Support XCTestExpectation creation APIs on XCTestCase from non-main threads

This matches a behavior change made in the Xcode copy of XCTest and allows tests which call `XCTestCase.expectation(description:)` — as well as other `XCTestCase` APIs that return an `XCTestExpectation` — to do so from any thread. This removes a previous requirement that tests call these APIs from the main thread, and it allows existing tests using them to adopt `async` without requiring `@MainActor`.

rdar://85344643
rdar://79163972
rdar://81024086

Author: stmontgomery

Sources/XCTest/Public/Asynchronous/XCTestCase+Asynchronous.swift

@@ -41,6 +41,7 @@ public extension XCTestCase {
     ///   these environments. To ensure compatibility of tests between
     ///   swift-corelibs-xctest and Apple XCTest, it is not recommended to pass
     ///   explicit values for `file` and `line`.
+    // FIXME: This should have `@MainActor` to match Xcode XCTest, but adding it causes errors in tests authored pre-Swift Concurrency which don't typically have `@MainActor`.
     func waitForExpectations(timeout: TimeInterval, file: StaticString = #file, line: Int = #line, handler: XCWaitCompletionHandler? = nil) {
         precondition(Thread.isMainThread, "\(#function) must be called on the main thread")
         if currentWaiter != nil {
@@ -58,7 +59,7 @@ public extension XCTestCase {
 
         currentWaiter = nil
 
-        cleanUpExpectations()
+        cleanUpExpectations(expectations)
 
         // The handler is invoked regardless of whether the test passed.
         if let handler = handler {

Sources/XCTest/Public/XCTestCase.swift

@@ -48,6 +48,7 @@ open class XCTestCase: XCTest {
         return 1
     }
 
+    // FIXME: Once `waitForExpectations(timeout:...handler:)` gains `@MainActor`, this may be able to add it as well.
     internal var currentWaiter: XCTWaiter?
 
     /// The set of expectations made upon this test case.
@@ -60,9 +61,6 @@ open class XCTestCase: XCTest {
     }
 
     internal func addExpectation(_ expectation: XCTestExpectation) {
-        precondition(Thread.isMainThread, "\(#function) must be called on the main thread")
-        precondition(currentWaiter == nil, "API violation - creating an expectation while already in waiting mode.")
-
         XCTWaiter.subsystemQueue.sync {
             _allExpectations.append(expectation)
         }
@@ -99,7 +97,10 @@ open class XCTestCase: XCTest {
         XCTCurrentTestCase = self
         testRun.start()
         invokeTest()
-        failIfExpectationsNotWaitedFor(_allExpectations)
+
+        let allExpectations = XCTWaiter.subsystemQueue.sync { _allExpectations }
+        failIfExpectationsNotWaitedFor(allExpectations)
+
         testRun.stop()
         XCTCurrentTestCase = nil
     }

Tests/Functional/Asynchronous/Expectations/main.swift

@@ -482,6 +482,45 @@ class ExpectationsTestCase: XCTestCase {
         waitForExpectations(timeout: 1)
     }
 
+// CHECK: Test Case 'ExpectationsTestCase.test_expectationCreationOnSecondaryThread' started at \d+-\d+-\d+ \d+:\d+:\d+\.\d+
+// CHECK: Test Case 'ExpectationsTestCase.test_expectationCreationOnSecondaryThread' passed \(\d+\.\d+ seconds\)
+    func test_expectationCreationOnSecondaryThread() {
+        DispatchQueue.global().sync {
+            let foo = self.expectation(description: "foo")
+            foo.fulfill()
+        }
+        waitForExpectations(timeout: 1)
+
+        // Also test using an "explicit" wait API (which accepts an array of expectations) to wait on an
+        // expectation created on a non-main thread.
+        let foo: XCTestExpectation = DispatchQueue.global().sync {
+            let foo = self.expectation(description: "foo")
+            foo.fulfill()
+            return foo
+        }
+        wait(for: [foo], timeout: 1)
+    }
+
+// CHECK: Test Case 'ExpectationsTestCase.test_expectationCreationWhileWaiting' started at \d+-\d+-\d+ \d+:\d+:\d+\.\d+
+// CHECK: Test Case 'ExpectationsTestCase.test_expectationCreationWhileWaiting' passed \(\d+\.\d+ seconds\)
+    func test_expectationCreationWhileWaiting() {
+        let foo = expectation(description: "foo")
+
+        DispatchQueue.main.async {
+            let bar = self.expectation(description: "bar")
+            bar.fulfill()
+
+            foo.fulfill()
+        }
+
+        // Waits on `foo` with a generous timeout, since we want to ensure we don't proceed until the dispatched block has completed.
+        waitForExpectations(timeout: 100)
+
+        // Waits on `bar` with a zero timeout, since by this point we expect the dispatched block to have completed and thus
+        // the second expectation should already be fulfilled.
+        waitForExpectations(timeout: 0)
+    }
+
 // CHECK: Test Case 'ExpectationsTestCase.test_runLoopInsideDispatch' started at \d+-\d+-\d+ \d+:\d+:\d+\.\d+
 // CHECK: .*[/\\]Tests[/\\]Functional[/\\]Asynchronous[/\\]Expectations[/\\]main.swift:[[@LINE+8]]: error: ExpectationsTestCase.test_runLoopInsideDispatch : Asynchronous wait failed - Exceeded timeout of 0.5 seconds, with unfulfilled expectations: foo
 // CHECK: Test Case 'ExpectationsTestCase.test_runLoopInsideDispatch' failed \(\d+\.\d+ seconds\)
@@ -545,16 +584,18 @@ class ExpectationsTestCase: XCTestCase {
 
             // Regressions
             ("test_fulfillmentOnSecondaryThread", test_fulfillmentOnSecondaryThread),
+            ("test_expectationCreationOnSecondaryThread", test_expectationCreationOnSecondaryThread),
+            ("test_expectationCreationWhileWaiting", test_expectationCreationWhileWaiting),
             ("test_runLoopInsideDispatch", test_runLoopInsideDispatch),
         ]
     }()
 }
 // CHECK: Test Suite 'ExpectationsTestCase' failed at \d+-\d+-\d+ \d+:\d+:\d+\.\d+
-// CHECK: \t Executed 32 tests, with 16 failures \(2 unexpected\) in \d+\.\d+ \(\d+\.\d+\) seconds
+// CHECK: \t Executed 34 tests, with 16 failures \(2 unexpected\) in \d+\.\d+ \(\d+\.\d+\) seconds
 
 XCTMain([testCase(ExpectationsTestCase.allTests)])
 
 // CHECK: Test Suite '.*\.xctest' failed at \d+-\d+-\d+ \d+:\d+:\d+\.\d+
-// CHECK: \t Executed 32 tests, with 16 failures \(2 unexpected\) in \d+\.\d+ \(\d+\.\d+\) seconds
+// CHECK: \t Executed 34 tests, with 16 failures \(2 unexpected\) in \d+\.\d+ \(\d+\.\d+\) seconds
 // CHECK: Test Suite 'All tests' failed at \d+-\d+-\d+ \d+:\d+:\d+\.\d+
-// CHECK: \t Executed 32 tests, with 16 failures \(2 unexpected\) in \d+\.\d+ \(\d+\.\d+\) seconds
+// CHECK: \t Executed 34 tests, with 16 failures \(2 unexpected\) in \d+\.\d+ \(\d+\.\d+\) seconds