Catch race condition in PostHogFileBackedQueue.deleteFiles (#218)

* Catch race condition in PostHogFileBackedQueue.deleteFiles

In rare cases, `deleteFiles(_:)` can be invoked such that, between the check for items.isEmpty and the call to items.remove, the task gets pre-empted and items is mutated. Once the thread resumes, the `.remove(at: 0)` operation causes a crash.

* Update ReadWriteLock.

* Address feedback.

PostHog/PostHogFileBackedQueue.swift

@@ -100,10 +100,14 @@ class PostHogFileBackedQueue {
 
     private func deleteFiles(_ count: Int) {
         for _ in 0 ..< count {
-            if items.isEmpty { return }
-            let removed = items.remove(at: 0) // We always remove from the top of the queue
-
-            deleteSafely(queue.appendingPathComponent(removed))
+            if let removed: String = _items.mutate({ items in
+                if items.isEmpty {
+                    return nil
+                }
+                return items.remove(at: 0) // We always remove from the top of the queue
+            }) {
+                deleteSafely(queue.appendingPathComponent(removed))
+            }
         }
     }
 }

PostHog/Utils/ReadWriteLock.swift

@@ -50,10 +50,13 @@ public final class ReadWriteLock<Value> {
     /// The lock will be acquired once for writing before invoking the closure.
     ///
     /// - Parameter closure: The closure with the mutable value.
-    public func mutate(_ closure: (inout Value) -> Void) {
+    @discardableResult
+    public func mutate<T>(_ closure: (inout Value) -> T) -> T {
         pthread_rwlock_wrlock(&rwlock)
-        closure(&value)
-        pthread_rwlock_unlock(&rwlock)
+        defer {
+            pthread_rwlock_unlock(&rwlock)
+        }
+        return closure(&value)
     }
 }