Add flatSubmit method to EventLoop (#426) (#1174)

Motivation:

Calling EventLoop's submit method with a task that returns an EventLoopFuture results in a double future. flatSubmit provides an alternative which does not return a double future.

Modifications:

Added flatSubmit() to the EventLoop protocol and provided a default implementation that returns the result of calling flatMap { $0 } on the EventLoopFuture returned from task.

Result:

Additive change only. flatSubmit method on EventLoop will be available for use.

Sources/NIO/EventLoop.swift

@@ -465,6 +465,18 @@ extension EventLoop {
         return promise.futureResult
     }
 
+    /// Submit `task` to be run on this `EventLoop`.
+    ///
+    /// The returned `EventLoopFuture` will be completed when `task` has finished running. It will be identical to
+    /// the `EventLoopFuture` returned by `task`.
+    ///
+    /// - parameters:
+    ///     - task: The synchronous task to run. As everything that runs on the `EventLoop`, it must not block.
+    /// - returns: An `EventLoopFuture` identical to the `EventLooopFuture` returned from `task`.
+    public func flatSubmit<T>(_ task: @escaping () -> EventLoopFuture<T>) -> EventLoopFuture<T> {
+        return submit(task).flatMap { $0 }
+    }
+
     /// Creates and returns a new `EventLoopPromise` that will be notified using this `EventLoop` as execution `NIOThread`.
     public func makePromise<T>(of type: T.Type = T.self, file: StaticString = #file, line: UInt = #line) -> EventLoopPromise<T> {
         return EventLoopPromise<T>(eventLoop: self, file: file, line: line)

Tests/NIOTests/EventLoopTest+XCTest.swift

@@ -60,6 +60,8 @@ extension EventLoopTest {
                 ("testEdgeCasesNIODeadlineMinusNIODeadline", testEdgeCasesNIODeadlineMinusNIODeadline),
                 ("testEdgeCasesNIODeadlinePlusTimeAmount", testEdgeCasesNIODeadlinePlusTimeAmount),
                 ("testEdgeCasesNIODeadlineMinusTimeAmount", testEdgeCasesNIODeadlineMinusTimeAmount),
+                ("testSuccessfulFlatSubmit", testSuccessfulFlatSubmit),
+                ("testFailingFlatSubmit", testFailingFlatSubmit),
            ]
    }
 }

Tests/NIOTests/EventLoopTest.swift

@@ -928,4 +928,26 @@ public final class EventLoopTest : XCTestCase {
             }
         }
     }
+
+    func testSuccessfulFlatSubmit() {
+        let eventLoop = EmbeddedEventLoop()
+        let future = eventLoop.flatSubmit {
+            eventLoop.makeSucceededFuture(1)
+        }
+        eventLoop.run()
+        XCTAssertNoThrow(XCTAssertEqual(1, try future.wait()))
+    }
+
+    func testFailingFlatSubmit() {
+        enum TestError: Error { case failed }
+
+        let eventLoop = EmbeddedEventLoop()
+        let future = eventLoop.flatSubmit { () -> EventLoopFuture<Int> in
+            eventLoop.makeFailedFuture(TestError.failed)
+        }
+        eventLoop.run()
+        XCTAssertThrowsError(try future.wait()) { error in
+            XCTAssertEqual(.failed, error as? TestError)
+        }
+    }
 }