Support exporting current container to image (#1172)

This PR adds a command to export container to image (#1019).

## Type of Change
- [ ] Bug fix
- [X] New feature  
- [ ] Breaking change
- [ ] Documentation update

## Motivation and Context
Users can export their container to image.

## Testing
- [X] Tested locally
- [ ] Added/updated tests
- [ ] Added/updated docs

Author: JaewonHur

Package.swift

@@ -128,6 +128,7 @@ let package = Package(
                 .product(name: "Containerization", package: "containerization"),
                 .product(name: "ContainerizationExtras", package: "containerization"),
                 .product(name: "ContainerizationOS", package: "containerization"),
+                .product(name: "ContainerizationEXT4", package: "containerization"),
                 .product(name: "GRPC", package: "grpc-swift"),
                 .product(name: "Logging", package: "swift-log"),
                 "ContainerAPIService",

Sources/ContainerCommands/Application.swift

@@ -63,6 +63,7 @@ public struct Application: AsyncLoggableCommand {
                     ContainerStats.self,
                     ContainerStop.self,
                     ContainerPrune.self,
+                    ContainerExport.self,
                 ]
             ),
             CommandGroup(

Sources/ContainerCommands/Container/ContainerExport.swift

@@ -0,0 +1,67 @@
+//===----------------------------------------------------------------------===//
+// Copyright © 2026 Apple Inc. and the container project authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//   https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//===----------------------------------------------------------------------===//
+
+import ArgumentParser
+import ContainerAPIClient
+import ContainerizationError
+import Foundation
+import TerminalProgress
+
+extension Application {
+    public struct ContainerExport: AsyncLoggableCommand {
+        public init() {}
+        public static var configuration: CommandConfiguration {
+            CommandConfiguration(
+                commandName: "export",
+                abstract: "Export a container state to an image",
+            )
+        }
+
+        @OptionGroup
+        public var logOptions: Flags.Logging
+
+        @Option(name: .long, help: "image name")
+        var image: String?
+
+        @Argument(help: "container ID")
+        var id: String
+
+        public func run() async throws {
+            let client = ContainerClient()
+            let tempDir = FileManager.default.temporaryDirectory.appendingPathComponent(UUID().uuidString)
+
+            try FileManager.default.createDirectory(at: tempDir, withIntermediateDirectories: true)
+            defer {
+                try? FileManager.default.removeItem(at: tempDir)
+            }
+
+            let imageName = image ?? id
+
+            let archive = tempDir.appendingPathComponent("archive.tar")
+            try await client.export(id: id, archive: archive)
+
+            let dockerfile = """
+                FROM scratch
+                ADD archive.tar .
+                """
+            try dockerfile.data(using: .utf8)!.write(to: tempDir.appendingPathComponent("Dockerfile"), options: .atomic)
+
+            let builder = try BuildCommand.parse(["-t", imageName, tempDir.absolutePath()])
+
+            try await builder.run()
+        }
+    }
+}

Sources/ContainerResource/Container/Bundle.swift

@@ -38,7 +38,7 @@ public struct Bundle: Sendable {
         self.path.appendingPathComponent("vminitd.log")
     }
 
-    private var containerRootfsBlock: URL {
+    public var containerRootfsBlock: URL {
         self.path.appendingPathComponent(Self.containerRootFsBlockFilename)
     }
 

Sources/Helpers/APIServer/APIServer+Start.swift

@@ -246,6 +246,7 @@ extension APIServer {
             routes[XPCRoute.containerKill] = harness.kill
             routes[XPCRoute.containerStats] = harness.stats
             routes[XPCRoute.containerDiskUsage] = harness.diskUsage
+            routes[XPCRoute.containerExport] = harness.export
 
             return service
         }

Sources/Services/ContainerAPIService/Client/ContainerClient.swift

@@ -321,4 +321,20 @@ public struct ContainerClient: Sendable {
             )
         }
     }
+
+    public func export(id: String, archive: URL) async throws {
+        let request = XPCMessage(route: .containerExport)
+        request.set(key: .id, value: id)
+        request.set(key: .archive, value: archive.absolutePath())
+
+        do {
+            try await xpcClient.send(request)
+        } catch {
+            throw ContainerizationError(
+                .internalError,
+                message: "failed to export container",
+                cause: error
+            )
+        }
+    }
 }

Sources/Services/ContainerAPIService/Client/XPC+.swift

@@ -54,6 +54,8 @@ public enum XPCKeys: String {
     case pluginName
     case plugins
     case plugin
+    /// Archive path to export rootfs
+    case archive
 
     /// Health check request.
     case ping
@@ -148,6 +150,7 @@ public enum XPCRoute: String {
     case containerEvent
     case containerStats
     case containerDiskUsage
+    case containerExport
 
     case pluginLoad
     case pluginGet

Sources/Services/ContainerAPIService/Server/Containers/ContainersHarness.swift

@@ -305,4 +305,26 @@ public struct ContainersHarness: Sendable {
         reply.set(key: .statistics, value: data)
         return reply
     }
+
+    @Sendable
+    public func export(_ message: XPCMessage) async throws -> XPCMessage {
+        let id = message.string(key: .id)
+        guard let id else {
+            throw ContainerizationError(
+                .invalidArgument,
+                message: "id cannot be empty"
+            )
+        }
+        let archive = message.string(key: .archive)
+        guard let archive else {
+            throw ContainerizationError(
+                .invalidArgument,
+                message: "archive cannot be empty"
+            )
+        }
+        let archiveUrl = URL(fileURLWithPath: archive)
+
+        try await service.exportRootfs(id: id, archive: archiveUrl)
+        return message.reply()
+    }
 }

Sources/Services/ContainerAPIService/Server/Containers/ContainersService.swift

@@ -21,12 +21,14 @@ import ContainerResource
 import ContainerSandboxServiceClient
 import ContainerXPC
 import Containerization
+import ContainerizationEXT4
 import ContainerizationError
 import ContainerizationExtras
 import ContainerizationOCI
 import ContainerizationOS
 import Foundation
 import Logging
+import SystemPackage
 
 public actor ContainersService {
     struct ContainerState {
@@ -584,6 +586,20 @@ public actor ContainersService {
         return Self.calculateDirectorySize(at: containerPath)
     }
 
+    public func exportRootfs(id: String, archive: URL) async throws {
+        self.log.debug("\(#function)")
+
+        let state = try self._getContainerState(id: id)
+        guard state.snapshot.status == .stopped else {
+            throw ContainerizationError(.invalidState, message: "container is not stopped")
+        }
+
+        let path = self.containerRoot.appendingPathComponent(id)
+        let bundle = ContainerResource.Bundle(path: path)
+        let rootfs = bundle.containerRootfsBlock
+        try EXT4.EXT4Reader(blockDevice: FilePath(rootfs)).export(archive: FilePath(archive))
+    }
+
     private func handleContainerExit(id: String, code: ExitStatus? = nil) async throws {
         try await self.lock.withLock { [self] context in
             try await handleContainerExit(id: id, code: code, context: context)

Tests/CLITests/Subcommands/Containers/TestCLIExport.swift

@@ -0,0 +1,66 @@
+//===----------------------------------------------------------------------===//
+// Copyright © 2026 Apple Inc. and the container project authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//   https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//===----------------------------------------------------------------------===//
+
+import Foundation
+import Testing
+
+class TestCLIExportCommand: CLITest {
+    private func getTestName() -> String {
+        Test.current!.name.trimmingCharacters(in: ["(", ")"]).lowercased()
+    }
+
+    @Test func testExportCommand() throws {
+        let name = getTestName()
+        try doLongRun(name: name, autoRemove: false)
+        defer {
+            try? doStop(name: name)
+            try? doRemove(name: name)
+        }
+
+        let mustBeInImage = "must-be-in-image"
+        _ = try doExec(name: name, cmd: ["sh", "-c", "echo \(mustBeInImage) > /foo"])
+
+        _ = try doExec(name: name, cmd: ["sh", "-c", "mkdir -p /parent/child"])
+        let hardlinkMustRemain = "hardlink-must-remain"
+        _ = try doExec(name: name, cmd: ["sh", "-c", "echo \(hardlinkMustRemain) > /parent/child/bar"])
+        _ = try doExec(name: name, cmd: ["sh", "-c", "ln /parent/child/bar /bar"])
+
+        let symlinkMustRemain = "symlink-must-remain"
+        _ = try doExec(name: name, cmd: ["sh", "-c", "echo \(symlinkMustRemain) > /parent/child/baz"])
+        _ = try doExec(name: name, cmd: ["sh", "-c", "ln /parent/child/baz /baz"])
+
+        try doStop(name: name)
+        try doExport(name: name, image: name)
+        defer {
+            try? doRemoveImages(images: [name])
+        }
+
+        let exported = "\(name)-from-exported"
+        try doLongRun(name: exported, image: name)
+        defer {
+            try? doStop(name: exported)
+        }
+
+        let foo = try doExec(name: exported, cmd: ["cat", "/foo"])
+        #expect(foo == mustBeInImage + "\n")
+
+        let bar = try doExec(name: exported, cmd: ["cat", "/bar"])
+        #expect(bar == hardlinkMustRemain + "\n")
+
+        let baz = try doExec(name: exported, cmd: ["cat", "/baz"])
+        #expect(baz == symlinkMustRemain + "\n")
+    }
+}