feat: Add testing utilities that make a number of nio-related tests much easier to run.

Author: scottmarchant

Tests/NIOAsyncRuntimeTests/TestUtils.swift

@@ -0,0 +1,103 @@
+//===----------------------------------------------------------------------===//
+//
+// Copyright (c) 2025 PassiveLogic, Inc.
+// Licensed under Apache License v2.0
+//
+// See LICENSE.txt for license information
+//
+// SPDX-License-Identifier: Apache-2.0
+//
+//===----------------------------------------------------------------------===//
+
+// swift-format-ignore: AmbiguousTrailingClosureOverload
+
+import NIOConcurrencyHelpers
+import XCTest
+
+@testable import NIOCore
+
+func assertNoThrowWithValue<T>(
+  _ body: @autoclosure () throws -> T,
+  defaultValue: T? = nil,
+  message: String? = nil,
+  file: StaticString = #filePath,
+  line: UInt = #line
+) throws -> T {
+  do {
+    return try body()
+  } catch {
+    XCTFail(
+      "\(message.map { $0 + ": " } ?? "")unexpected error \(error) thrown", file: (file), line: line
+    )
+    if let defaultValue = defaultValue {
+      return defaultValue
+    } else {
+      throw error
+    }
+  }
+}
+
+func assert(
+  _ condition: @autoclosure () -> Bool,
+  within time: TimeAmount,
+  testInterval: TimeAmount? = nil,
+  _ message: String = "condition not satisfied in time",
+  file: StaticString = #filePath,
+  line: UInt = #line
+) {
+  let testInterval = testInterval ?? TimeAmount.nanoseconds(time.nanoseconds / 5)
+  let endTime = NIODeadline.now() + time
+
+  repeat {
+    if condition() { return }
+    usleep(UInt32(testInterval.nanoseconds / 1000))
+  } while NIODeadline.now() < endTime
+
+  if !condition() {
+    XCTFail(message, file: (file), line: line)
+  }
+}
+
+func assertSuccess<Value>(
+  _ result: Result<Value, Error>, file: StaticString = #filePath, line: UInt = #line
+) {
+  guard case .success = result else {
+    return XCTFail("Expected result to be successful", file: (file), line: line)
+  }
+}
+
+func assertFailure<Value>(
+  _ result: Result<Value, Error>, file: StaticString = #filePath, line: UInt = #line
+) {
+  guard case .failure = result else {
+    return XCTFail("Expected result to be a failure", file: (file), line: line)
+  }
+}
+
+extension EventLoopFuture {
+  var isFulfilled: Bool {
+    if self.eventLoop.inEventLoop {
+      // Easy, we're on the EventLoop. Let's just use our knowledge that we run completed future callbacks
+      // immediately.
+      var fulfilled = false
+      self.assumeIsolated().whenComplete { _ in
+        fulfilled = true
+      }
+      return fulfilled
+    } else {
+      let fulfilledBox = NIOLockedValueBox(false)
+      let group = DispatchGroup()
+
+      group.enter()
+      self.eventLoop.execute {
+        let isFulfilled = self.isFulfilled  // This will now enter the above branch.
+        fulfilledBox.withLockedValue {
+          $0 = isFulfilled
+        }
+        group.leave()
+      }
+      group.wait()  // this is very nasty but this is for tests only, so...
+      return fulfilledBox.withLockedValue { $0 }
+    }
+  }
+}