tar: Extract the tar writer into its own module

Preparation for refactoring and unit testing

Author: euanh

Package.swift

@@ -40,7 +40,7 @@ let package = Package(
         .executableTarget(
             name: "containertool",
             dependencies: [
-                .target(name: "ContainerRegistry"), .target(name: "VendorCNIOExtrasZlib"),
+                .target(name: "ContainerRegistry"), .target(name: "VendorCNIOExtrasZlib"), .target(name: "Tar"),
                 .product(name: "ArgumentParser", package: "swift-argument-parser"),
             ],
             swiftSettings: [.swiftLanguageMode(.v5)]
@@ -51,7 +51,7 @@ let package = Package(
             dependencies: [],
             path: "Vendor/github.com/apple/swift-nio-extras/Sources/CNIOExtrasZlib",
             linkerSettings: [.linkedLibrary("z")]
-        ),
+        ), .target(name: "Tar"),
         .target(
             // Vendored from https://github.com/apple/swift-package-manager with modifications
             name: "Basics",

Sources/containertool/tar.swift renamed to Sources/Tar/tar.swift

@@ -143,7 +143,12 @@ let CONTTYPE = "7"  // reserved
 let XHDTYPE = "x"  // Extended header referring to the next file in the archive
 let XGLTYPE = "g"  // Global extended header
 
-func tar(_ bytes: [UInt8], filename: String = "app") -> [UInt8] {
+/// Creates a tar archive containing a single file
+/// - Parameters:
+///   - bytes: The file's body data
+///   - filename: The file's name in the archive
+/// - Returns: A tar archive containing the file
+public func tar(_ bytes: [UInt8], filename: String = "app") -> [UInt8] {
     // A file entry consists of a file header followed by the
     // contents of the file. The header includes information such as
     // the file name, size and permissions.   Different versions of
@@ -187,4 +192,9 @@ func tar(_ bytes: [UInt8], filename: String = "app") -> [UInt8] {
     return hdr
 }
 
-func tar(_ data: Data, filename: String) -> [UInt8] { tar([UInt8](data), filename: filename) }
+/// Creates a tar archive containing a single file
+/// - Parameters:
+///   - data: The file's body data
+///   - filename: The file's name in the archive
+/// - Returns: A tar archive containing the file
+public func tar(_ data: Data, filename: String) -> [UInt8] { tar([UInt8](data), filename: filename) }

Sources/containertool/containertool.swift

@@ -15,6 +15,7 @@
 import ArgumentParser
 import Foundation
 import ContainerRegistry
+import Tar
 import Basics
 
 extension Swift.String: Swift.Error {}