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

Sources/SocketForwarder/ConnectHandler.swift

@@ -81,6 +81,11 @@ extension ConnectHandler {
             .whenComplete { result in
                 switch result {
                 case .success(let channel):
+                    guard context.channel.isActive else {
+                        self.log?.trace("backend - frontend channel closed, closing backend connection")
+                        channel.close(promise: nil)
+                        return
+                    }
                     self.log?.trace("backend - connected")
                     self.glue(channel, context: context)
                 case .failure(let error):

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()
+    }
+}