add a future-returning shutdown method

Author: Johannes Weiss

Sources/AsyncHTTPClient/AsyncAwait/HTTPClient+shutdown.swift

@@ -12,6 +12,27 @@
 //
 //===----------------------------------------------------------------------===//
 
+import NIOCore
+
+extension HTTPClient {
+    public func shutdown() -> EventLoopFuture<Void> {
+        switch self.eventLoopGroupProvider {
+        case .shared(let group):
+            let promise = group.any().makePromise(of: Void.self)
+            self.shutdown(queue: .global()) { error in
+                if let error = error {
+                    promise.fail(error)
+                } else {
+                    promise.succeed(())
+                }
+            }
+            return promise.futureResult
+        case .createNew:
+            preconditionFailure("Cannot use the shutdown() method which returns a future when owning the EventLoopGroup. Please use the one of the other shutdown methods.")
+        }
+    }
+}
+
 #if compiler(>=5.5.2) && canImport(_Concurrency)
 
 extension HTTPClient {

Tests/AsyncHTTPClientTests/HTTPClientTests.swift

@@ -3463,4 +3463,9 @@ class HTTPClientTests: XCTestCase {
         XCTAssertEqual(response?.version, .http1_1)
         XCTAssertEqual(response?.body?.readableBytes, 10_000)
     }
+
+    func testShutdownWithFutures() {
+        let httpClient = HTTPClient(eventLoopGroupProvider: .shared(self.clientGroup))
+        XCTAssertNoThrow(try httpClient.shutdown().wait())
+    }
 }