add AcceptHandler after serverChannelInitializer runs (#1393)

Motivation:

For compatibility with 2.13.1 and earlier, we're adding the
AcceptHandler after the serverChannelInitializer runs. If that's the
best solution is discussed in #1392.

Modifications:

- add AcceptHandler after the serverChannelInitializer
- give the AcceptHandler the name AcceptHandler

Result:

- better compatibility

Sources/NIO/Bootstrap.swift

@@ -234,10 +234,11 @@ public final class ServerBootstrap {
 
         return eventLoop.submit {
             serverChannelOptions.applyAllChannelOptions(to: serverChannel).flatMap {
-                serverChannel.pipeline.addHandler(AcceptHandler(childChannelInitializer: childChannelInit,
-                                                                childChannelOptions: childChannelOptions))
-            }.flatMap {
                 serverChannelInit(serverChannel)
+            }.flatMap {
+                serverChannel.pipeline.addHandler(AcceptHandler(childChannelInitializer: childChannelInit,
+                                                                childChannelOptions: childChannelOptions),
+                                                  name: "AcceptHandler")
             }.flatMap {
                 register(eventLoop, serverChannel)
             }.map {

Tests/NIOTests/BootstrapTest+XCTest.swift

@@ -39,6 +39,7 @@ extension BootstrapTest {
                 ("testPreConnectedSocketSetsChannelOptionsBeforeChannelInitializer", testPreConnectedSocketSetsChannelOptionsBeforeChannelInitializer),
                 ("testDatagramBootstrapSetsChannelOptionsBeforeChannelInitializer", testDatagramBootstrapSetsChannelOptionsBeforeChannelInitializer),
                 ("testPipeBootstrapSetsChannelOptionsBeforeChannelInitializer", testPipeBootstrapSetsChannelOptionsBeforeChannelInitializer),
+                ("testServerBootstrapAddsAcceptHandlerAfterServerChannelInitialiser", testServerBootstrapAddsAcceptHandlerAfterServerChannelInitialiser),
            ]
    }
 }

Tests/NIOTests/BootstrapTest.swift

@@ -388,6 +388,43 @@ class BootstrapTest: XCTestCase {
             return []
         })
     }
+
+    func testServerBootstrapAddsAcceptHandlerAfterServerChannelInitialiser() {
+        // It's unclear if this is the right solution, see https://github.com/apple/swift-nio/issues/1392
+        let group = MultiThreadedEventLoopGroup(numberOfThreads: 1)
+        defer {
+            XCTAssertNoThrow(try group.syncShutdownGracefully())
+        }
+
+        struct FoundHandlerThatWasNotSupposedToBeThereError: Error {}
+
+        var maybeServer: Channel? = nil
+        XCTAssertNoThrow(maybeServer = try ServerBootstrap(group: group)
+            .serverChannelInitializer { channel in
+                // Here, we test that we can't find the AcceptHandler
+                return channel.pipeline.context(name: "AcceptHandler").flatMap { context -> EventLoopFuture<Void> in
+                    XCTFail("unexpectedly found \(context)")
+                    return channel.eventLoop.makeFailedFuture(FoundHandlerThatWasNotSupposedToBeThereError())
+                }.flatMapError { error -> EventLoopFuture<Void> in
+                    XCTAssertEqual(.notFound, error as? ChannelPipelineError)
+                    if case .some(.notFound) = error as? ChannelPipelineError {
+                        return channel.eventLoop.makeSucceededFuture(())
+                    }
+                    return channel.eventLoop.makeFailedFuture(error)
+                }
+            }
+            .bind(host: "127.0.0.1", port: 0)
+            .wait())
+
+        guard let server = maybeServer else {
+            XCTFail("couldn't bootstrap server")
+            return
+        }
+
+        // But now, it should be there.
+        XCTAssertNoThrow(_ = try server.pipeline.context(name: "AcceptHandler").wait())
+        XCTAssertNoThrow(try server.close().wait())
+    }
 }
 
 private final class MakeSureAutoReadIsOffInChannelInitializer:  ChannelInboundHandler {