automatically start/upgrade buildkitd

- Automatically start a buildkit daemon if no BUILDKIT_HOST is
  provided (and if not already started)
- Customization of BUILDKIT_HOST is still possible, just like before
- Automatically upgrade the managed daemon to the version used by dagger
  if necessary
- Add CI test to make sure the managed buildkit and the vendored
  buildkit versions match

Signed-off-by: Andrea Luzzardi <aluzzardi@gmail.com>

pkg/buildkitd/buildkitd.go

@@ -0,0 +1,168 @@
+package buildkitd
+
+import (
+	"context"
+	"fmt"
+	"os/exec"
+	"strings"
+
+	"github.com/docker/distribution/reference"
+	"github.com/rs/zerolog/log"
+)
+
+const (
+	image         = "moby/buildkit"
+	version       = "v0.8.2"
+	imageVersion  = image + ":" + version
+	containerName = "dagger-buildkitd"
+)
+
+func Start(ctx context.Context) (string, error) {
+	lg := log.Ctx(ctx)
+
+	// Attempt to detect the current buildkit version
+	currentVersion, err := getBuildkitVersion(ctx)
+	if err != nil {
+		// If that failed, it might either be because buildkitd is not running
+		// or because the docker CLI is out of service.
+		if err := checkDocker(ctx); err != nil {
+			return "", err
+		}
+
+		currentVersion = ""
+		lg.Debug().Msg("no buildkit daemon detected")
+	} else {
+		lg.Debug().Str("version", currentVersion).Msg("detected buildkit version")
+	}
+
+	if currentVersion != version {
+		if currentVersion != "" {
+			lg.
+				Info().
+				Str("version", version).
+				Msg("upgrading buildkit")
+			if err := remvoveBuildkit(ctx); err != nil {
+				return "", err
+			}
+		} else {
+			lg.
+				Info().
+				Str("version", version).
+				Msg("starting buildkit")
+		}
+		if err := startBuildkit(ctx); err != nil {
+			return "", err
+		}
+	}
+
+	return fmt.Sprintf("docker-container://%s", containerName), nil
+}
+
+// ensure the docker CLI is available and properly set up (e.g. permissions to
+// communicate with the daemon, etc)
+func checkDocker(ctx context.Context) error {
+	cmd := exec.CommandContext(ctx, "docker", "info")
+	output, err := cmd.CombinedOutput()
+	if err != nil {
+		log.
+			Ctx(ctx).
+			Error().
+			Err(err).
+			Bytes("output", output).
+			Msg("failed to run docker")
+		return err
+	}
+
+	return nil
+}
+
+func startBuildkit(ctx context.Context) error {
+	lg := log.
+		Ctx(ctx).
+		With().
+		Str("version", version).
+		Logger()
+
+	lg.Debug().Msg("pulling buildkit image")
+	cmd := exec.CommandContext(ctx,
+		"docker",
+		"pull",
+		imageVersion,
+	)
+	output, err := cmd.CombinedOutput()
+	if err != nil {
+		lg.
+			Error().
+			Err(err).
+			Bytes("output", output).
+			Msg("failed to pull buildkit image")
+		return err
+	}
+
+	cmd = exec.CommandContext(ctx,
+		"docker",
+		"run",
+		"-d",
+		"--restart", "always",
+		"--name", containerName,
+		"--privileged",
+		imageVersion,
+	)
+	output, err = cmd.CombinedOutput()
+	if err != nil {
+		log.
+			Ctx(ctx).
+			Error().
+			Err(err).
+			Bytes("output", output).
+			Msg("unable to start buildkitd")
+		return err
+	}
+	return nil
+}
+
+func remvoveBuildkit(ctx context.Context) error {
+	lg := log.
+		Ctx(ctx)
+
+	cmd := exec.CommandContext(ctx,
+		"docker",
+		"rm",
+		"-fv",
+		containerName,
+	)
+	output, err := cmd.CombinedOutput()
+	if err != nil {
+		lg.
+			Error().
+			Err(err).
+			Bytes("output", output).
+			Msg("failed to stop buildkit")
+		return err
+	}
+
+	return nil
+}
+
+func getBuildkitVersion(ctx context.Context) (string, error) {
+	cmd := exec.CommandContext(ctx,
+		"docker",
+		"inspect",
+		"--format",
+		"{{.Config.Image}}",
+		containerName,
+	)
+	output, err := cmd.CombinedOutput()
+	if err != nil {
+		return "", err
+	}
+	ref, err := reference.ParseNormalizedNamed(strings.TrimSpace(string(output)))
+	if err != nil {
+		return "", err
+	}
+	tag, ok := ref.(reference.Tagged)
+	if !ok {
+		return "", fmt.Errorf("failed to parse image: %s", output)
+	}
+	return tag.Tag(), nil
+}