Add Windows service support

This branch adds the abilitity to run buildkitd as a Windows service.
Most of this code is shared with dockerd and containerd, with slight
changes to adapt it to buildkit.

Signed-off-by: Gabriel Adrian Samfira <gsamfira@cloudbasesolutions.com>

Author: gabriel-samfira

cmd/buildkitd/main.go

@@ -199,6 +199,7 @@ func main() {
 		},
 	)
 	app.Flags = append(app.Flags, appFlags...)
+	app.Flags = append(app.Flags, serviceFlags()...)
 
 	app.Action = func(c *cli.Context) error {
 		// TODO: On Windows this always returns -1. The actual "are you admin" check is very Windows-specific.
@@ -254,6 +255,15 @@ func main() {
 			return errors.Wrapf(err, "failed to create %s", root)
 		}
 
+		// Stop if we are registering or unregistering against Windows SCM.
+		stop, err := registerUnregisterService(cfg.Root)
+		if err != nil {
+			logrus.Fatal(err)
+		}
+		if stop {
+			return nil
+		}
+
 		lockPath := filepath.Join(root, "buildkitd.lock")
 		lock := flock.New(lockPath)
 		locked, err := lock.TryLock()
@@ -291,6 +301,12 @@ func main() {
 				}
 			}
 		}
+
+		// Launch as a Windows Service if necessary
+		if err := launchService(server); err != nil {
+			logrus.Fatal(err)
+		}
+
 		errCh := make(chan error, 1)
 		if err := serveGRPC(cfg.GRPC, server, errCh); err != nil {
 			return err
@@ -486,6 +502,8 @@ func applyMainFlags(c *cli.Context, cfg *config.Config) error {
 	if tlsca := c.String("tlscacert"); tlsca != "" {
 		cfg.GRPC.TLS.CA = tlsca
 	}
+	applyPlatformFlags(c)
+
 	return nil
 }
 

cmd/buildkitd/service_unix.go

@@ -0,0 +1,29 @@
+//go:build !windows
+// +build !windows
+
+package main
+
+import (
+	"github.com/urfave/cli"
+	"google.golang.org/grpc"
+)
+
+// serviceFlags returns an array of flags for configuring buildkitd to run
+// as a service. Only relevant on Windows.
+func serviceFlags() []cli.Flag {
+	return []cli.Flag{}
+}
+
+// applyPlatformFlags applies platform-specific flags.
+func applyPlatformFlags(context *cli.Context) {
+}
+
+// registerUnregisterService is only relevant on Windows.
+func registerUnregisterService(root string) (bool, error) {
+	return false, nil
+}
+
+// launchService is only relevant on Windows.
+func launchService(s *grpc.Server) error {
+	return nil
+}