fix EventLoopFuture.and's serious threading issues

Motivation:

EventLoopFuture.and had serious threading issues if the EventLoops
weren't the same.

Modifications:

Fixed the threading issues and tested them properly.

Result:

Hopefully `and` and `andAll` now don't crash if you use them across
EventLoops.

Sources/NIO/EventLoopFuture.swift

@@ -635,43 +635,75 @@ extension EventLoopFuture {
     /// of results. If either one fails, the combined `EventLoopFuture` will fail with
     /// the first error encountered.
     public func and<U>(_ other: EventLoopFuture<U>, file: StaticString = #file, line: UInt = #line) -> EventLoopFuture<(T,U)> {
-        let andlock = Lock()
         let promise = EventLoopPromise<(T,U)>(eventLoop: eventLoop, file: file, line: line)
         var tvalue: T?
         var uvalue: U?
-
-        _whenComplete { () -> CallbackList in
-            switch self.value! {
-            case .failure(let error):
-                return promise._setValue(value: .failure(error))
-            case .success(let t):
-                andlock.lock()
-                if let u = uvalue {
-                    andlock.unlock()
-                    return promise._setValue(value: .success((t, u)))
-                } else {
-                    andlock.unlock()
-                    tvalue = t
+        
+        if self.eventLoop === other.eventLoop {
+            assert(self.eventLoop === other.eventLoop)
+            _whenComplete { () -> CallbackList in
+                switch self.value! {
+                case .failure(let error):
+                    return promise._setValue(value: .failure(error))
+                case .success(let t):
+                    if let u = uvalue {
+                        return promise._setValue(value: .success((t, u)))
+                    } else {
+                        tvalue = t
+                    }
                 }
+                return CallbackList()
             }
-            return CallbackList()
-        }
-
-        other._whenComplete { () -> CallbackList in
-            switch other.value! {
-            case .failure(let error):
-                return promise._setValue(value: .failure(error))
-            case .success(let u):
-                andlock.lock()
-                if let t = tvalue {
-                    andlock.unlock()
-                    return promise._setValue(value: .success((t, u)))
-                } else {
-                    andlock.unlock()
-                    uvalue = u
+
+            other._whenComplete { () -> CallbackList in
+                switch other.value! {
+                case .failure(let error):
+                    return promise._setValue(value: .failure(error))
+                case .success(let u):
+                    if let t = tvalue {
+                        return promise._setValue(value: .success((t, u)))
+                    } else {
+                        uvalue = u
+                    }
                 }
+                return CallbackList()
+            }
+        } else {
+            let andlock = Lock()
+
+            _whenComplete { () -> CallbackList in
+                switch self.value! {
+                case .failure:
+                    self.cascadeFailure(promise: promise)
+                case .success(let t):
+                    andlock.lock()
+                    if let u = uvalue {
+                        andlock.unlock()
+                        promise.succeed(result: (t, u))
+                    } else {
+                        tvalue = t
+                        andlock.unlock()
+                    }
+                }
+                return CallbackList()
+            }
+
+            other._whenComplete { () -> CallbackList in
+                switch other.value! {
+                case .failure:
+                    other.cascadeFailure(promise: promise)
+                case .success(let u):
+                    andlock.lock()
+                    if let t = tvalue {
+                        andlock.unlock()
+                        promise.succeed(result: (t, u))
+                    } else {
+                        uvalue = u
+                        andlock.unlock()
+                    }
+                }
+                return CallbackList()
             }
-            return CallbackList()
         }
 
         return promise.futureResult

Tests/NIOTests/EventLoopFutureTest+XCTest.swift

@@ -38,6 +38,9 @@ extension EventLoopFutureTest {
                 ("testOrderOfFutureCompletion", testOrderOfFutureCompletion),
                 ("testEventLoopHoppingInThen", testEventLoopHoppingInThen),
                 ("testEventLoopHoppingInThenWithFailures", testEventLoopHoppingInThenWithFailures),
+                ("testEventLoopHoppingAndAll", testEventLoopHoppingAndAll),
+                ("testEventLoopHoppingAndAllWithFailures", testEventLoopHoppingAndAllWithFailures),
+                ("testFutureInVariousScenarios", testFutureInVariousScenarios),
            ]
    }
 }

Tests/NIOTests/EventLoopFutureTest.swift

@@ -213,7 +213,7 @@ class EventLoopFutureTest : XCTestCase {
         let n = 20
         let elg = MultiThreadedEventLoopGroup(numThreads: n)
         var prev: EventLoopFuture<Int> = elg.next().newSucceededFuture(result: 0)
-        (1..<20).forEach { (i: Int) in
+        (1..<n).forEach { (i: Int) in
             let p: EventLoopPromise<Int> = elg.next().newPromise()
             prev.then { (i2: Int) -> EventLoopFuture<Int> in
                 XCTAssertEqual(i - 1, i2)
@@ -241,4 +241,143 @@ class EventLoopFutureTest : XCTestCase {
         }
         XCTAssertNoThrow(try elg.syncShutdownGracefully())
     }
+
+    func testEventLoopHoppingAndAll() throws {
+        let n = 20
+        let elg = MultiThreadedEventLoopGroup(numThreads: n)
+        let ps = (0..<n).map { (_: Int) -> EventLoopPromise<()> in
+            elg.next().newPromise()
+        }
+        let allOfEm = EventLoopFuture<()>.andAll(ps.map { $0.futureResult }, eventLoop: elg.next())
+        ps.reversed().forEach { p in
+            DispatchQueue.global().async {
+                p.succeed(result: ())
+            }
+        }
+        try allOfEm.wait()
+        XCTAssertNoThrow(try elg.syncShutdownGracefully())
+    }
+
+    func testEventLoopHoppingAndAllWithFailures() throws {
+        enum DummyError: Error { case dummy }
+        let n = 20
+        let fireBackEl = MultiThreadedEventLoopGroup(numThreads: 1)
+        let elg = MultiThreadedEventLoopGroup(numThreads: n)
+        let ps = (0..<n).map { (_: Int) -> EventLoopPromise<()> in
+            elg.next().newPromise()
+        }
+        let allOfEm = EventLoopFuture<()>.andAll(ps.map { $0.futureResult }, eventLoop: fireBackEl.next())
+        ps.reversed().enumerated().forEach { idx, p in
+            DispatchQueue.global().async {
+                if idx == n / 2 {
+                    p.fail(error: DummyError.dummy)
+                } else {
+                    p.succeed(result: ())
+                }
+            }
+        }
+        do {
+            try allOfEm.wait()
+            XCTFail("unexpected failure")
+        } catch _ as DummyError {
+            // ok
+        } catch {
+            XCTFail("unexpected error: \(error)")
+        }
+        XCTAssertNoThrow(try elg.syncShutdownGracefully())
+        XCTAssertNoThrow(try fireBackEl.syncShutdownGracefully())
+    }
+
+    func testFutureInVariousScenarios() throws {
+        enum DummyError: Error { case dummy0; case dummy1 }
+        let elg = MultiThreadedEventLoopGroup(numThreads: 2)
+        let el1 = elg.next()
+        let el2 = elg.next()
+        precondition(el1 !== el2)
+        let q1 = DispatchQueue(label: "q1")
+        let q2 = DispatchQueue(label: "q2")
+
+        // this determines which promise is fulfilled first (and (true, true) meaning they race)
+        for whoGoesFirst in [(false, true), (true, false), (true, true)] {
+            // this determines what EventLoops the Promises are created on
+            for eventLoops in [(el1, el1), (el1, el2), (el2, el1), (el2, el2)] {
+                // this determines if the promises fail or succeed
+                for whoSucceeds in [(false, false), (false, true), (true, false), (true, true)] {
+                    let p0: EventLoopPromise<Int> = eventLoops.0.newPromise()
+                    let p1: EventLoopPromise<String> = eventLoops.1.newPromise()
+                    let fAll = p0.futureResult.and(p1.futureResult)
+
+                    // preheat both queues so we have a better chance of racing
+                    let sem1 = DispatchSemaphore(value: 0)
+                    let sem2 = DispatchSemaphore(value: 0)
+                    let g = DispatchGroup()
+                    q1.async(group: g) {
+                        sem2.signal()
+                        sem1.wait()
+                    }
+                    q2.async(group: g) {
+                        sem1.signal()
+                        sem2.wait()
+                    }
+                    g.wait()
+
+                    if whoGoesFirst.0 {
+                        q1.async {
+                            if whoSucceeds.0 {
+                                p0.succeed(result: 7)
+                            } else {
+                                p0.fail(error: DummyError.dummy0)
+                            }
+                            if !whoGoesFirst.1 {
+                                q2.asyncAfter(deadline: .now() + 0.1) {
+                                    if whoSucceeds.1 {
+                                        p1.succeed(result: "hello")
+                                    } else {
+                                        p1.fail(error: DummyError.dummy1)
+                                    }
+                                }
+                            }
+                        }
+                    }
+                    if whoGoesFirst.1 {
+                        q2.async {
+                            if whoSucceeds.1 {
+                                p1.succeed(result: "hello")
+                            } else {
+                                p1.fail(error: DummyError.dummy1)
+                            }
+                            if !whoGoesFirst.0 {
+                                q1.asyncAfter(deadline: .now() + 0.1) {
+                                    if whoSucceeds.0 {
+                                        p0.succeed(result: 7)
+                                    } else {
+                                        p0.fail(error: DummyError.dummy0)
+                                    }
+                                }
+                            }
+                        }
+                    }
+                    do {
+                        let result = try fAll.wait()
+                        if !whoSucceeds.0 || !whoSucceeds.1 {
+                            XCTFail("unexpected success")
+                        } else {
+                            XCTAssert((7, "hello") == result)
+                        }
+                    } catch let e as DummyError {
+                        switch e {
+                        case .dummy0:
+                            XCTAssertFalse(whoSucceeds.0)
+                        case .dummy1:
+                            XCTAssertFalse(whoSucceeds.1)
+                        }
+                    } catch {
+                        XCTFail("unexpected error: \(error)")
+                    }
+                }
+            }
+        }
+
+        XCTAssertNoThrow(try elg.syncShutdownGracefully())
+    }
 }