Add async version of NIOThreadPool.runIfActive (#2566)

* Add async version of NIOThreadPool.runIfActive

* Changes from comments in PR

Remove no longer supported code
Add tests for errors being thrown, and the thread pool not being active

* Collapse async runIfActive into one function

* remove whitespace

* Make T Sendable

---------

Co-authored-by: Cory Benfield <lukasa@apple.com>

Sources/NIOPosix/NIOThreadPool.swift

@@ -290,6 +290,29 @@ extension NIOThreadPool {
         }
         return promise.futureResult
     }
+
+    /// Runs the submitted closure if the thread pool is still active, otherwise throw an error.
+    /// The closure will be run on the thread pool so can do blocking work.
+    ///
+    /// - parameters:
+    ///     - body: The closure which performs some blocking work to be done on the thread pool.
+    /// - returns: result of the passed closure.
+    @available(macOS 10.15, iOS 13, tvOS 13, watchOS 6, *)
+    public func runIfActive<T: Sendable>(_ body: @escaping @Sendable () throws -> T) async throws -> T {
+        try await withCheckedThrowingContinuation { (cont: CheckedContinuation<T, Error>) in
+            self.submit { shouldRun in
+                guard case shouldRun = NIOThreadPool.WorkItemState.active else {
+                    cont.resume(throwing: NIOThreadPoolError.ThreadPoolInactive())
+                    return
+                }
+                do {
+                    try cont.resume(returning: body())
+                } catch {
+                    cont.resume(throwing: error)
+                }
+            }
+        }
+    }
 }
 
 extension NIOThreadPool {

Tests/NIOPosixTests/NIOThreadPoolTest.swift

@@ -14,6 +14,7 @@
 
 import XCTest
 @testable import NIOPosix
+import Atomics
 import Dispatch
 import NIOConcurrencyHelpers
 import NIOEmbedded
@@ -110,6 +111,54 @@ class NIOThreadPoolTest: XCTestCase {
         }
     }
 
+    func testAsyncThreadPool() async throws {
+        guard #available(macOS 10.15, iOS 13.0, watchOS 6.0, tvOS 13.0, *) else { throw XCTSkip() }
+        let numberOfThreads = 1
+        let pool = NIOThreadPool(numberOfThreads: numberOfThreads)
+        pool.start()
+        do {
+            let hitCount = ManagedAtomic(false)
+            try await pool.runIfActive {
+                hitCount.store(true, ordering: .relaxed)
+            }
+            XCTAssertEqual(hitCount.load(ordering: .relaxed), true)
+        } catch {}
+        try await pool.shutdownGracefully()
+    }
+
+    func testAsyncThreadPoolErrorPropagation() async throws {
+        guard #available(macOS 10.15, iOS 13.0, watchOS 6.0, tvOS 13.0, *) else { throw XCTSkip() }
+        struct ThreadPoolError: Error {}
+        let numberOfThreads = 1
+        let pool = NIOThreadPool(numberOfThreads: numberOfThreads)
+        pool.start()
+        do {
+            try await pool.runIfActive {
+                throw ThreadPoolError()
+            }
+            XCTFail("Should not get here as closure sent to runIfActive threw an error")
+        } catch {
+            XCTAssertNotNil(error as? ThreadPoolError, "Error thrown should be of type ThreadPoolError")
+        }
+        try await pool.shutdownGracefully()
+    }
+
+    func testAsyncThreadPoolNotActiveError() async throws {
+        guard #available(macOS 10.15, iOS 13.0, watchOS 6.0, tvOS 13.0, *) else { throw XCTSkip() }
+        struct ThreadPoolError: Error {}
+        let numberOfThreads = 1
+        let pool = NIOThreadPool(numberOfThreads: numberOfThreads)
+        do {
+            try await pool.runIfActive {
+                throw ThreadPoolError()
+            }
+            XCTFail("Should not get here as thread pool isn't active")
+        } catch {
+            XCTAssertNotNil(error as? NIOThreadPoolError.ThreadPoolInactive, "Error thrown should be of type ThreadPoolError")
+        }
+        try await pool.shutdownGracefully()
+    }
+
     func testAsyncShutdownWorks() async throws {
         guard #available(macOS 10.15, iOS 13.0, watchOS 6.0, tvOS 13.0, *) else { throw XCTSkip() }
         let threadPool = NIOThreadPool(numberOfThreads: 17)