tart clone: clone VM and generate MAC under a file lock (#215)

* tart clone: clone VM and generate MAC under a file lock

* Lock concurrent "tart pull"'s for the same host

* Config: ensure Tart's home and cache directories always exist

Sources/tart/Commands/Clone.swift

@@ -32,12 +32,17 @@ struct Clone: AsyncParsableCommand {
       }
 
       let sourceVM = try VMStorageHelper.open(sourceName)
-      let generateMAC = try localStorage.hasVMsWithMACAddress(macAddress: sourceVM.macAddress())
 
       let tmpVMDir = try VMDirectory.temporary()
       try await withTaskCancellationHandler(operation: {
+        let lock = try FileLock(lockURL: Config().tartHomeDir)
+        try lock.lock()
+
+        let generateMAC = try localStorage.hasVMsWithMACAddress(macAddress: sourceVM.macAddress())
         try sourceVM.clone(to: tmpVMDir, generateMAC: generateMAC)
         try localStorage.move(newName, from: tmpVMDir)
+
+        try lock.unlock()
       }, onCancel: {
         try? FileManager.default.removeItem(at: tmpVMDir.baseURL)
       })

Sources/tart/Config.swift

@@ -4,7 +4,7 @@ struct Config {
   let tartHomeDir: URL
   let tartCacheDir: URL
 
-  init() {
+  init() throws {
     var tartHomeDir: URL
 
     if let customTartHome = ProcessInfo.processInfo.environment["TART_HOME"] {
@@ -17,6 +17,8 @@ struct Config {
 
     self.tartHomeDir = tartHomeDir
     tartCacheDir = tartHomeDir.appendingPathComponent("cache", isDirectory: true)
+
+    try FileManager.default.createDirectory(at: tartCacheDir, withIntermediateDirectories: true)
   }
 
   static func jsonEncoder() -> JSONEncoder {

Sources/tart/FileLock.swift

@@ -0,0 +1,48 @@
+import Foundation
+import System
+
+enum FileLockError: Error, Equatable {
+    case Failed(_ message: String)
+    case AlreadyLocked
+}
+
+class FileLock {
+    let url: URL
+    let fd: Int32
+
+    init(lockURL: URL) throws {
+        url = lockURL
+        fd = open(lockURL.path, 0)
+    }
+
+    deinit {
+        close(fd)
+    }
+
+    func trylock() throws -> Bool {
+        try flockWrapper(LOCK_EX | LOCK_NB)
+    }
+
+    func lock() throws {
+        _ = try flockWrapper(LOCK_EX)
+    }
+
+    func unlock() throws {
+        _ = try flockWrapper(LOCK_UN)
+    }
+
+    func flockWrapper(_ operation: Int32) throws -> Bool {
+        let ret = flock(fd, operation)
+        if ret != 0 {
+            let details = Errno(rawValue: CInt(errno))
+
+            if (operation & LOCK_NB) != 0 && details == .wouldBlock {
+                return false
+            }
+
+            throw FileLockError.Failed("failed to lock \(url): \(details)")
+        }
+
+        return true
+    }
+}

Sources/tart/IPSWCache.swift

@@ -5,7 +5,7 @@ class IPSWCache: PrunableStorage {
   let baseURL: URL
 
   init() throws {
-    baseURL = Config().tartCacheDir.appendingPathComponent("IPSWs", isDirectory: true)
+    baseURL = try Config().tartCacheDir.appendingPathComponent("IPSWs", isDirectory: true)
     try FileManager.default.createDirectory(at: baseURL, withIntermediateDirectories: true)
   }
 

Sources/tart/VMStorageLocal.swift

@@ -1,7 +1,7 @@
 import Foundation
 
 class VMStorageLocal {
-  let baseURL: URL = Config().tartHomeDir.appendingPathComponent("vms", isDirectory: true)
+  let baseURL: URL = try! Config().tartHomeDir.appendingPathComponent("vms", isDirectory: true)
 
   private func vmURL(_ name: String) -> URL {
     baseURL.appendingPathComponent(name, isDirectory: true)

Sources/tart/VMStorageOCI.swift

@@ -1,12 +1,16 @@
 import Foundation
 
 class VMStorageOCI: PrunableStorage {
-  let baseURL = Config().tartCacheDir.appendingPathComponent("OCIs", isDirectory: true)
+  let baseURL = try! Config().tartCacheDir.appendingPathComponent("OCIs", isDirectory: true)
 
   private func vmURL(_ name: RemoteName) -> URL {
     baseURL.appendingRemoteName(name)
   }
 
+  private func hostDirectoryURL(_ name: RemoteName) -> URL {
+    baseURL.appendingHost(name)
+  }
+
   func exists(_ name: RemoteName) -> Bool {
     VMDirectory(baseURL: vmURL(name)).initialized
   }
@@ -125,6 +129,24 @@ class VMStorageOCI: PrunableStorage {
     let digestName = RemoteName(host: name.host, namespace: name.namespace,
             reference: Reference(digest: Digest.hash(manifestData)))
 
+    // Ensure that host directory for given RemoteName exists in OCI storage
+    let hostDirectoryURL = hostDirectoryURL(digestName)
+    try FileManager.default.createDirectory(at: hostDirectoryURL, withIntermediateDirectories: true)
+
+    // Acquire a lock on it to prevent concurrent pulls for a single host
+    let lock = try FileLock(lockURL: hostDirectoryURL)
+
+    let sucessfullyLocked = try lock.trylock()
+    if !sucessfullyLocked {
+      print("waiting for lock...")
+      try lock.lock()
+    }
+    defer { try! lock.unlock() }
+
+    if Task.isCancelled {
+      throw CancellationError()
+    }
+
     if !exists(digestName) {
       let tmpVMDir = try VMDirectory.temporary()
 
@@ -181,4 +203,8 @@ extension URL {
 
     return result
   }
+
+  func appendingHost(_ name: RemoteName) -> URL {
+    self.appendingPathComponent(name.host, isDirectory: true)
+  }
 }

Tests/TartTests/FileLockTests.swift

@@ -0,0 +1,34 @@
+import XCTest
+@testable import tart
+
+final class FileLockTests: XCTestCase {
+    func testSimple() throws {
+        // Create a temporary file that will be used as a lock
+        let url = temporaryFile()
+
+        // Make sure this file can be locked and unlocked
+        let lock = try FileLock(lockURL: url)
+        try lock.lock()
+        try lock.unlock()
+    }
+
+    func testDoubleLockResultsInError() throws {
+        // Create a temporary file that will be used as a lock
+        let url = temporaryFile()
+
+        // Create two locks on a same file and ensure one of them fails
+        let firstLock = try FileLock(lockURL: url)
+        try firstLock.lock()
+
+        let secondLock = try! FileLock(lockURL: url)
+        XCTAssertFalse(try secondLock.trylock())
+    }
+
+    private func temporaryFile() -> URL {
+        let url = URL(fileURLWithPath: NSTemporaryDirectory()).appendingPathComponent(UUID().uuidString)
+
+        FileManager.default.createFile(atPath: url.path, contents: nil)
+
+        return url
+    }
+}