Support setting root disk synchronization mode (#875)

* Support setting root disk synchronization mode

Adds a new VMConfig parameter (tart get / tart set) called 'sync' which
can be set to 'full' (default), 'fsync', or 'none', corresponding with
the values of VZDiskImageSynchronizationMode and allowing a tradeoff
between data integrity and speed.

* Remove unused import

* Fix formatting

* Make root disk sync behaviour a commandline option

Sources/tart/Commands/Run.swift

@@ -12,6 +12,34 @@ var vm: VM?
 struct IPNotFound: Error {
 }
 
+extension VZDiskImageSynchronizationMode : LosslessStringConvertible {
+  public init?(_ description: String) {
+    switch description {
+    case "none":
+      self = .none
+    case "fsync":
+      self = .fsync
+    case "full":
+      self = .full
+    default:
+      return nil
+    }
+  }
+
+  public var description: String {
+    switch self {
+    case .none:
+      return "none"
+    case .fsync:
+      return "fsync"
+    case .full:
+      return "full"
+    @unknown default:
+      return "unknown"
+    }
+  }
+}
+
 struct Run: AsyncParsableCommand {
   static var configuration = CommandConfiguration(abstract: "Run a VM")
 
@@ -140,6 +168,14 @@ struct Run: AsyncParsableCommand {
   @Flag(help: ArgumentHelp("Restrict network access to the host-only network"))
   var netHost: Bool = false
 
+  @Option(help: ArgumentHelp("Set the root disk synchronization mode (Linux images only). Possible values: none, fsync, full.",
+                             discussion: """
+                             'full' synchronizes data with the drive and ensures that it is written to permanent storage.
+                             'fsync' synchronizes data with the drive but doesn't guarantee that it is written to permanent storage.
+                             'none' doesn't synchronize data with the drive.
+                             """))
+  var sync: String = "full"
+
   #if arch(arm64)
     @Flag(help: ArgumentHelp("Disables audio and entropy devices and switches to only Mac-specific input devices.", discussion: "Useful for running a VM that can be suspended via \"tart suspend\"."))
   #endif
@@ -195,6 +231,10 @@ struct Run: AsyncParsableCommand {
         throw ValidationError("Seems you have a disk targeting x86 architecture (hence amd64 in the name). Please use an 'arm64' version of the disk.")
       }
     }
+
+    if(VZDiskImageSynchronizationMode(sync) == nil) {
+      throw ValidationError("Invalid disk synchronization mode: \(sync)")
+    }
   }
 
   @MainActor
@@ -247,7 +287,8 @@ struct Run: AsyncParsableCommand {
       serialPorts: serialPorts,
       suspendable: suspendable,
       audio: !noAudio,
-      clipboard: !noClipboard
+      clipboard: !noClipboard,
+      sync: VZDiskImageSynchronizationMode(sync) ?? .full
     )
 
     let vncImpl: VNC? = try {

Sources/tart/VM.swift

@@ -48,7 +48,8 @@ class VM: NSObject, VZVirtualMachineDelegate, ObservableObject {
        serialPorts: [VZSerialPortConfiguration] = [],
        suspendable: Bool = false,
        audio: Bool = true,
-       clipboard: Bool = true
+       clipboard: Bool = true,
+       sync: VZDiskImageSynchronizationMode = .full
   ) throws {
     name = vmDir.name
     config = try VMConfig.init(fromURL: vmDir.configURL)
@@ -66,7 +67,8 @@ class VM: NSObject, VZVirtualMachineDelegate, ObservableObject {
                                                 serialPorts: serialPorts,
                                                 suspendable: suspendable,
                                                 audio: audio,
-                                                clipboard: clipboard
+                                                clipboard: clipboard,
+                                                sync: sync
     )
     virtualMachine = VZVirtualMachine(configuration: configuration)
 
@@ -294,7 +296,8 @@ class VM: NSObject, VZVirtualMachineDelegate, ObservableObject {
     serialPorts: [VZSerialPortConfiguration],
     suspendable: Bool = false,
     audio: Bool = true,
-    clipboard: Bool = true
+    clipboard: Bool = true,
+    sync: VZDiskImageSynchronizationMode = .full
   ) throws -> VZVirtualMachineConfiguration {
     let configuration = VZVirtualMachineConfiguration()
 
@@ -350,7 +353,7 @@ class VM: NSObject, VZVirtualMachineDelegate, ObservableObject {
     // Storage
     let attachment: VZDiskImageStorageDeviceAttachment = vmConfig.os == .linux ?
       // Use "cached" caching mode for virtio drive to prevent fs corruption on linux
-      try VZDiskImageStorageDeviceAttachment(url: diskURL, readOnly: false, cachingMode: .cached, synchronizationMode: .full) :
+      try VZDiskImageStorageDeviceAttachment(url: diskURL, readOnly: false, cachingMode: .cached, synchronizationMode: sync) :
       try VZDiskImageStorageDeviceAttachment(url: diskURL, readOnly: false)
 
     var device: VZStorageDeviceConfiguration