Report progress when downloading IPSW files (#768)

The URLSession async/await functions do not report progress through
the normal URLSessionTaskDelegate callbacks, as reported in:

 https://developer.apple.com/forums/thread/723015

We don't want to use URLSession.bytes, as that results in a much
slower download speed compared to URLSession.download, but we can
work around the lack of progress callbacks by observing the
progress on the URLSessionTask itself.

Fixes #767

Sources/tart/Fetcher.swift

@@ -3,19 +3,32 @@ import AsyncAlgorithms
 
 fileprivate let urlSession = createURLSession()
 
-class Fetcher {
-  static func fetch(_ request: URLRequest, viaFile: Bool = false) async throws -> (AsyncThrowingChannel<Data, Error>, HTTPURLResponse) {
-    if viaFile {
-      return try await fetchViaFile(request)
-    }
-
-    return try await fetchViaMemory(request)
+class DownloadDelegate: NSObject, URLSessionTaskDelegate {
+  let progress: Progress
+  init(_ progress: Progress) throws {
+    self.progress = progress
   }
 
-  private static func fetchViaMemory(_ request: URLRequest) async throws -> (AsyncThrowingChannel<Data, Error>, HTTPURLResponse) {
+  func urlSession(_ session: URLSession, didCreateTask task: URLSessionTask) {
+    self.progress.addChild(task.progress, withPendingUnitCount: self.progress.totalUnitCount)
+  }
+}
+
+class Fetcher {
+  static func fetch(_ request: URLRequest, viaFile: Bool = false, progress: Progress? = nil) async throws -> (AsyncThrowingChannel<Data, Error>, HTTPURLResponse) {
+    let delegate = progress != nil ? try DownloadDelegate(progress!) : nil
+
+    if viaFile {
+      return try await fetchViaFile(request, delegate: delegate)
+    }
+
+    return try await fetchViaMemory(request, delegate: delegate)
+  }
+
+  private static func fetchViaMemory(_ request: URLRequest, delegate: URLSessionTaskDelegate? = nil) async throws -> (AsyncThrowingChannel<Data, Error>, HTTPURLResponse) {
     let dataCh = AsyncThrowingChannel<Data, Error>()
 
-    let (data, response) = try await urlSession.data(for: request)
+    let (data, response) = try await urlSession.data(for: request, delegate: delegate)
 
     Task {
       await dataCh.send(data)
@@ -26,10 +39,10 @@ class Fetcher {
     return (dataCh, response as! HTTPURLResponse)
   }
 
-  private static func fetchViaFile(_ request: URLRequest) async throws -> (AsyncThrowingChannel<Data, Error>, HTTPURLResponse) {
+  private static func fetchViaFile(_ request: URLRequest, delegate: URLSessionTaskDelegate? = nil) async throws -> (AsyncThrowingChannel<Data, Error>, HTTPURLResponse) {
     let dataCh = AsyncThrowingChannel<Data, Error>()
 
-    let (fileURL, response) = try await urlSession.download(for: request)
+    let (fileURL, response) = try await urlSession.download(for: request, delegate: delegate)
 
     // Acquire a handle to the downloaded file and then remove it.
     //

Sources/tart/VM.swift

@@ -91,12 +91,17 @@ class VM: NSObject, VZVirtualMachineDelegate, ObservableObject {
     // Download the IPSW
     defaultLogger.appendNewLine("Fetching \(remoteURL.lastPathComponent)...")
 
-    let (channel, response) = try await Fetcher.fetch(URLRequest(url: remoteURL), viaFile: true)
+    let downloadProgress = Progress(totalUnitCount: 100)
+    ProgressObserver(downloadProgress).log(defaultLogger)
 
-    let progress = Progress(totalUnitCount: response.expectedContentLength)
-    ProgressObserver(progress).log(defaultLogger)
+    let request = URLRequest(url: remoteURL)
+    let (channel, response) = try await Fetcher.fetch(request, viaFile: true, progress: downloadProgress)
 
     let temporaryLocation = try Config().tartTmpDir.appendingPathComponent(UUID().uuidString + ".ipsw")
+    defaultLogger.appendNewLine("Computing digest for \(temporaryLocation.path)...")
+    let digestProgress = Progress(totalUnitCount: response.expectedContentLength)
+    ProgressObserver(digestProgress).log(defaultLogger)
+
     FileManager.default.createFile(atPath: temporaryLocation.path, contents: nil)
     let lock = try FileLock(lockURL: temporaryLocation)
     try lock.lock()
@@ -108,7 +113,7 @@ class VM: NSObject, VZVirtualMachineDelegate, ObservableObject {
       let chunkAsData = Data(chunk)
       fileHandle.write(chunkAsData)
       digest.update(chunkAsData)
-      progress.completedUnitCount += Int64(chunk.count)
+      digestProgress.completedUnitCount += Int64(chunk.count)
     }
 
     try fileHandle.close()