Fix remaining issues in async/sync mode switching.

Sources/Segment/Analytics.swift

@@ -437,16 +437,11 @@ 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/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 = {