Allow mounting a single directory without a name (#555)

* Allow mounting a single directory without a name

To utilize `VZSingleDirectoryShare` which seems more stable than `VZMultipleDirectoryShare`.

We've been having reports from users that mounted directories occasionally return "no such file" errors when building large projects. I took a stab at reproducing the issue by running https://github.com/devMEremenko/XcodeBenchmark in a mounted directory:

```bash
tart run --dir=workdir-test:~/workspace-temp/XcodeBenchmark ventura-xcode
```

And I was able to reproduce the "no such file" error on the first try! After looking into the issue I decided to try `VZSingleDirectoryShare` as this PR changes and to my pleasant surprise it all worked like a charm the next run. So it seems there is a bug in `VZMultipleDirectoryShare` integration with virtiofs. Since in most cases users only mount a single directory it makes sense to allow doing it wihtout providing a `name`.

So now it will be possible to run the following command:

```bash
tart run --dir=~/workspace-temp/XcodeBenchmark ventura-xcode
```

Which will make `~/workspace-temp/XcodeBenchmark` available under `/Volumes/My Shared Files/` without any intermediate directories.

* Reformat

* Updated description

Sources/tart/Commands/Run.swift

@@ -73,13 +73,17 @@ struct Run: AsyncParsableCommand {
   var rosettaTag: String?
 
   @Option(help: ArgumentHelp("""
-  Additional directory shares with an optional read-only specifier\n(e.g. --dir=\"build:~/src/build\" --dir=\"sources:~/src/sources:ro\")
+  Additional directory shares with an optional read-only specifier\n(e.g. --dir=\"~/src/build\" or --dir=\"~/src/sources:ro\")
   """, discussion: """
   Requires host to be macOS 13.0 (Ventura) or newer.
-  All shared directories are automatically mounted to "/Volumes/My Shared Files" directory on macOS,
+  A shared directory is automatically mounted to "/Volumes/My Shared Files" directory on macOS,
   while on Linux you have to do it manually: "mount -t virtiofs com.apple.virtio-fs.automount /mount/point".
   For macOS guests, they must be running macOS 13.0 (Ventura) or newer.
-  """, valueName: "name:path[:ro]"))
+
+  In case of passing multiple directories it is required to prefix them with names e.g. --dir=\"build:~/src/build\" --dir=\"sources:~/src/sources:ro\"
+  These names will be used as directory names under the mounting point inside guests. For the example above it will be
+  "/Volumes/My Shared Files/build" and "/Volumes/My Shared Files/sources" respectively.
+  """, valueName: "[name:]path[:ro]"))
   var dir: [String] = []
 
   @Option(help: ArgumentHelp("""
@@ -390,46 +394,31 @@ struct Run: AsyncParsableCommand {
       throw UnsupportedOSError("directory sharing", "is")
     }
 
-    struct DirectoryShare {
-      let name: String
-      let path: URL
-      let readOnly: Bool
-    }
-
     var directoryShares: [DirectoryShare] = []
 
+    var allNamedShares = true
     for rawDir in dir {
-      let splits = rawDir.split(maxSplits: 2) { $0 == ":" }
-
-      if splits.count < 2 {
-        throw ValidationError("invalid --dir syntax: should at least include name and path, colon-separated")
+      let directoryShare = try DirectoryShare(parseFrom: rawDir)
+      if (directoryShare.name == nil) {
+        allNamedShares = false
       }
-
-      var readOnly: Bool = false
-
-      if splits.count == 3 {
-        if splits[2] == "ro" {
-          readOnly = true
-        } else {
-          throw ValidationError("invalid --dir syntax: optional read-only specifier can only be \"ro\"")
-        }
-      }
-
-      let (name, path) = (String(splits[0]), String(splits[1]))
-
-      directoryShares.append(DirectoryShare(
-        name: name,
-        path: URL(fileURLWithPath: NSString(string: path).expandingTildeInPath),
-        readOnly: readOnly)
-      )
+      directoryShares.append(directoryShare)
     }
 
-    var directories: [String : VZSharedDirectory] = Dictionary()
-    directoryShares.forEach { directories[$0.name] = VZSharedDirectory(url: $0.path, readOnly: $0.readOnly) }
 
     let automountTag = VZVirtioFileSystemDeviceConfiguration.macOSGuestAutomountTag
     let sharingDevice = VZVirtioFileSystemDeviceConfiguration(tag: automountTag)
-    sharingDevice.share = VZMultipleDirectoryShare(directories: directories)
+    if allNamedShares {
+      var directories: [String : VZSharedDirectory] = Dictionary()
+      directoryShares.forEach { directories[$0.name!] = VZSharedDirectory(url: $0.path, readOnly: $0.readOnly) }
+      sharingDevice.share = VZMultipleDirectoryShare(directories: directories)
+    } else if dir.count > 1 {
+      throw ValidationError("invalid --dir syntax: for multiple directory shares each one of them should be named")
+    } else if dir.count == 1 {
+      let directoryShare = directoryShares.first!
+      let singleDirectoryShare = VZSingleDirectoryShare(directory: VZSharedDirectory(url: directoryShare.path, readOnly: directoryShare.readOnly))
+      sharingDevice.share = singleDirectoryShare
+    }
 
     return [sharingDevice]
   }
@@ -595,3 +584,43 @@ struct VMView: NSViewRepresentable {
     nsView.virtualMachine = vm.virtualMachine
   }
 }
+
+struct DirectoryShare {
+  let name: String?
+  let path: URL
+  let readOnly: Bool
+
+  init(parseFrom: String) throws {
+    let splits = parseFrom.split(maxSplits: 2) { $0 == ":" }
+
+    if splits.count == 3 {
+      if splits[2] == "ro" {
+        readOnly = true
+      } else {
+        throw ValidationError("invalid --dir syntax: optional read-only specifier can only be \"ro\"")
+      }
+      name = String(splits[0])
+      path = String(splits[1]).toFilePathURL()
+    } else if splits.count == 2 {
+      if splits[1] == "ro" {
+        name = nil
+        path = String(splits[0]).toFilePathURL()
+        readOnly = true
+      } else {
+        name = String(splits[0])
+        path = String(splits[1]).toFilePathURL()
+        readOnly = false
+      }
+    } else {
+      name = nil
+      path = String(splits[0]).toFilePathURL()
+      readOnly = false
+    }
+  }
+}
+
+extension String {
+  func toFilePathURL() -> URL {
+    URL(fileURLWithPath: NSString(string: self).expandingTildeInPath)
+  }
+}

Tests/TartTests/DirecotryShareTests.swift

@@ -0,0 +1,32 @@
+import XCTest
+@testable import tart
+
+final class DirectoryShareTests: XCTestCase {
+  func testNamedParsing() throws {
+    let share = try DirectoryShare(parseFrom: "build:/Users/admin/build")
+    XCTAssertEqual(share.name, "build")
+    XCTAssertEqual(share.path, URL(filePath: "/Users/admin/build"))
+    XCTAssertFalse(share.readOnly)
+  }
+
+  func testNamedReadOnlyParsing() throws {
+    let share = try DirectoryShare(parseFrom: "build:/Users/admin/build:ro")
+    XCTAssertEqual(share.name, "build")
+    XCTAssertEqual(share.path, URL(filePath: "/Users/admin/build"))
+    XCTAssertTrue(share.readOnly)
+  }
+
+  func testOptionalNameParsing() throws {
+    let share = try DirectoryShare(parseFrom: "/Users/admin/build")
+    XCTAssertNil(share.name)
+    XCTAssertEqual(share.path, URL(filePath: "/Users/admin/build"))
+    XCTAssertFalse(share.readOnly)
+  }
+
+  func testOptionalNameReadOnlyParsing() throws {
+    let share = try DirectoryShare(parseFrom: "/Users/admin/build:ro")
+    XCTAssertNil(share.name)
+    XCTAssertEqual(share.path, URL(filePath: "/Users/admin/build"))
+    XCTAssertTrue(share.readOnly)
+  }
+}