Correctly hop across event loops during child channel setup. (#17)

Motivation:

We were inadvertently modifying the registration information of
NIOTSEventLoop objects from multiple threads, which is very not good.

Modifications:

Add an event loop execute to hop loops.

Result:

Thread safe!

Sources/NIOTransportServices/NIOTSEventLoop.swift

@@ -196,13 +196,13 @@ extension NIOTSEventLoop {
             throw EventLoopError.shutdown
         }
 
-        assert(channel.eventLoop === self)
+        channel.eventLoop.assertInEventLoop()
         self.registeredChannels[ObjectIdentifier(channel)] = channel
     }
 
     // We don't allow deregister to fail, as it doesn't make any sense.
     internal func deregister(_ channel: Channel) {
-        assert(channel.eventLoop === self)
+        channel.eventLoop.assertInEventLoop()
         let oldChannel = self.registeredChannels.removeValue(forKey: ObjectIdentifier(channel))
         assert(oldChannel != nil)
     }

Sources/NIOTransportServices/NIOTSListenerChannel.swift

@@ -339,11 +339,15 @@ extension NIOTSListenerChannel: StateManagedChannel {
     }
 
     public func channelRead0(_ data: NIOAny) {
+        self.eventLoop.assertInEventLoop()
+
         let channel = self.unwrapData(data, as: NIOTSConnectionChannel.self)
-        let p: EventLoopPromise<Void> = self.eventLoop.newPromise()
-        channel.registerAlreadyConfigured0(promise: p)
-        p.futureResult.whenFailure { (_: Error) in
-            channel.close(promise: nil)
+        let p: EventLoopPromise<Void> = channel.eventLoop.newPromise()
+        channel.eventLoop.execute {
+            channel.registerAlreadyConfigured0(promise: p)
+            p.futureResult.whenFailure { (_: Error) in
+                channel.close(promise: nil)
+            }
         }
     }
 

Tests/NIOTransportServicesTests/NIOTSListenerChannelTests.swift

@@ -156,4 +156,32 @@ class NIOTSListenerChannelTests: XCTestCase {
             XCTFail("Unexpected error")
         }
     }
+
+    func testCanSafelyInvokeChannelsAcrossThreads() throws {
+        // This is a test that aims to trigger TSAN violations.
+        let childGroup = NIOTSEventLoopGroup(loopCount: 2)
+        let childChannelPromise: EventLoopPromise<Channel> = childGroup.next().newPromise()
+        let activePromise: EventLoopPromise<Void> = childGroup.next().newPromise()
+
+        let listener = try NIOTSListenerBootstrap(group: self.group, childGroup: childGroup)
+            .childChannelInitializer { channel in
+                childChannelPromise.succeed(result: channel)
+                return channel.pipeline.add(handler: PromiseOnActiveHandler(activePromise))
+            }.bind(host: "localhost", port: 0).wait()
+        defer {
+            XCTAssertNoThrow(try listener.close().wait())
+        }
+
+        // Connect to the listener.
+        let channel = try NIOTSConnectionBootstrap(group: self.group)
+            .connect(to: listener.localAddress!).wait()
+
+        // Wait for the child channel to become active.
+        let childChannel = try childChannelPromise.futureResult.wait()
+        XCTAssertNoThrow(try activePromise.futureResult.wait())
+
+        // Now close the child channel.
+        XCTAssertNoThrow(try childChannel.close().wait())
+        XCTAssertNoThrow(try channel.closeFuture.wait())
+    }
 }