Fix BlockOperation with multiple execution blocks

Steps:
1. Create BlockOperation with an init block.
2. Add another one with addExecutionBlock.
3. Access the executionBlocks property, it contains 3 blocks.

Author: dannliu

Sources/Foundation/Operation.swift

@@ -668,22 +668,16 @@ open class BlockOperation : Operation {
         }
         _lock()
         defer { _unlock() }
-        if _block == nil && _executionBlocks == nil {
+        if _block == nil {
             _block = block
+        } else if _executionBlocks == nil {
+            _executionBlocks = [block]
         } else {
-            if _executionBlocks == nil {
-                if let existing = _block {
-                    _executionBlocks = [existing, block]
-                } else {
-                    _executionBlocks = [block]
-                }
-            } else {
-                _executionBlocks?.append(block)
-            }
+            _executionBlocks?.append(block)
         }
     }
 
-    open var executionBlocks: [@convention(block) () -> Void] {
+    open var executionBlocks: [() -> Void] {
         get {
             _lock()
             defer { _unlock() }

Tests/Foundation/Tests/TestOperationQueue.swift

@@ -40,6 +40,7 @@ class TestOperationQueue : XCTestCase {
             ("test_CustomOperationReady", test_CustomOperationReady),
             ("test_DependencyCycleBreak", test_DependencyCycleBreak),
             ("test_Lifecycle", test_Lifecycle),
+            ("test_BlockOperationAddExecutionBlock", test_BlockOperationAddExecutionBlock),
         ]
     }
 
@@ -699,6 +700,21 @@ class TestOperationQueue : XCTestCase {
         Thread.sleep(forTimeInterval: 1) // Let queue to be deallocated
         XCTAssertNil(weakQueue, "Queue should be deallocated at this point")
     }
+
+    func test_BlockOperationAddExecutionBlock() {
+        var msgOperations = [String]()
+        let blockOperation = BlockOperation {
+            msgOperations.append("block1 executed")
+        }
+        blockOperation.addExecutionBlock {
+            msgOperations.append("block2 executed")
+        }
+        XCTAssert(blockOperation.executionBlocks.count == 2)
+        let queue = OperationQueue()
+        queue.addOperation(blockOperation)
+        queue.waitUntilAllOperationsAreFinished()
+        XCTAssertEqual(msgOperations, ["block1 executed", "block2 executed"])
+    }
 }
 
 class AsyncOperation: Operation {