Add a helper for setting or cascading optional promises (#2697)

Motivation:

Many operations accept an optional promise. It's not uncommon to batch
operations (which may each have their own promise) and complete them
as a single operation. Combining these optional promises is slightly
tedious.

Modifications:

- Add an extension to `Optional` to set or cascade a promise
- If a promise exists, its result is cascaded to the provided promise.
  Otherwise the optional is set to the provided promise.

Result:

It's easier to combine optional promises.

Sources/NIOCore/EventLoopFuture.swift

@@ -1840,3 +1840,23 @@ extension EventLoopPromise where Value == Void {
  succeed(Void())
  }
 }
+
+extension Optional {
+ /// Sets or cascades the future result of self to the provided promise, if present.
+ ///
+ /// If `promise` is `nil` then this function is a no-op. Otherwise, if `self` is `nil` then
+ /// `self` is set to `promise`. If `self` isn't `nil` then its `futureResult` will be cascaded
+ /// to `promise`.
+ ///
+ /// - Parameter promise: The promise to set or cascade to.
+ public mutating func setOrCascade<Value>(to promise: EventLoopPromise<Value>?) where Wrapped == EventLoopPromise<Value> {
+ guard let promise = promise else { return }
+
+ switch self {
+ case .none:
+ self = .some(promise)
+ case .some(let existing):
+ existing.futureResult.cascade(to: promise)
+ }
+ }
+}

Tests/NIOPosixTests/EventLoopFutureTest.swift

@@ -1541,4 +1541,33 @@ class EventLoopFutureTest : XCTestCase {
  }
  }
 
+ func testSetOrCascadeReplacesNil() throws {
+ let eventLoop = EmbeddedEventLoop()
+
+ var promise: EventLoopPromise<Void>? = nil
+ let other = eventLoop.makePromise(of: Void.self)
+ promise.setOrCascade(to: other)
+ XCTAssertNotNil(promise)
+ promise?.succeed()
+ try other.futureResult.wait()
+ }
+
+ func testSetOrCascadeCascadesToExisting() throws {
+ let eventLoop = EmbeddedEventLoop()
+
+ var promise: EventLoopPromise<Void>? = eventLoop.makePromise(of: Void.self)
+ let other = eventLoop.makePromise(of: Void.self)
+ promise.setOrCascade(to: other)
+ promise?.succeed()
+ try other.futureResult.wait()
+ }
+
+ func testSetOrCascadeNoOpOnNil() throws {
+ let eventLoop = EmbeddedEventLoop()
+
+ var promise: EventLoopPromise<Void>? = eventLoop.makePromise(of: Void.self)
+ promise.setOrCascade(to: nil)
+ XCTAssertNotNil(promise)
+ promise?.succeed()
+ }
 }