chore: import upstream snapshot with attribution

Tests/ContainerizationNetlinkTests/MockNetlinkSocket.swift

@@ -0,0 +1,57 @@
+//===----------------------------------------------------------------------===//
+// Copyright © 2025-2026 Apple Inc. and the Containerization 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.
+//===----------------------------------------------------------------------===//
+
+//
+
+@testable import ContainerizationNetlink
+
+class MockNetlinkSocket: NetlinkSocket {
+    static let ENOMEM: Int32 = 12
+    static let EOVERFLOW: Int32 = 75
+
+    var pid: UInt32 = 0
+
+    var requests: [[UInt8]] = []
+    var responses: [[UInt8]] = []
+
+    var responseIndex = 0
+
+    public init() throws {}
+
+    public func send(buf: UnsafeRawPointer!, len: Int, flags: Int32) throws -> Int {
+        let ptr = buf.bindMemory(to: UInt8.self, capacity: len)
+        requests.append(Array(UnsafeBufferPointer(start: ptr, count: len)))
+        return len
+    }
+
+    public func recv(buf: UnsafeMutableRawPointer!, len: Int, flags: Int32) throws -> Int {
+        guard responseIndex < responses.count else {
+            throw NetlinkSocketError.recvFailure(rc: Self.ENOMEM)
+        }
+
+        let response = responses[responseIndex]
+        guard len >= response.count else {
+            throw NetlinkSocketError.recvFailure(rc: 75)
+        }
+
+        response.withUnsafeBytes { bytes in
+            buf.copyMemory(from: bytes.baseAddress!, byteCount: response.count)
+        }
+
+        responseIndex += 1
+        return response.count
+    }
+}