Fix remaining issues in async/sync mode switching.

Sources/Segment/Analytics.swift

@@ -212,40 +212,49 @@ extension Analytics {
         flushGroup.enter()
 
         apply { plugin in
+            // we want to enter as soon as possible.  waiting to do it from
+            // another queue just takes too long.
             operatingMode.run(queue: configuration.values.flushQueue) {
                 if let p = plugin as? FlushCompletion {
-                    // this is async
-                    // flush(group:completion:) handles the enter/leave.
+                    // flush handles the groups enter/leave calls
                     p.flush(group: flushGroup) { plugin in
                         // we don't really care about the plugin value .. yet.
                     }
                 } else if let p = plugin as? EventPlugin {
-                    // we have no idea if this will be async or not, assume it's sync.
                     flushGroup.enter()
+                    // we have no idea if this will be async or not, assume it's sync.
                     p.flush()
                     flushGroup.leave()
                 }
             }
         }
 
-        // if we're not in server mode, we need to be notified when it's done.
-        if let completion, operatingMode != .synchronous {
-            // set up our callback to know when the group has completed, if we're not
-            // in .server operating mode.
-            flushGroup.notify(queue: configuration.values.flushQueue) {
-                DispatchQueue.main.async { completion() }
-            }
-        }
-
         flushGroup.leave() // matches our initial enter().
 
-        // if we ARE in server mode, we need to wait on the group.
+        // if we ARE in sync mode, we need to wait on the group.
         // This effectively ends up being a `sync` operation.
         if operatingMode == .synchronous {
             flushGroup.wait()
             // we need to call completion on our own since
-            // we skipped setting up notify.
-            if let completion { DispatchQueue.main.async { completion() }}
+            // we skipped setting up notify.  we don't need to do it on
+            // .main since we are in synchronous mode.
+            if let completion { completion() }
+        } else if operatingMode == .asynchronous {
+            // if we're not, flip over to our serial queue, tell it to wait on the flush
+            // group to complete if we have a completion to hit.  Otherwise, no need to
+            // wait on completion.
+            if let completion {
+                // NOTE: DispatchGroup's `notify` method on linux ended up getting called
+                // before the tasks have actually completed, so we went with this instead.
+                OperatingMode.defaultQueue.async { [weak self] in
+                    let timedOut = flushGroup.wait(timeout: .now() + 15 /*seconds*/)
+                    if timedOut == .timedOut {
+                        self?.log(message: "flush(completion:) timed out waiting for completion.")
+                    }
+                    completion()
+                    //DispatchQueue.main.async { completion() }
+                }
+            }
         }
     }
 
@@ -437,16 +446,11 @@ extension OperatingMode {
                 task()
             }
         case .synchronous:
-            // if for some reason, we're told to do all this stuff on
-            // main, ignore it, and use the default queue.  this prevents
-            // a possible deadlock.
-            if queue === DispatchQueue.main {
-                OperatingMode.defaultQueue.asyncAndWait {
-                    task()
-                }
-            } else {
-                queue.asyncAndWait { task() }
-            }
+            // in synchronous mode, always use our own queue to
+            // prevent deadlocks.
+            let workItem = DispatchWorkItem(block: task)
+            OperatingMode.defaultQueue.asyncAndWait(execute: workItem)
         }
     }
 }
+

Sources/Segment/Plugins/SegmentDestination.swift

@@ -124,8 +124,11 @@ public class SegmentDestination: DestinationPlugin, Subscriber, FlushCompletion
         // don't flush if analytics is disabled.
         guard analytics.enabled == true else { return }
 
+        // enter for the high level flush, allow us time to run through any existing files..
+        group.enter()
+
         // Read events from file system
-        guard let data = storage.read(Storage.Constants.events) else { return }
+        guard let data = storage.read(Storage.Constants.events) else { group.leave(); return }
 
         eventCount = 0
         cleanupUploads()
@@ -134,9 +137,9 @@ public class SegmentDestination: DestinationPlugin, Subscriber, FlushCompletion
 
         if pendingUploads == 0 {
             for url in data {
-                analytics.log(message: "Processing Batch:\n\(url.lastPathComponent)")
-                // enter the dispatch group
+                // enter for this url we're going to kick off
                 group.enter()
+                analytics.log(message: "Processing Batch:\n\(url.lastPathComponent)")
                 // set up the task
                 let uploadTask = httpClient.startBatchUpload(writeKey: analytics.configuration.values.writeKey, batch: url) { (result) in
                     switch result {
@@ -154,7 +157,7 @@ public class SegmentDestination: DestinationPlugin, Subscriber, FlushCompletion
                     self.cleanupUploads()
                     // call the completion
                     completion(self)
-                    // leave the dispatch group
+                    // leave for the url we kicked off.
                     group.leave()
                 }
                 // we have a legit upload in progress now, so add it to our list.
@@ -165,6 +168,9 @@ public class SegmentDestination: DestinationPlugin, Subscriber, FlushCompletion
         } else {
             analytics.log(message: "Skipping processing; Uploads in progress.")
         }
+
+        // leave for the high level flush
+        group.leave()
     }
 }
 

Sources/Segment/Utilities/Utils.swift

@@ -7,6 +7,17 @@
 
 import Foundation
 
+#if os(Linux)
+extension DispatchQueue {
+    func asyncAndWait(execute workItem: DispatchWorkItem) {
+        async {
+            workItem.perform()
+        }
+        workItem.wait()
+    }
+}
+#endif
+
 /// Inquire as to whether we are within a Unit Testing environment.
 #if DEBUG
 internal var isUnitTesting: Bool = {
@@ -58,3 +69,36 @@ extension Optional: Flattenable {
     }
 }
 
+class TrackingDispatchGroup: CustomStringConvertible {
+    internal let group = DispatchGroup()
+
+    var description: String {
+        return "DispatchGroup Enters: \(enters), Leaves: \(leaves)"
+    }
+
+    var enters: Int = 0
+    var leaves: Int = 0
+    var current: Int = 0
+
+    func enter() {
+        enters += 1
+        current += 1
+        group.enter()
+    }
+
+    func leave() {
+        leaves += 1
+        current -= 1
+        group.leave()
+    }
+
+    init() { }
+
+    func wait() {
+        group.wait()
+    }
+
+    public func notify(qos: DispatchQoS = .unspecified, flags: DispatchWorkItemFlags = [], queue: DispatchQueue, execute work: @escaping @convention(block) () -> Void) {
+        group.notify(qos: qos, flags: flags, queue: queue, execute: work)
+    }
+}

Tests/Segment-Tests/Analytics_Tests.swift

@@ -692,9 +692,38 @@ final class Analytics_Tests: XCTestCase {
 
     }
 
-    func testServerOperatingMode() {
+    func testAsyncOperatingMode() {
         // Use a specific writekey to this test so we do not collide with other cached items.
-        let analytics = Analytics(configuration: Configuration(writeKey: "testFlush_serverMode")
+        let analytics = Analytics(configuration: Configuration(writeKey: "testFlush_asyncMode")
+            .flushInterval(9999)
+            .flushAt(9999)
+            .operatingMode(.asynchronous))
+
+        waitUntilStarted(analytics: analytics)
+
+        analytics.storage.hardReset(doYouKnowHowToUseThis: true)
+
+        @Atomic var completionCalled = false
+
+        // put an event in the pipe ...
+        analytics.track(name: "completion test1")
+        // flush it, that'll get us an upload going
+        analytics.flush {
+            // verify completion is called.
+            completionCalled = true
+        }
+
+        while !completionCalled {
+            RunLoop.main.run(until: Date.distantPast)
+        }
+
+        XCTAssertTrue(completionCalled)
+        XCTAssertEqual(analytics.pendingUploads!.count, 0)
+    }
+
+    func testSyncOperatingMode() {
+        // Use a specific writekey to this test so we do not collide with other cached items.
+        let analytics = Analytics(configuration: Configuration(writeKey: "testFlush_syncMode")
             .flushInterval(9999)
             .flushAt(9999)
             .operatingMode(.synchronous))

Tests/Segment-Tests/StressTests.swift

@@ -69,7 +69,8 @@ class StressTests: XCTestCase {
                 let event = "write queue 1: \(eventsWritten)"
                 analytics.track(name: event)
                 eventsWritten += 1
-                usleep(0001)
+                //usleep(0001)
+                RunLoop.main.run(until: Date.distantPast)
             }
             print("queue 1 wrote \(eventsWritten) events.")
             queue1Done = true
@@ -82,7 +83,8 @@ class StressTests: XCTestCase {
                 let event = "write queue 2: \(eventsWritten)"
                 analytics.track(name: event)
                 eventsWritten += 1
-                usleep(0001)
+                //usleep(0001)
+                RunLoop.main.run(until: Date.distantPast)
             }
             print("queue 2 wrote \(eventsWritten) events.")
             queue2Done = true
@@ -91,10 +93,12 @@ class StressTests: XCTestCase {
         flushQueue.async {
             while (ready == false) { usleep(1) }
             var counter = 0
-            sleep(1)
+            //sleep(1)
+            RunLoop.main.run(until: Date(timeIntervalSinceNow: 1))
             while (queue1Done == false || queue2Done == false) {
                 let sleepTime = UInt32.random(in: 1..<3000)
-                usleep(sleepTime)
+                //usleep(sleepTime)
+                RunLoop.main.run(until: Date(timeIntervalSinceNow: Double(sleepTime / 1000) ))
                 analytics.flush()
                 counter += 1
             }