cli: Add support for multiple backends for "moby run"

- Move HyperKit code into a separate file. It should be compilable
  on all supported OSes now.
- Add a (optional) subcommand to "moby run" to select a backend
  i.e., "moby run hyperkit [options] [prefix]"
- On macOS the default is "hyperkit" so that:
  "moby run [options] [prefix]"
  just works
- Add enough command line parsing to make it easy to add new
  backends to the run command
Update help messages.

Signed-off-by: Rolf Neugebauer <rolf.neugebauer@docker.com>

src/cmd/moby/run.go

@@ -0,0 +1,45 @@
+package main
+
+import (
+	"fmt"
+	"os"
+	"runtime"
+
+	log "github.com/Sirupsen/logrus"
+)
+
+func runUsage() {
+	fmt.Printf("USAGE: %s run [backend] [options] [prefix]\n\n", os.Args[0])
+
+	fmt.Printf("'backend' specifies the run backend.\n")
+	fmt.Printf("If not specified the platform specific default will be used\n")
+	fmt.Printf("Supported backends are (default platform in brackets):\n")
+	fmt.Printf("  hyperkit [macOS]\n")
+	fmt.Printf("\n")
+	fmt.Printf("'options' are the backend specific options.\n")
+	fmt.Printf("See 'moby run [backend] --help' for details.\n\n")
+	fmt.Printf("'prefix' specifies the path to the VM image.\n")
+	fmt.Printf("It defaults to './moby'.\n")
+}
+
+func run(args []string) {
+	if len(args) < 1 {
+		runUsage()
+		os.Exit(1)
+	}
+
+	switch args[0] {
+	case "help", "-h", "-help", "--help":
+		runUsage()
+		os.Exit(0)
+	case "hyperkit":
+		runHyperKit(args[1:])
+	default:
+		switch runtime.GOOS {
+		case "darwin":
+			runHyperKit(args)
+		default:
+			log.Errorf("There currently is no default 'run' backend for your platform.")
+		}
+	}
+}

src/cmd/moby/run_fallback.go

@@ -1,11 +0,0 @@
-// +build !darwin
-
-package main
-
-import (
-	log "github.com/Sirupsen/logrus"
-)
-
-func run(args []string) {
-	log.Fatalf("'run' is not supported yet on your OS")
-}

src/cmd/moby/run_hyperkit.go

@@ -1,5 +1,3 @@
-// +build darwin
-
 package main
 
 import (
@@ -13,33 +11,33 @@ import (
 )
 
 // Process the run arguments and execute run
-func run(args []string) {
-	runCmd := flag.NewFlagSet("run", flag.ExitOnError)
-	runCmd.Usage = func() {
-		fmt.Printf("USAGE: %s run [options] [prefix]\n\n", os.Args[0])
+func runHyperKit(args []string) {
+	hyperkitCmd := flag.NewFlagSet("hyperkit", flag.ExitOnError)
+	hyperkitCmd.Usage = func() {
+		fmt.Printf("USAGE: %s run hyperkit [options] [prefix]\n\n", os.Args[0])
 		fmt.Printf("'prefix' specifies the path to the VM image.\n")
 		fmt.Printf("It defaults to './moby'.\n")
 		fmt.Printf("\n")
 		fmt.Printf("Options:\n")
-		runCmd.PrintDefaults()
+		hyperkitCmd.PrintDefaults()
 	}
-	runCPUs := runCmd.Int("cpus", 1, "Number of CPUs")
-	runMem := runCmd.Int("mem", 1024, "Amount of memory in MB")
-	runDiskSz := runCmd.Int("disk-size", 0, "Size of Disk in MB")
-	runDisk := runCmd.String("disk", "", "Path to disk image to used")
+	runCPUs := hyperkitCmd.Int("cpus", 1, "Number of CPUs")
+	runMem := hyperkitCmd.Int("mem", 1024, "Amount of memory in MB")
+	runDiskSz := hyperkitCmd.Int("disk-size", 0, "Size of Disk in MB")
+	runDisk := hyperkitCmd.String("disk", "", "Path to disk image to used")
 
-	runCmd.Parse(args)
-	remArgs := runCmd.Args()
+	hyperkitCmd.Parse(args)
+	remArgs := hyperkitCmd.Args()
 
 	prefix := "moby"
 	if len(remArgs) > 0 {
 		prefix = remArgs[0]
 	}
 
-	runInternal(*runCPUs, *runMem, *runDiskSz, *runDisk, prefix)
+	runHyperKitInternal(*runCPUs, *runMem, *runDiskSz, *runDisk, prefix)
 }
 
-func runInternal(cpus, mem, diskSz int, disk, prefix string) {
+func runHyperKitInternal(cpus, mem, diskSz int, disk, prefix string) {
 	cmdline, err := ioutil.ReadFile(prefix + "-cmdline")
 	if err != nil {
 		log.Fatalf("Cannot open cmdline file: %v", err)