monitor: add shell escape feature

Debuggers like GDB support prefixing a command with an exclamation mark
"!" to pass the rest of the command to the shell. We can mimic this
functionality.

Additionally, we can enable shlex parsing easily by using the
google/shlex utility.

Signed-off-by: Justin Chadwell <me@jedevc.com>

monitor/monitor.go

@@ -2,8 +2,11 @@ package monitor
 
 import (
 	"context"
+	"errors"
 	"fmt"
 	"io"
+	"os/exec"
+	"runtime"
 	"sort"
 	"strings"
 	"sync"
@@ -12,6 +15,7 @@ import (
 	"github.com/containerd/console"
 	controllerapi "github.com/docker/buildx/commands/controller/pb"
 	"github.com/docker/buildx/util/ioset"
+	"github.com/google/shlex"
 	"github.com/sirupsen/logrus"
 	"golang.org/x/term"
 )
@@ -116,13 +120,30 @@ func RunMonitor(ctx context.Context, curRef string, options controllerapi.BuildO
 					}
 					return
 				}
-				args := strings.Fields(l) // TODO: use shlex
+
+				if strings.HasPrefix(l, "!") {
+					raw := defaultShell(runtime.GOOS)
+					raw = append(raw, l[1:])
+					cmd := exec.Command(raw[0], raw[1:]...)
+
+					// cmd.Stdin = stdin // TODO: support stdin
+					cmd.Stdout = stdout
+					cmd.Stderr = stderr
+					if err := cmd.Run(); err != nil && !errors.Is(err, &exec.ExitError{}) {
+						fmt.Fprintf(stdout, "command failed: %v\n", err)
+					}
+					continue
+				}
+
+				args, err := shlex.Split(l)
+				if err != nil {
+					fmt.Fprintf(stdout, "failed to parse command: %v", err)
+					continue
+				}
 				if len(args) == 0 {
 					continue
 				}
 				switch args[0] {
-				case "":
-					// nop
 				case "reload":
 					if curRef != "" {
 						if err := c.Disconnect(ctx, curRef); err != nil {
@@ -255,3 +276,10 @@ type nopCloser struct {
 }
 
 func (c nopCloser) Close() error { return nil }
+
+func defaultShell(os string) []string {
+	if os == "windows" {
+		return []string{"cmd", "/S", "/C"}
+	}
+	return []string{"/bin/sh", "-c"}
+}