on the grounds that the attempt is over, so an expectation carried from another platform may not hold
+     here.
+
+     This blocks the thread it is called on until the answer is in or the budget runs out, so where the answer is
+     not needed, `flush` queues the same delivery and returns at once.
+
+     This is not a crash-time mechanism. It is safe to call from the main thread, but keep the budget well below
+     15 seconds there.
+
+     - parameter timeout: How long to wait, in seconds. Zero or less does not wait, and so reports `NO`. There is
+       no upper limit: a delivery always ends, by its own request timeouts if nothing else, so a very long timeout
+       waits for that.
+     - returns: `YES` if LaunchDarkly accepted the events, or there were none to send. `NO` if the timeout expired
+       first, the client is offline or closed, or any of the events were lost: refused by LaunchDarkly, still failing
+       after one retry, or unable to be serialized. Lost events are not kept for a later flush, so calling this again
+       does not resend them. A refusal also stops event delivery until the client is next set online, and until then
+       this returns `NO` even with nothing to send. A `NO` because the timeout expired does not mean the events were
+       not sent: the delivery is left running, and may still arrive.
+     */
+    @objc public func flushAndWait(timeout: TimeInterval) -> Bool {
+        ldClient.flushAndWait(timeout: timeout)
+    }
+
    /**
      Starts the LDClient using the passed in `config` & `context`. Call this before requesting feature flag values. The LDClient will not go online until you call this method.
      Starting the LDClient means setting the `config` & `context`, setting the client online if `config.startOnline` is true (the default setting), and starting event recording. The client app must start the LDClient before it will report feature flag values. If a client does not call `start`, no methods will work.