limactl start: add --foreground flag

This change adds the `--foreground` flag to `limactl start`.

This flag:

- Launches `limactl hostagent` with `syscall.Exec`, which allows `limactl start --foreground` to be used in macOS's `launchd.plist`, making it possible to hide `limactl hostagent`.
- When launched from the terminal, it uses the terminal for standard output and standard error instead of the default `ha.std{out,err}.log` files. In this case, a message indicating that the default log files are not used is written to the log files.
- When launched from something other than the terminal, such as `launchd`, it uses the `ha.std{out,err}.log` files as before.
- Cannot be used on Windows.

The following are the updated steps to set up the Docker instance of Lima to start at login using launchd on macOS:
```bash
# Create the docker instance
limactl create template://docker --vm-type vz --rosetta --network vzNAT --tty=false

# Create the launchd plist
echo '<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
	<key>Label</key>
	<string>io.lima-vm.docker</string>
	<key>ProgramArguments</key>
	<array>
		<string>'"$(which limactl)"'</string>
		<string>start</string>
		<string>--foreground</string>
		<string>docker</string>
	</array>
	<key>RunAtLoad</key>
	<true/>
	<key>StandardErrorPath</key>
	<string>launchd.stderr.log</string>
	<key>StandardOutPath</key>
	<string>launchd.stdout.log</string>
	<key>WorkingDirectory</key>
	<string>'"$(limactl list docker --format '{{.Dir}}')"'</string>
</dict>
</plist>' > ~/Library/LaunchAgents/io.lima-vm.docker.plist

# Load the launchd plist and launchd will start the docker instance
launchctl load ~/Library/LaunchAgents/io.lima-vm.docker.plist
```
that changed from #2140:

- Changed to `limactl create` because it's no longer necessary to run `limactl start` before loading into `launchd`.
- Changed from `limactl hostagent` to `limactl start --foreground`.
- Changed `Standard{Error,Out}Path` to `launchd.std{err,out}.log`, so that only the output of `limactl start` is written.

Signed-off-by: Norio Nomura <norio.nomura@gmail.com>

Update cmd/limactl/start.go

Change `instance` to `hostagent`

Co-authored-by: Akihiro Suda <suda.kyoto@gmail.com>

Add `launchHostAgentForeground` to `start.Start()`

Avoid using `Context` to passing `launchHostAgentForeground`

Remove `start.DefaultLaunchHostAgentForeground`

Author: norio-nomura

cmd/limactl/edit.go

@@ -122,7 +122,7 @@ func editAction(cmd *cobra.Command, args []string) error {
 	if err != nil {
 		return err
 	}
-	return start.Start(ctx, inst)
+	return start.Start(ctx, inst, false)
 }
 
 func askWhetherToStart() (bool, error) {

cmd/limactl/start.go

@@ -8,6 +8,7 @@ import (
 	"net/http"
 	"os"
 	"path/filepath"
+	"runtime"
 	"strings"
 
 	"github.com/containerd/containerd/identifiers"
@@ -92,6 +93,9 @@ See the examples in 'limactl create --help'.
 		RunE:              startAction,
 	}
 	registerCreateFlags(startCommand, "[limactl create] ")
+	if runtime.GOOS != "windows" {
+		startCommand.Flags().Bool("foreground", false, "run the hostagent in the foreground")
+	}
 	startCommand.Flags().Duration("timeout", start.DefaultWatchHostAgentEventsTimeout, "duration to wait for the instance to be running before timing out")
 	return startCommand
 }
@@ -510,6 +514,14 @@ func startAction(cmd *cobra.Command, args []string) error {
 		return err
 	}
 
+	launchHostAgentForeground := false
+	if runtime.GOOS != "windows" {
+		foreground, err := cmd.Flags().GetBool("foreground")
+		if err != nil {
+			return err
+		}
+		launchHostAgentForeground = foreground
+	}
 	timeout, err := cmd.Flags().GetDuration("timeout")
 	if err != nil {
 		return err
@@ -518,7 +530,7 @@ func startAction(cmd *cobra.Command, args []string) error {
 		ctx = start.WithWatchHostAgentTimeout(ctx, timeout)
 	}
 
-	return start.Start(ctx, inst)
+	return start.Start(ctx, inst, launchHostAgentForeground)
 }
 
 func createBashComplete(cmd *cobra.Command, _ []string, _ string) ([]string, cobra.ShellCompDirective) {

pkg/osutil/osutil_unix.go

@@ -9,6 +9,10 @@ import (
 	"golang.org/x/sys/unix"
 )
 
+func Dup2(oldfd, newfd int) (err error) {
+	return unix.Dup2(oldfd, newfd)
+}
+
 func Ftruncate(fd int, length int64) (err error) {
 	return unix.Ftruncate(fd, length)
 }

pkg/osutil/osutil_windows.go

@@ -34,6 +34,10 @@ func SysKill(pid int, _ Signal) error {
 	return windows.GenerateConsoleCtrlEvent(syscall.CTRL_BREAK_EVENT, uint32(pid))
 }
 
+func Dup2(oldfd int, newfd syscall.Handle) (err error) {
+	return fmt.Errorf("unimplemented")
+}
+
 func Ftruncate(_ int, _ int64) (err error) {
 	return fmt.Errorf("unimplemented")
 }

pkg/start/start.go

@@ -10,6 +10,7 @@ import (
 	"os/exec"
 	"path/filepath"
 	"runtime"
+	"syscall"
 	"text/template"
 	"time"
 
@@ -19,6 +20,7 @@ import (
 	"github.com/lima-vm/lima/pkg/osutil"
 	"github.com/lima-vm/lima/pkg/qemu"
 	"github.com/lima-vm/lima/pkg/qemu/entitlementutil"
+	"github.com/mattn/go-isatty"
 
 	"github.com/lima-vm/lima/pkg/downloader"
 	"github.com/lima-vm/lima/pkg/fileutils"
@@ -115,7 +117,7 @@ func Prepare(ctx context.Context, inst *store.Instance) (*Prepared, error) {
 
 // Start starts the instance.
 // Start calls Prepare by itself, so you do not need to call Prepare manually before calling Start.
-func Start(ctx context.Context, inst *store.Instance) error {
+func Start(ctx context.Context, inst *store.Instance, launchHostAgentForeground bool) error {
 	haPIDPath := filepath.Join(inst.Dir, filenames.HostAgentPID)
 	if _, err := os.Stat(haPIDPath); !errors.Is(err, os.ErrNotExist) {
 		return fmt.Errorf("instance %q seems running (hint: remove %q if the instance is not actually running)", inst.Name, haPIDPath)
@@ -194,7 +196,29 @@ func Start(ctx context.Context, inst *store.Instance) error {
 
 	begin := time.Now() // used for logrus propagation
 
-	if err := haCmd.Start(); err != nil {
+	if launchHostAgentForeground {
+		logrus.Info("Running the host agent in the foreground")
+		if isatty.IsTerminal(os.Stdin.Fd()) || isatty.IsCygwinTerminal(os.Stdin.Fd()) {
+			// Write message to standard log files to avoid confusing users
+			message := "This log file is not used because `limactl start` was launched in the terminal with the `--foreground` option."
+			if _, err := haStdoutW.WriteString(message); err != nil {
+				return err
+			}
+			if _, err := haStderrW.WriteString(message); err != nil {
+				return err
+			}
+		} else {
+			if err := osutil.Dup2(int(haStdoutW.Fd()), syscall.Stdout); err != nil {
+				return err
+			}
+			if err := osutil.Dup2(int(haStderrW.Fd()), syscall.Stderr); err != nil {
+				return err
+			}
+		}
+		if err := syscall.Exec(self, haCmd.Args, haCmd.Environ()); err != nil {
+			return err
+		}
+	} else if err := haCmd.Start(); err != nil {
 		return err
 	}