Monitor Softnet process and throw if it terminates prematurely (#270)

Package.resolved

@@ -54,6 +54,15 @@
         "revision" : "f05e450f0b909c0e80670a47516c4b9700b9e5da"
       }
     },
+    {
+      "identity" : "swift-atomics",
+      "kind" : "remoteSourceControl",
+      "location" : "https://github.com/apple/swift-atomics.git",
+      "state" : {
+        "revision" : "919eb1d83e02121cdb434c7bfc1f0c66ef17febe",
+        "version" : "1.0.2"
+      }
+    },
     {
       "identity" : "swift-collections",
       "kind" : "remoteSourceControl",

Package.swift

@@ -17,6 +17,7 @@ let package = Package(
     .package(url: "https://github.com/malcommac/SwiftDate", from: "6.3.1"),
     .package(url: "https://github.com/sushichop/Puppy", from: "0.5.1"),
     .package(url: "https://github.com/antlr/antlr4", branch: "dev"),
+    .package(url: "https://github.com/apple/swift-atomics.git", .upToNextMajor(from: "1.0.0")),
   ],
   targets: [
     .executableTarget(name: "tart", dependencies: [
@@ -27,6 +28,7 @@ let package = Package(
       .product(name: "SwiftDate", package: "SwiftDate"),
       .product(name: "Puppy", package: "Puppy"),
       .product(name: "Antlr4Static", package: "Antlr4"),
+      .product(name: "Atomics", package: "swift-atomics"),
     ], exclude: [
       "OCI/Reference/Makefile",
       "OCI/Reference/Reference.g4",

Sources/tart/Network/Network.swift

@@ -2,6 +2,6 @@ import Virtualization
 
 protocol Network {
   func attachment() -> VZNetworkDeviceAttachment
-  func run() throws
-  func stop() throws
+  func run(_ sema: DispatchSemaphore) throws
+  func stop() async throws
 }

Sources/tart/Network/NetworkBridged.swift

@@ -12,11 +12,11 @@ class NetworkBridged: Network {
         VZBridgedNetworkDeviceAttachment(interface: interface)
     }
 
-    func run() throws {
+    func run(_ sema: DispatchSemaphore) throws {
         // no-op, only used for Softnet
     }
 
-    func stop() throws {
+    func stop() async throws {
         // no-op, only used for Softnet
     }
 }

Sources/tart/Network/NetworkShared.swift

@@ -6,11 +6,11 @@ class NetworkShared: Network {
         VZNATNetworkDeviceAttachment()
     }
 
-    func run() throws {
+    func run(_ sema: DispatchSemaphore) throws {
         // no-op, only used for Softnet
     }
 
-    func stop() throws {
+    func stop() async throws {
         // no-op, only used for Softnet
     }
 }

Sources/tart/Network/Softnet.swift

@@ -1,12 +1,16 @@
 import Foundation
 import Virtualization
+import Atomics
 
 enum SoftnetError: Error {
   case InitializationFailed(why: String)
+  case RuntimeFailed(why: String)
 }
 
 class Softnet: Network {
   private let process = Process()
+  private var monitorTask: Task<Void, Error>? = nil
+  private let monitorTaskFinished = ManagedAtomic<Bool>(false)
 
   let vmFD: Int32
 
@@ -35,13 +39,33 @@ class Softnet: Network {
     process.standardInput = FileHandle(fileDescriptor: softnetFD, closeOnDealloc: false)
   }
 
-  func run() throws {
+  func run(_ sema: DispatchSemaphore) throws {
     try process.run()
+
+    monitorTask = Task {
+      // Wait for the Softnet to finish
+      process.waitUntilExit()
+
+      // Signal to the caller that the Softnet has finished
+      sema.signal()
+
+      // Signal to ourselves that the Softnet has finished
+      monitorTaskFinished.store(true, ordering: .sequentiallyConsistent)
+    }
   }
 
-  func stop() throws {
-    process.interrupt()
-    process.waitUntilExit()
+  func stop() async throws {
+    if monitorTaskFinished.load(ordering: .sequentiallyConsistent) {
+      // Consume the monitor task's value to ensure the task has finished
+      _ = try await monitorTask?.value
+
+      throw SoftnetError.RuntimeFailed(why: "Softnet process terminated prematurely")
+    } else {
+      process.interrupt()
+
+      // Consume the monitor task's value to ensure the task has finished
+      _ = try await monitorTask?.value
+    }
   }
 
   private func setSocketBuffers(_ fd: Int32, _ sizeBytes: Int) throws {

Sources/tart/VM.swift

@@ -209,7 +209,7 @@ class VM: NSObject, VZVirtualMachineDelegate, ObservableObject {
   }
 
   func run(_ recovery: Bool) async throws {
-    try network.run()
+    try network.run(sema)
 
     DispatchQueue.main.sync {
       Task {
@@ -239,7 +239,7 @@ class VM: NSObject, VZVirtualMachineDelegate, ObservableObject {
       }
     }
 
-    try network.stop()
+    try await network.stop()
   }
 
   static func craftConfiguration(