mesh-controller/internal/builder/builder.go

package builder

import (
	"archive/tar"
	"compress/gzip"
	"context"
	"crypto/sha256"
	"encoding/hex"
	"fmt"
	"io"
	"os"
	"os/exec"
	"path/filepath"
	"sort"
	"strings"

	"github.com/novox/mesh-control/internal/catalogue"
)

// Turning a repository into artifacts the mesh can pin.
//
// **This runs on a node, not in the control plane.** Building needs a container runtime and a
// working tree, and the control plane deliberately cannot run commands on a machine — what it may
// send is bounded by the declaration language (novox/hq ADR 0005), and "run this build" is not in
// it. So the builder is something a node runs *as a module*, given work over the broker like
// anything else, and this package is what it does when it gets some.
//
// The alternative — the control plane holding a docker socket — would make it the one component
// that can do anything on a machine, which is the property the whole design is arranged to avoid.

// Runner runs a command in a directory and returns what it said. Injected so the tests do not
// need docker and git, and so the failure of either is reported rather than assumed.
type Runner func(ctx context.Context, dir string, name string, args ...string) (string, error)

// Publisher puts an artifact somewhere a machine can fetch it, and says how to refer to it.
type Publisher interface {
	// PublishImage pushes a locally built image and returns a reference pinned by digest.
	PublishImage(ctx context.Context, localTag, repository string) (string, error)
	// PublishArchive stores bytes and returns where to fetch them from.
	PublishArchive(ctx context.Context, repository string, body []byte, digest string) (string, error)
}

// Result is everything one build produced.
type Result struct {
	// Manifest is the module as the mesh should hold it: artifacts resolved to digests.
	Manifest catalogue.Manifest
	// Commit is what was built, so "is this current?" is answerable without building again.
	Commit string
	// Built is each artifact, for reporting.
	Built []catalogue.Built
}

// Build clones a repository at a ref, reads its manifest, produces what it declares, publishes
// each, and returns the manifest the mesh should hold.
//
// **Nothing is published until everything is built.** A module whose image succeeded and whose
// archive failed would otherwise leave half of itself in the store under a digest the mesh never
// records — reachable, unreferenced, and indistinguishable from something in use.
func Build(ctx context.Context, run Runner, publish Publisher,
	repository, ref, workspace string) (Result, error) {

	// Made rather than required. A builder that fails because the directory it was told to work
	// in does not exist is a builder that needs a setup step nobody documented.
	if err := os.MkdirAll(workspace, 0o755); err != nil {
		return Result{}, err
	}
	tree := filepath.Join(workspace, "source")
	if err := os.RemoveAll(tree); err != nil {
		return Result{}, err
	}
	// A fresh clone every time rather than a fetch into a tree that is already there. A build
	// that reuses a working tree can succeed because of something a previous build left behind,
	// and that is a build nobody can reproduce.
	if _, err := run(ctx, workspace, "git", "clone", "--quiet", repository, tree); err != nil {
		return Result{}, fmt.Errorf("cannot clone %s: %w", repository, err)
	}
	if ref != "" {
		if _, err := run(ctx, tree, "git", "checkout", "--quiet", ref); err != nil {
			return Result{}, fmt.Errorf("%s has no %s: %w", repository, ref, err)
		}
	}
	commit, err := run(ctx, tree, "git", "rev-parse", "HEAD")
	if err != nil {
		return Result{}, err
	}
	commit = strings.TrimSpace(commit)

	raw, err := os.ReadFile(filepath.Join(tree, ManifestName))
	if err != nil {
		return Result{}, fmt.Errorf(
			"%s has no %s at its root, so there is nothing saying what it is: %w",
			repository, ManifestName, err)
	}
	manifest, err := catalogue.ParseManifest(raw)
	if err != nil {
		return Result{}, err
	}

	var built []catalogue.Built
	if manifest.Build != nil {
		artifacts := append([]catalogue.Artifact{}, manifest.Build.Artifacts...)
		// Ordered, so two builds of one commit do the same work in the same sequence and their
		// logs can be compared.
		sort.Slice(artifacts, func(i, j int) bool { return artifacts[i].Name < artifacts[j].Name })
		for _, a := range artifacts {
			made, err := one(ctx, run, publish, manifest.Module, tree, commit, a)
			if err != nil {
				return Result{}, err
			}
			built = append(built, made)
		}
	}

	resolved, err := manifest.Resolve(built)
	if err != nil {
		return Result{}, err
	}
	return Result{Manifest: resolved, Commit: commit, Built: built}, nil
}

// ManifestName is the one file a module repository must have.
//
// At the root, and named the same in every repository. A convention somebody can look for beats a
// setting somebody has to find.
const ManifestName = "module.json"

func one(ctx context.Context, run Runner, publish Publisher,
	module, tree, commit string, a catalogue.Artifact) (catalogue.Built, error) {

	switch a.Kind {
	case catalogue.ArtifactImage:
		// Tagged by commit rather than by version, because a version is what a person calls a
		// release and a commit is what was actually built. The mesh pins the digest anyway; this
		// is only so a person looking at the build node can tell what is there.
		local := fmt.Sprintf("%s-%s:%s", module, a.Name, short(commit))
		if _, err := run(ctx, tree, "docker", "build", "-f", a.From, "-t", local, "."); err != nil {
			return catalogue.Built{}, fmt.Errorf("%s: building %s failed: %w", module, a.Name, err)
		}
		reference, err := publish.PublishImage(ctx, local, module+"/"+a.Name)
		if err != nil {
			return catalogue.Built{}, err
		}
		return catalogue.Built{Name: a.Name, Kind: a.Kind, Reference: reference}, nil

	case catalogue.ArtifactArchive:
		body, err := pack(filepath.Join(tree, a.From))
		if err != nil {
			return catalogue.Built{}, fmt.Errorf("%s: packing %s failed: %w", module, a.Name, err)
		}
		sum := sha256.Sum256(body)
		digest := "sha256:" + hex.EncodeToString(sum[:])
		where, err := publish.PublishArchive(ctx, module+"/"+a.Name, body, digest)
		if err != nil {
			return catalogue.Built{}, err
		}
		return catalogue.Built{Name: a.Name, Kind: a.Kind, Reference: where, Digest: digest}, nil
	}
	return catalogue.Built{}, fmt.Errorf("%s: %q is a %q, which is not something this builds",
		module, a.Name, a.Kind)
}

// pack tars and gzips a directory.
//
// **Deterministically**: entries sorted, and no timestamps, uid, gid or original names carried
// through. Two builds of one commit must produce one digest, or nothing downstream can tell "this
// changed" from "this was built again" — and every rebuild would look like a change to every
// machine holding it.
func pack(root string) ([]byte, error) {
	info, err := os.Stat(root)
	if err != nil {
		return nil, err
	}
	if !info.IsDir() {
		return nil, fmt.Errorf("%s is not a directory", root)
	}

	var paths []string
	err = filepath.Walk(root, func(path string, info os.FileInfo, err error) error {
		if err != nil {
			return err
		}
		if info.IsDir() || !info.Mode().IsRegular() {
			// Only files. A symlink or a device in an archive is refused by the host that unpacks
			// it, so putting one in would build something that cannot be applied.
			if !info.IsDir() && !info.Mode().IsRegular() {
				return fmt.Errorf("%s is neither a file nor a directory, and an archive carries "+
					"only those", path)
			}
			return nil
		}
		paths = append(paths, path)
		return nil
	})
	if err != nil {
		return nil, err
	}
	// filepath.Walk is documented to walk in lexical order, so this is belt and braces rather
	// than load-bearing — and no test distinguishes it, which is worth saying rather than
	// implying otherwise. It stays because the cost is nothing and the failure it guards against
	// is silent: an archive whose digest changes because the traversal did.
	sort.Strings(paths)

	var out strings.Builder
	zipped := gzip.NewWriter(&stringWriter{&out})
	writer := tar.NewWriter(zipped)
	for _, path := range paths {
		body, err := os.ReadFile(path)
		if err != nil {
			return nil, err
		}
		relative, err := filepath.Rel(root, path)
		if err != nil {
			return nil, err
		}
		info, err := os.Stat(path)
		if err != nil {
			return nil, err
		}
		mode := int64(info.Mode().Perm())
		if err := writer.WriteHeader(&tar.Header{
			Name: filepath.ToSlash(relative), Mode: mode, Size: int64(len(body)),
			Typeflag: tar.TypeReg,
			// Everything else left at its zero value on purpose — see the note above.
		}); err != nil {
			return nil, err
		}
		if _, err := writer.Write(body); err != nil {
			return nil, err
		}
	}
	if err := writer.Close(); err != nil {
		return nil, err
	}
	if err := zipped.Close(); err != nil {
		return nil, err
	}
	return []byte(out.String()), nil
}

type stringWriter struct{ to *strings.Builder }

func (w *stringWriter) Write(p []byte) (int, error) { return w.to.Write(p) }

func short(commit string) string {
	if len(commit) > 8 {
		return commit[:8]
	}
	return commit
}

// Command is a Runner that actually runs things.
func Command(ctx context.Context, dir, name string, args ...string) (string, error) {
	cmd := exec.CommandContext(ctx, name, args...)
	cmd.Dir = dir
	out, err := cmd.CombinedOutput()
	if err != nil {
		return string(out), fmt.Errorf("%s %s: %w\n%s",
			name, strings.Join(args, " "), err, strings.TrimSpace(string(out)))
	}
	return string(out), nil
}

var _ io.Writer = (*stringWriter)(nil)