containertool: Add special-case image source for scratch images, instead of optional source

Author: euanh

Sources/ContainerRegistry/ImageSource.swift

@@ -35,4 +35,17 @@ public protocol ImageSource {
         repository: ImageReference.Repository,
         reference: any ImageReference.Reference
     ) async throws -> ImageIndex
+
+    /// Get an image configuration record from the registry.
+    /// - Parameters:
+    ///   - image: Reference to the image containing the record.
+    ///   - digest: Digest of the record.
+    /// - Returns: The image confguration record stored in `repository` with digest `digest`.
+    /// - Throws: If the blob cannot be decoded as an `ImageConfiguration`.
+    ///
+    /// Image configuration records are stored as blobs in the registry.  This function retrieves the requested blob and tries to decode it as a configuration record.
+    func getImageConfiguration(
+        forImage image: ImageReference,
+        digest: ImageReference.Digest
+    ) async throws -> ImageConfiguration
 }

Sources/ContainerRegistry/RegistryClient+ImageConfiguration.swift

@@ -14,7 +14,7 @@
 
 import class Foundation.JSONDecoder
 
-extension ImageSource {
+extension RegistryClient {
     /// Get an image configuration record from the registry.
     /// - Parameters:
     ///   - image: Reference to the image containing the record.

Sources/ContainerRegistry/ScratchImageSource.swift

@@ -0,0 +1,105 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the SwiftContainerPlugin open source project
+//
+// Copyright (c) 2025 Apple Inc. and the SwiftContainerPlugin project authors
+// Licensed under Apache License v2.0
+//
+// See LICENSE.txt for license information
+// See CONTRIBUTORS.txt for the list of SwiftContainerPlugin project authors
+//
+// SPDX-License-Identifier: Apache-2.0
+//
+//===----------------------------------------------------------------------===//
+
+import struct Foundation.Data
+import class Foundation.JSONDecoder
+import class Foundation.JSONEncoder
+
+public struct ScratchImageSource {
+    var architecture: String
+    var os: String
+
+    public var decoder: JSONDecoder
+    var encoder: JSONEncoder
+
+    public init(architecture: String, os: String) {
+        self.architecture = architecture
+        self.os = os
+        self.decoder = JSONDecoder()
+        self.encoder = JSONEncoder()
+        self.encoder.outputFormatting = [.sortedKeys, .prettyPrinted, .withoutEscapingSlashes]
+        self.encoder.dateEncodingStrategy = .iso8601
+    }
+}
+
+extension ScratchImageSource: ImageSource {
+    /// Fetches an unstructured blob of data from the registry.
+    ///
+    /// - Parameters:
+    ///   - repository: Name of the repository containing the blob.
+    ///   - digest: Digest of the blob.
+    /// - Returns: The downloaded data.
+    /// - Throws: If the blob download fails.
+    public func getBlob(repository: ImageReference.Repository, digest: ImageReference.Digest) async throws -> Data {
+        Data()  // fatalError?
+    }
+
+    public func getManifest(
+        repository: ImageReference.Repository,
+        reference: any ImageReference.Reference
+    ) async throws -> (ImageManifest, ContentDescriptor) {
+        let config = ImageConfiguration(
+            architecture: architecture,
+            os: os,
+            rootfs: .init(_type: "layers", diff_ids: [])
+        )
+        let encodedConfig = try encoder.encode(config)
+
+        let manifest = ImageManifest(
+            schemaVersion: 2,
+            config: ContentDescriptor(
+                mediaType: "application/vnd.oci.image.config.v1+json",
+                digest: "\(ImageReference.Digest(of: encodedConfig))",
+                size: Int64(encodedConfig.count)
+            ),
+            layers: []
+        )
+        let encodedManifest = try encoder.encode(manifest)
+
+        return (
+            manifest,
+            ContentDescriptor(
+                mediaType: "application/vnd.oci.image.manifest.v1+json",
+                digest: "\(ImageReference.Digest(of: encodedManifest))",
+                size: Int64(encodedManifest.count)
+            )
+        )
+    }
+
+    public func getIndex(
+        repository: ImageReference.Repository,
+        reference: any ImageReference.Reference
+    ) async throws -> ImageIndex {
+        fatalError()
+    }
+
+    /// Get an image configuration record from the registry.
+    /// - Parameters:
+    ///   - image: Reference to the image containing the record.
+    ///   - digest: Digest of the record.
+    /// - Returns: The image confguration record stored in `repository` with digest `digest`.
+    /// - Throws: If the blob cannot be decoded as an `ImageConfiguration`.
+    ///
+    /// Image configuration records are stored as blobs in the registry.  This function retrieves the requested blob and tries to decode it as a configuration record.
+    public func getImageConfiguration(
+        forImage image: ImageReference,
+        digest: ImageReference.Digest
+    ) async throws -> ImageConfiguration {
+        ImageConfiguration(
+            architecture: architecture,
+            os: os,
+            rootfs: .init(_type: "layers", diff_ids: [])
+        )
+    }
+}

Sources/containertool/containertool.swift

@@ -191,9 +191,9 @@ enum AllowHTTP: String, ExpressibleByArgument, CaseIterable { case source, desti
 
         // The base image may be stored on a different registry to the final destination, so two clients are needed.
         // `scratch` is a special case and requires no source client.
-        let source: RegistryClient?
+        let source: ImageSource
         if from == "scratch" {
-            source = nil
+            source = ScratchImageSource(architecture: architecture, os: os)
         } else {
             source = try await RegistryClient(
                 registry: baseImage.registry,
@@ -236,7 +236,7 @@ enum AllowHTTP: String, ExpressibleByArgument, CaseIterable { case source, desti
 func publishContainerImage<Source: ImageSource, Destination: ImageDestination>(
     baseImage: ImageReference,
     destinationImage: ImageReference,
-    source: Source?,
+    source: Source,
     destination: Destination,
     architecture: String,
     os: String,
@@ -248,34 +248,17 @@ func publishContainerImage<Source: ImageSource, Destination: ImageDestination>(
 
     // MARK: Find the base image
 
-    let baseImageManifest: ImageManifest
-    let baseImageConfiguration: ImageConfiguration
-    let baseImageDescriptor: ContentDescriptor
-    if let source {
-        (baseImageManifest, baseImageDescriptor) = try await source.getImageManifest(
-            forImage: baseImage,
-            architecture: architecture
-        )
-        try log("Found base image manifest: \(ImageReference.Digest(baseImageDescriptor.digest))")
+    let (baseImageManifest, baseImageDescriptor) = try await source.getImageManifest(
+        forImage: baseImage,
+        architecture: architecture
+    )
+    try log("Found base image manifest: \(ImageReference.Digest(baseImageDescriptor.digest))")
 
-        baseImageConfiguration = try await source.getImageConfiguration(
-            forImage: baseImage,
-            digest: ImageReference.Digest(baseImageManifest.config.digest)
-        )
-        log("Found base image configuration: \(baseImageManifest.config.digest)")
-    } else {
-        baseImageManifest = .init(
-            schemaVersion: 2,
-            config: .init(mediaType: "scratch", digest: "scratch", size: 0),
-            layers: []
-        )
-        baseImageConfiguration = .init(
-            architecture: architecture,
-            os: os,
-            rootfs: .init(_type: "layers", diff_ids: [])
-        )
-        if verbose { log("Using scratch as base image") }
-    }
+    let baseImageConfiguration = try await source.getImageConfiguration(
+        forImage: baseImage,
+        digest: ImageReference.Digest(baseImageManifest.config.digest)
+    )
+    log("Found base image configuration: \(baseImageManifest.config.digest)")
 
     // MARK: Upload resource layers
 
@@ -357,15 +340,13 @@ func publishContainerImage<Source: ImageSource, Destination: ImageDestination>(
     // Copy the base image layers to the destination repository
     // Layers could be checked and uploaded concurrently
     // This could also happen in parallel with the application image build
-    if let source {
-        for layer in baseImageManifest.layers {
-            try await source.copyBlob(
-                digest: ImageReference.Digest(layer.digest),
-                fromRepository: baseImage.repository,
-                toClient: destination,
-                toRepository: destinationImage.repository
-            )
-        }
+    for layer in baseImageManifest.layers {
+        try await source.copyBlob(
+            digest: ImageReference.Digest(layer.digest),
+            fromRepository: baseImage.repository,
+            toClient: destination,
+            toRepository: destinationImage.repository
+        )
     }
 
     // MARK: Upload application manifest