Introduce "tart fqn" command

Sources/tart/Commands/Pull.swift

@@ -23,17 +23,24 @@ struct Pull: AsyncParsableCommand {
   @Option(help: "network concurrency to use when pulling a remote VM from the OCI-compatible registry")
   var concurrency: UInt = 4
 
+  @Flag(help: .hidden)
+  var jsonDigest: Bool = false
+
   func validate() throws {
     if concurrency < 1 {
       throw ValidationError("network concurrency cannot be less than 1")
     }
   }
 
   func run() async throws {
+    if jsonDigest {
+      jsonLogger = SimpleConsoleLogger()
+    }
+
     // Be more liberal when accepting local image as argument,
     // see https://github.com/cirruslabs/tart/issues/36
     if VMStorageLocal().exists(remoteName) {
-      print("\"\(remoteName)\" is a local image, nothing to pull here!")
+      defaultLogger.appendNewLine("\"\(remoteName)\" is a local image, nothing to pull here!")
 
       return
     }

Sources/tart/Logging/Logger.swift

@@ -12,16 +12,13 @@ var defaultLogger: Logger = {
     return InteractiveConsoleLogger()
   }
 }()
+var jsonLogger: Logger = NopLogger()
 
 public class InteractiveConsoleLogger: Logger {
   private let eraseCursorDown = "\u{001B}[J" // clear entire line
   private let moveUp = "\u{001B}[1A" // move one line up
   private let moveBeginningOfLine = "\r" // 
 
-  public init() {
-
-  }
-
   public func appendNewLine(_ line: String) {
     print(line, terminator: "\n")
   }
@@ -32,15 +29,21 @@ public class InteractiveConsoleLogger: Logger {
 }
 
 public class SimpleConsoleLogger: Logger {
-  public init() {
+  public func appendNewLine(_ line: String) {
+    print(line, terminator: "\n")
+  }
 
+  public func updateLastLine(_ line: String) {
+    print(line, terminator: "\n")
   }
+}
 
+public class NopLogger: Logger {
   public func appendNewLine(_ line: String) {
-    print(line, terminator: "\n")
+    // do nothing
   }
 
   public func updateLastLine(_ line: String) {
-    print(line, terminator: "\n")
+    // do nothing
   }
 }

Sources/tart/VMStorageOCI.swift

@@ -147,6 +147,7 @@ class VMStorageOCI: PrunableStorage {
     if exists(name) && exists(digestName) && linked(from: name, to: digestName) {
       // optimistically check if we need to do anything at all before locking
       defaultLogger.appendNewLine("\(digestName) image is already cached and linked!")
+      jsonLogger.appendNewLine("{\"digest\": \"\(digestName)\"}")
       return
     }
 
@@ -159,7 +160,7 @@ class VMStorageOCI: PrunableStorage {
 
     let sucessfullyLocked = try lock.trylock()
     if !sucessfullyLocked {
-      print("waiting for lock...")
+      defaultLogger.appendNewLine("waiting for lock...")
       try lock.lock()
     }
     defer { try! lock.unlock() }
@@ -199,6 +200,8 @@ class VMStorageOCI: PrunableStorage {
       defaultLogger.appendNewLine("\(digestName) image is already cached! creating a symlink...")
     }
 
+    jsonLogger.appendNewLine("{\"digest\": \"\(digestName)\"}")
+
     if name != digestName {
       // Create new or overwrite the old symbolic link
       try link(from: name, to: digestName)