Fix: Kubes Cluster in Container Crashing Container (IS#923) (#930)

- Fixes issue #923 
- I fixed a race condition in `ConnectHandler.swift` where
  an asynchronous network connection could complete
  after the handler had already been removed from the
  pipeline.
- This prevents the EXC_BREAKPOINT crash in
  container-runtime-linux that occurred when kinc
  (Kubernetes in Container) created rapid connections.
- The actual fix was inadvertently applied in #957, so this
  PR contains only the test code.

Tests/SocketForwarderTests/ConnectHandlerRaceTest.swift

@@ -0,0 +1,66 @@
+//===----------------------------------------------------------------------===//
+// Copyright © 2025 Apple Inc. and the container project authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//   https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//===----------------------------------------------------------------------===//
+
+import NIO
+import Testing
+
+@testable import SocketForwarder
+
+struct ConnectHandlerRaceTest {
+    let eventLoopGroup = MultiThreadedEventLoopGroup(numberOfThreads: System.coreCount)
+
+    @Test
+    func testRapidConnectDisconnect() async throws {
+        let requestCount = 500
+
+        let serverAddress = try SocketAddress(ipAddress: "127.0.0.1", port: 0)
+        let server = TCPEchoServer(serverAddress: serverAddress, eventLoopGroup: eventLoopGroup)
+        let serverChannel = try await server.run().get()
+        let actualServerAddress = try #require(serverChannel.localAddress)
+
+        let proxyAddress = try SocketAddress(ipAddress: "127.0.0.1", port: 0)
+        let forwarder = try TCPForwarder(
+            proxyAddress: proxyAddress,
+            serverAddress: actualServerAddress,
+            eventLoopGroup: eventLoopGroup
+        )
+        let forwarderResult = try await forwarder.run().get()
+        let actualProxyAddress = try #require(forwarderResult.proxyAddress)
+
+        try await withThrowingTaskGroup(of: Void.self) { group in
+            for _ in 0..<requestCount {
+                group.addTask {
+                    do {
+                        let channel = try await ClientBootstrap(group: self.eventLoopGroup)
+                            .connect(to: actualProxyAddress)
+                            .get()
+
+                        try await channel.close()
+                    } catch {
+                        // Going to ignore connection errors as we are intentionally stressing it
+                    }
+                }
+            }
+            try await group.waitForAll()
+        }
+
+        serverChannel.eventLoop.execute { _ = serverChannel.close() }
+        try await serverChannel.closeFuture.get()
+
+        forwarderResult.close()
+        try await forwarderResult.wait()
+    }
+}