OCI: store uncompressed disk size in manifest (#128)

Sources/tart/OCI/Manifest.swift

@@ -3,15 +3,38 @@ import Foundation
 let ociManifestMediaType = "application/vnd.oci.image.manifest.v1+json"
 let ociConfigMediaType = "application/vnd.oci.image.config.v1+json"
 
+// Annotations
+let uncompressedDiskSizeAnnotation = "org.cirruslabs.tart.uncompressed-disk-size"
+
 struct OCIManifest: Codable, Equatable {
   var schemaVersion: Int = 2
   var mediaType: String = ociManifestMediaType
   var config: OCIManifestConfig
   var layers: [OCIManifestLayer] = Array()
+  var annotations: Dictionary<String, String> = Dictionary()
+
+  init(config: OCIManifestConfig, layers: [OCIManifestLayer], uncompressedDiskSize: UInt64? = nil) {
+    self.config = config
+    self.layers = layers
+
+    if let uncompressedDiskSize = uncompressedDiskSize {
+      annotations = [
+        uncompressedDiskSizeAnnotation: String(uncompressedDiskSize)
+      ]
+    }
+  }
 
   func digest() throws -> String {
     try Digest.hash(JSONEncoder().encode(self))
   }
+
+  func uncompressedDiskSize() -> UInt64? {
+    guard let value = annotations[uncompressedDiskSizeAnnotation] else {
+      return nil
+    }
+
+    return UInt64(value)
+  }
 }
 
 struct OCIManifestConfig: Codable, Equatable {

Sources/tart/VMDirectory+OCI.swift

@@ -117,8 +117,10 @@ extension VMDirectory {
     // Read VM's compressed disk as chunks
     // and sequentially upload them as blobs
     let disk = try FileHandle(forReadingFrom: diskURL)
+    var diskReadBytes: UInt64 = 0
     let compressingFilter = try InputFilter<Data>(.compress, using: .lz4, bufferCapacity: Self.bufferSizeBytes) { _ in
       let data = try disk.read(upToCount: Self.bufferSizeBytes)
+      diskReadBytes += UInt64(data?.count ?? 0)
 
       progress.completedUnitCount += Int64(data?.count ?? 0)
 
@@ -146,7 +148,8 @@ extension VMDirectory {
     let ociConfigDigest = try await registry.pushBlob(fromData: ociConfigJSON)
     let manifest = OCIManifest(
             config: OCIManifestConfig(size: ociConfigJSON.count, digest: ociConfigDigest),
-            layers: layers
+            layers: layers,
+            uncompressedDiskSize: diskReadBytes
     )
 
     // Manifest