chore: import upstream snapshot with attribution

Sources/ContainerXPC/XPCClientSession.swift

@@ -0,0 +1,53 @@
+//===----------------------------------------------------------------------===//
+// Copyright © 2026 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.
+//===----------------------------------------------------------------------===//
+
+#if os(macOS)
+import Synchronization
+
+/// Represents a long-lived connection to an XPC service on the client side.
+///
+/// Obtain one via `XPCClient.openSession()`. The disconnect handler is
+/// installed at initialisation time, before the first `send()`, so there is
+/// no window in which a server crash goes undetected.
+public final class XPCClientSession: Sendable {
+    private let client: XPCClient
+    private let handlers: Mutex<[@Sendable () async -> Void]> = Mutex([])
+
+    init(client: XPCClient) {
+        self.client = client
+        client.setDisconnectHandler { [weak self] in
+            guard let self else { return }
+            let snapshot = self.handlers.withLock { $0 }
+            Task { for handler in snapshot { await handler() } }
+        }
+    }
+
+    /// Register a handler to be called when the server disconnects.
+    public func onDisconnect(_ handler: @Sendable @escaping () async -> Void) {
+        handlers.withLock { $0.append(handler) }
+    }
+
+    /// Send a message over the persistent connection.
+    @discardableResult
+    public func send(_ message: XPCMessage, responseTimeout: Duration? = nil) async throws -> XPCMessage {
+        try await client.send(message, responseTimeout: responseTimeout)
+    }
+
+    /// Cancel the underlying connection.
+    public func close() { client.close() }
+}
+
+#endif