chore: import upstream snapshot with attribution

Sources/Containerization/BridgeStateFile.swift

@@ -0,0 +1,75 @@
+//===----------------------------------------------------------------------===//
+// Copyright © 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.
+//===----------------------------------------------------------------------===//
+
+import Foundation
+
+/// On-disk record of state `BridgeManager.create()` modified, used by
+/// `delete()` to restore the host. Stored at
+/// `/run/containerization/bridge-<name>.state` (tmpfs — gone after host
+/// reboot, which is fine because reboot already clears `ip_forward` and
+/// the bridge link itself).
+struct BridgeState: Codable, Equatable {
+    /// Whether `create()` programmed NAT (iptables MASQUERADE/FORWARD +
+    /// `ip_forward`). When `false`, the only thing `create()` did was bring
+    /// the bridge up — `delete()` only needs to remove the link, not roll
+    /// back NAT. State files predating this field are decoded as
+    /// `natEnabled = true` for back-compat.
+    let natEnabled: Bool
+
+    /// Value of `/proc/sys/net/ipv4/ip_forward` read at the *first*
+    /// `create()` call. Preserved across re-runs so `delete()` can restore
+    /// the host's true original value. Only set when `natEnabled`.
+    let prevIpForward: String?
+
+    /// Egress interface that `create()` used in the iptables rules — passed
+    /// explicitly by the caller, or auto-detected from `/proc/net/route`.
+    /// Only set when `natEnabled`. Recorded for debug / observability and
+    /// to scope the FORWARD rule's `-o` clause; rule removal is keyed off
+    /// subnet, bridge name, and egress.
+    let egressInterface: String?
+
+    init(natEnabled: Bool, prevIpForward: String? = nil, egressInterface: String? = nil) {
+        self.natEnabled = natEnabled
+        self.prevIpForward = prevIpForward
+        self.egressInterface = egressInterface
+    }
+
+    enum CodingKeys: String, CodingKey {
+        case natEnabled
+        case prevIpForward
+        case egressInterface
+    }
+
+    init(from decoder: Decoder) throws {
+        let container = try decoder.container(keyedBy: CodingKeys.self)
+        // Default natEnabled to true when missing so files written by older
+        // versions (which always programmed NAT) still describe themselves
+        // accurately — delete() will roll back ip_forward / iptables.
+        self.natEnabled = try container.decodeIfPresent(Bool.self, forKey: .natEnabled) ?? true
+        self.prevIpForward = try container.decodeIfPresent(String.self, forKey: .prevIpForward)
+        self.egressInterface = try container.decodeIfPresent(String.self, forKey: .egressInterface)
+    }
+
+    func encode() throws -> Data {
+        let encoder = JSONEncoder()
+        encoder.outputFormatting = [.prettyPrinted, .sortedKeys]
+        return try encoder.encode(self)
+    }
+
+    static func decode(_ data: Data) throws -> BridgeState {
+        try JSONDecoder().decode(BridgeState.self, from: data)
+    }
+}