Merge pull request #2953 from readdle/operation-order-breach-5.3

[5.3] Fix OperationQueue scheduling logic to prevent disruption of operation lists

Author: spevans

Sources/Foundation/Operation.swift

@@ -993,7 +993,7 @@ open class OperationQueue : NSObject, ProgressReporting {
                 // if the cached state is possibly not valid then the isReady value needs to be re-updated
                 if Operation.__NSOperationState.enqueued == operation._state && operation._fetchCachedIsReady(&retest) {
                     if let previous = prev?.takeUnretainedValue() {
-                        previous.__nextOperation = next
+                        previous.__nextPriorityOperation = next
                     } else {
                         _setFirstPriorityOperation(prio, next)
                     }

Tests/Foundation/Tests/TestOperationQueue.swift

@@ -34,6 +34,7 @@ class TestOperationQueue : XCTestCase {
             ("test_CancelWhileSuspended", test_CancelWhileSuspended),
             ("test_OperationOrder", test_OperationOrder),
             ("test_OperationOrder2", test_OperationOrder2),
+            ("test_ExecutionOrder", test_ExecutionOrder),
             ("test_WaitUntilFinished", test_WaitUntilFinished),
             ("test_OperationWaitUntilFinished", test_OperationWaitUntilFinished),
             ("test_CustomOperationReady", test_CustomOperationReady),
@@ -531,6 +532,32 @@ class TestOperationQueue : XCTestCase {
         XCTAssertEqual(array, [5, 4, 3, 2, 1])
     }
 
+    func test_ExecutionOrder() {
+        let queue = OperationQueue()
+        
+        let didRunOp1 = expectation(description: "Did run first operation")
+        let didRunOp1Dependency = expectation(description: "Did run first operation dependency")
+        let didRunOp2 = expectation(description: "Did run second operation")
+        var didRunOp1DependencyFirst = false
+        
+        let op1 = BlockOperation {
+            didRunOp1.fulfill()
+            XCTAssertTrue(didRunOp1DependencyFirst, "Dependency should be executed first")
+        }
+        let op1Dependency = BlockOperation {
+            didRunOp1Dependency.fulfill()
+            didRunOp1DependencyFirst = true
+        }
+        op1.addDependency(op1Dependency)
+        queue.addOperations([op1, op1Dependency], waitUntilFinished: false)
+        
+        queue.addOperation {
+            didRunOp2.fulfill()
+        }
+        
+        waitForExpectations(timeout: 1.0)
+    }
+
     func test_WaitUntilFinished() {
         let queue1 = OperationQueue()
         let queue2 = OperationQueue()