Add an always method on EventLoopFuture

Sources/NIO/EventLoopFuture.swift

@@ -1267,3 +1267,18 @@ func executeAndComplete<Value>(_ promise: EventLoopPromise<Value>?, _ body: () t
         promise?.fail(e)
     }
 }
+
+
+extension EventLoopFuture {
+    /// Adds an observer callback to this `EventLoopFuture` that is called when the
+    /// `EventLoopFuture` has any result.
+    ///
+    /// - parameters:
+    ///     - callback: the callback that is called when the `EventLoopFuture` is fulfilled.   
+    /// - returns: the current `EventLoopFuture`
+    public func always(_ callback: @escaping () -> Void) -> EventLoopFuture<Value> {
+        self.whenComplete { _ in callback() }
+        return self
+    }
+
+}

Tests/NIOTests/EventLoopFutureTest+XCTest.swift

@@ -70,6 +70,8 @@ extension EventLoopFutureTest {
                 ("testWhenAllCompleteResultsWithFailuresStillSucceed", testWhenAllCompleteResultsWithFailuresStillSucceed),
                 ("testWhenAllCompleteResults", testWhenAllCompleteResults),
                 ("testWhenAllCompleteResolvesAfterFutures", testWhenAllCompleteResolvesAfterFutures),
+                ("testAlways", testAlways),
+                ("testAlwaysWithFailingPromise", testAlwaysWithFailingPromise),
            ]
    }
 }

Tests/NIOTests/EventLoopFutureTest.swift

@@ -1015,4 +1015,44 @@ class EventLoopFutureTest : XCTestCase {
         let results = try assertNoThrowWithValue(mainFuture.wait().map { try $0.get() })
         XCTAssertEqual(results, [0, 1, 2, 3, 4])
     }
+
+    struct DatabaseError: Error {}
+    struct Database {
+        let query: () -> EventLoopFuture<[String]>
+
+        var closed = false
+
+        init(query: @escaping () -> EventLoopFuture<[String]>) {
+            self.query = query
+        }
+
+        func runQuery() -> EventLoopFuture<[String]> {
+            return query()
+        }
+
+        mutating func close() {
+            self.closed = true
+        }
+    }
+
+    func testAlways() throws {
+        let group = EmbeddedEventLoop()
+        let loop = group.next()
+        var db = Database { loop.makeSucceededFuture(["Item 1", "Item 2", "Item 3"]) }
+
+        XCTAssertFalse(db.closed)
+        let _ = try assertNoThrowWithValue(db.runQuery().always { db.close() }.map { $0.map { $0.uppercased() }}.wait())
+        XCTAssertTrue(db.closed)
+    }
+
+    func testAlwaysWithFailingPromise() throws {
+        let group = EmbeddedEventLoop()
+        let loop = group.next()
+        var db = Database { loop.makeFailedFuture(DatabaseError()) }
+
+        XCTAssertFalse(db.closed)
+        let _ = try XCTAssertThrowsError(db.runQuery().always { db.close() }.map { $0.map { $0.uppercased() }}.wait()) { XCTAssertTrue($0 is DatabaseError) }
+        XCTAssertTrue(db.closed)
+
+    }
 }