Add utility function for waiting for a promise to complete

Sources/Pwomise/Extensions/Promise+Wait.swift

@@ -0,0 +1,38 @@
+//
+//  File.swift
+//  
+//
+//  Created by Eric Rabil on 7/25/22.
+//
+
+import Foundation
+
+public extension Promise {
+    enum TimeoutError: Error { case timedOut }
+
+    func wait(upTo time: DispatchTime) throws -> Output {
+        var completion: Completion!
+        let semaphore = DispatchSemaphore(value: 0)
+        var semaphoreLocked: Bool {
+            if semaphore.wait(timeout: .now()) == .success {
+                semaphore.signal()
+                return false
+            }
+            return true
+        }
+        always {
+            completion = $0
+            if semaphoreLocked {
+                semaphore.signal()
+            }
+        }.resolving(on: DispatchQueue.global())
+        if semaphore.wait(timeout: time) == .success {
+            // finished in time, signal and return
+            semaphore.signal()
+        } else {
+            // did not finish in time, throw
+            throw TimeoutError.timedOut
+        }
+        return try completion.get()
+    }
+}