mesh-controller/internal/builder/builder.go

package builder

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

	"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 {
	// Against is every pinned image this build was built on top of, read out of its own inputs.
	//
	// **Derived, not declared** (novox/hq ADR 0009): a declared list of dependencies drifts from
	// what the code actually uses, and an artifact is out of date when anything it was built
	// against moved. These are artifact references rather than module-versions, because that is
	// what a build input names; resolving them to modules is the catalogue's work, since it is
	// what knows which module-version published which artifact.
	Against []string

	// 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, path, ref, workspace string, held map[string]string, npmrc Npmrc, log Log) (Result, error) {

	say := logging(log)
	say("clone", "%s%s at %s", repository, describePath(path), refOrHead(ref))

	// 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 {
		say("clone", "FAILED: %v", err)
		return Result{}, fmt.Errorf("cannot clone %s: %w", repository, err)
	}
	say("clone", "done")
	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)
	say("commit", "%s", short(commit))

	// A module is a repository and a path within it (novox/hq ADR 0069). The ordinary case is an
	// empty path, meaning the repository's root; a repository holding several modules names each
	// by its own directory, which is what the catalogue is and what the system this replaces has
	// always done.
	within, err := inside(tree, path)
	if err != nil {
		return Result{}, err
	}

	raw, err := os.ReadFile(filepath.Join(within, ManifestName))
	if err != nil {
		return Result{}, fmt.Errorf(
			"%s has no %s at %s, so there is nothing saying what it is: %w",
			repository, ManifestName, describe(path), err)
	}
	manifest, err := catalogue.ParseManifest(raw)
	if err != nil {
		say("manifest", "INVALID: %v", err)
		return Result{}, err
	}
	say("manifest", "%s v%s — %d artifact(s)", manifest.Module, manifest.Version, artifactCount(manifest))

	// A build-time credential, written where a build can mount it but never where it can be copied
	// into an image or committed: under the workspace, beside the clone, not inside it. Absent when
	// this mesh has no package registry yet, which is the bootstrap case (novox/hq ADR 0076).
	var npmrcPath string
	if npmrc.Enabled() {
		content, err := npmrc.File()
		if err != nil {
			return Result{}, err
		}
		npmrcPath = filepath.Join(workspace, "npmrc")
		if err := os.WriteFile(npmrcPath, []byte(content), 0o600); err != nil {
			return Result{}, fmt.Errorf("cannot write the package-registry credential for the build: %w", err)
		}
		say("packages", "resolving %s from the mesh's package registry", npmrc.Scope)
	}

	var built []catalogue.Built
	if manifest.Build != nil {
		// What this module said it stands on, answered with what this mesh actually holds. Done
		// before anything is built, so a missing base is refused in front of the person who can
		// fix it rather than inside a build that stops on its own first line.
		args, err := standingOn(manifest, held)
		if err != nil {
			say("bases", "UNMET: %v", err)
			return Result{}, err
		}
		if len(args) > 0 {
			say("bases", "%d resolved from what the mesh holds", len(args)/2)
		}
		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 {
			say("artifact", "%s (%s%s) — starting", a.Name, a.Kind, langSuffix(a))
			made, err := one(ctx, run, publish, manifest.Module, within, commit, a, args, held, npmrcPath, say)
			if err != nil {
				say("artifact", "%s FAILED: %v", a.Name, err)
				return Result{}, err
			}
			say("artifact", "%s done — %s", a.Name, describeMade(made))
			built = append(built, made)
		}
	}

	resolved, err := manifest.Resolve(built)
	if err != nil {
		say("resolve", "FAILED: %v", err)
		return Result{}, err
	}
	say("done", "%s at %s — %d artifact(s) pinned", manifest.Module, short(commit), len(built))
	return Result{Manifest: resolved, Commit: commit, Built: built,
		Against: against(within, manifest)}, nil
}

// Log is where a build says what it is doing, step by step. Nil is silent — the tests pass none,
// and a build with nowhere to speak must still build.
type Log func(step, message string)

func logging(log Log) func(step, format string, args ...any) {
	if log == nil {
		return func(string, string, ...any) {}
	}
	return func(step, format string, args ...any) {
		log(step, fmt.Sprintf(format, args...))
	}
}

func describePath(path string) string {
	if path == "" {
		return ""
	}
	return " at " + path
}

func refOrHead(ref string) string {
	if ref == "" {
		return "HEAD"
	}
	return ref
}

func artifactCount(m catalogue.Manifest) int {
	if m.Build == nil {
		return 0
	}
	return len(m.Build.Artifacts)
}

func langSuffix(a catalogue.Artifact) string {
	if a.Language != "" {
		return ", " + a.Language
	}
	return ""
}

func describeMade(made catalogue.Built) string {
	if made.Digest != "" {
		return made.Kind + " " + short(strings.TrimPrefix(made.Digest, "sha256:"))
	}
	return made.Kind + " " + made.Reference
}

// inside resolves a module's path within a clone, and refuses one that leaves it.
//
// **A build reads only its own tree.** A path of `../../etc` would otherwise make a build read —
// and an archive artifact publish — whatever the build machine happens to hold, which is the one
// thing a machine that builds other people's repositories must not do.
func inside(tree, path string) (string, error) {
	if path == "" {
		return tree, nil
	}
	if filepath.IsAbs(path) {
		return "", fmt.Errorf(
			"a module's path is inside its repository, and %q is an absolute path", path)
	}
	within := filepath.Join(tree, path)
	rel, err := filepath.Rel(tree, within)
	if err != nil || rel == ".." || strings.HasPrefix(rel, ".."+string(filepath.Separator)) {
		return "", fmt.Errorf(
			"%q leaves the repository, and a build reads only its own tree", path)
	}
	return within, nil
}

// describe says where a manifest was looked for, in words a person can act on.
func describe(path string) string {
	if path == "" {
		return "its root"
	}
	return path
}

// pinnedImage matches an image reference pinned by digest, which is the only kind a build input is
// allowed to name — a tag is something somebody else can move under you.
var pinnedImage = regexp.MustCompile(`[A-Za-z0-9][A-Za-z0-9._/:-]*@sha256:[0-9a-f]{64}`)

// against reads what this module's image artifacts are built on top of, out of the files that
// build them. Nothing is guessed: a reference that is not written down is not reported.
func against(within string, manifest catalogue.Manifest) []string {
	if manifest.Build == nil {
		return nil
	}
	seen := map[string]bool{}
	var out []string
	for _, a := range manifest.Build.Artifacts {
		if a.Kind != catalogue.ArtifactImage || a.From == "" {
			continue
		}
		body, err := os.ReadFile(filepath.Join(within, a.From))
		if err != nil {
			// Not fatal: the build itself already failed if this file was needed and missing, and
			// reporting no edges is honest where inventing them would not be.
			continue
		}
		for _, found := range pinnedImage.FindAllString(string(body), -1) {
			if !seen[found] {
				seen[found] = true
				out = append(out, found)
			}
		}
	}
	sort.Strings(out)
	return out
}

// 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, args []string,
	held map[string]string, npmrc string, say func(step, format string, args ...any)) (catalogue.Built, error) {

	switch a.Kind {
	case catalogue.ArtifactUpstream:
		// Mirrored, not built. Pulled by the reference the module names and pushed under a name
		// of the mesh's own, so what a machine fetches is pinned by a digest this registry
		// assigned rather than by a tag somebody else can move.
		say("mirror", "pulling %s", a.From)
		if _, err := run(ctx, tree, "docker", "pull", a.From); err != nil {
			return catalogue.Built{}, fmt.Errorf("%s: cannot fetch %s: %w", module, a.From, err)
		}
		say("mirror", "publishing under the mesh's own name")
		reference, err := publish.PublishImage(ctx, a.From, module+"/"+a.Name)
		if err != nil {
			return catalogue.Built{}, err
		}
		return catalogue.Built{Name: a.Name, Kind: a.Kind, Reference: reference}, nil

	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))
		// The bases this module named, resolved to what this mesh holds. A recipe reads them as
		// build arguments, so a module says which module it stands on and never which copy.
		invocation := append([]string{"build", "-f", a.From, "-t", local}, args...)
		if a.Target != "" {
			invocation = append(invocation, "--target", a.Target)
		}
		if npmrc != "" {
			// Given to the build as a buildkit secret, so a RUN that needs the package registry mounts
			// it at that step and it is in no image layer. A Dockerfile that does not ask for it is
			// unaffected; the secret is simply not read (novox/hq ADR 0076).
			invocation = append(invocation, "--secret", "id=npmrc,src="+npmrc)
		}
		invocation = append(invocation, ".")
		say("image", "docker build -f %s", a.From)
		if _, err := run(ctx, tree, "docker", invocation...); err != nil {
			return catalogue.Built{}, fmt.Errorf("%s: building %s failed: %w", module, a.Name, err)
		}
		say("image", "built, publishing")
		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.ArtifactBundle:
		// **The one recipe that both builds and packs.** Everything else either produces an image
		// or packs what is already there; this compiles the module's own code first, in a
		// toolchain the mesh chose from what the module said it was written in, and packs the
		// result.
		//
		// The compiler runs in a container rather than on the build machine, for the reason every
		// other build does: what a build needs installed is the toolchain's business, and a build
		// machine that accumulated one toolchain per language would be a machine nobody could
		// reproduce.
		chain, err := ToolchainFor(a.Language)
		if err != nil {
			return catalogue.Built{}, fmt.Errorf("%s: %s: %w", module, a.Name, err)
		}
		base, ok := held[chain.Base+"/"+chain.Artifact]
		if !ok {
			// Named, not pinned: the mesh answers with the copy it holds. Refused before anything
			// is built, saying which module has to exist first, rather than failing inside a
			// compile with a message about an image (novox/hq 04-ISSUES/044).
			return catalogue.Built{}, fmt.Errorf(
				"%s: %s is written in %s, which is compiled by %s's %q artifact, and this mesh "+
					"holds no copy of it. Build %s first",
				module, a.Name, chain.Language, chain.Base, chain.Artifact, chain.Base)
		}
		say("bundle", "compiling %s in %s's toolchain", a.Language, chain.Base)
		compiled, err := compile(ctx, run, tree, chain, base, a)
		if err != nil {
			return catalogue.Built{}, fmt.Errorf("%s: compiling %s failed: %w", module, a.Name, err)
		}
		say("bundle", "compiled, packing")
		body, err := pack(compiled)
		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

	case catalogue.ArtifactPackage:
		// Built and published on a public base, to the mesh's package registry, by version
		// (novox/hq ADR 0076). Not an image, not an archive: nothing on a machine references it, so
		// there is no Publisher call — the container itself publishes, with the credential the
		// build was handed.
		say("package", "building and publishing %s (%s)", a.Name, a.Language)
		reference, err := publishPackage(ctx, run, module, tree, a, npmrc, say)
		if err != nil {
			return catalogue.Built{}, fmt.Errorf("%s: publishing %s failed: %w", module, a.Name, err)
		}
		say("package", "published %s", reference)
		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) {
	// **Every command is echoed before it runs**, with where. On a build that hangs, the last line
	// is exactly the command it is inside — which is the difference between "the builder did
	// nothing" and "git clone is waiting on a network that will not answer". Silent on success is
	// what made an empty workspace unreadable.
	started := timeNow()
	fmt.Fprintf(os.Stderr, "  $ (%s) %s %s\n", short(filepath.Base(dir)), name, strings.Join(args, " "))
	cmd := exec.CommandContext(ctx, name, args...)
	cmd.Dir = dir
	out, err := cmd.CombinedOutput()
	if err != nil {
		fmt.Fprintf(os.Stderr, "  ! %s %s failed after %s\n", name, args[0], since(started))
		return string(out), fmt.Errorf("%s %s: %w\n%s",
			name, strings.Join(args, " "), err, strings.TrimSpace(string(out)))
	}
	fmt.Fprintf(os.Stderr, "  ✓ %s %s (%s)\n", name, firstArg(args), since(started))
	return string(out), nil
}

func firstArg(args []string) string {
	if len(args) == 0 {
		return ""
	}
	return args[0]
}

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

// standingOn turns the bases a module named into build arguments for what this mesh holds.
//
// **Refused rather than defaulted** (novox/hq issue 044). A module naming a base the mesh has not
// built cannot be built here yet, and the useful sentence names which module is missing — not the
// one a container runtime produces when a recipe's first line refers to an image nobody has.
//
// The order is fixed so two builds of one commit invoke the same command.
func standingOn(manifest catalogue.Manifest, held map[string]string) ([]string, error) {
	if manifest.Build == nil || len(manifest.Build.On) == 0 {
		return nil, nil
	}
	on := append([]catalogue.BuildsOn{}, manifest.Build.On...)
	sort.Slice(on, func(i, j int) bool { return on[i].Arg < on[j].Arg })

	var args []string
	for _, base := range on {
		if base.Arg == "" || base.Module == "" || base.Artifact == "" {
			return nil, fmt.Errorf(
				"%s says its build stands on something, and does not say all of what: a base "+
					"needs the module, the artifact, and the build argument the recipe reads it "+
					"from", manifest.Module)
		}
		key := base.Module + "/" + base.Artifact
		reference, has := held[key]
		if !has {
			return nil, fmt.Errorf(
				"%s builds on %s, and this mesh has not built it. Build %s first — every module "+
					"in this toolchain stands on it, so it is the thing to have before anything "+
					"else", manifest.Module, key, base.Module)
		}
		args = append(args, "--build-arg", base.Arg+"="+reference)
	}
	return args, nil
}

// compile runs a module's own code through its toolchain, and says where the result is.
//
// **In the module's own directory, under the path the toolchain expects.** A module is compiled
// where its dependencies resolve upward into the base's own library directory, so what it is
// compiled against is exactly what it will run against — the reason every hand-written Dockerfile
// had to choose a working directory carefully, and the reason none of them has to now.
// publishPackage builds a `package` artifact on a public base image and publishes it to the mesh's
// package registry by version. The credential arrives as an .npmrc file the build was handed
// (novox/hq ADR 0076); it is mounted read-only into the container rather than baked, because a
// package build produces no image to leak it into. The reference returned is name@version, read from
// the module's own package.json — the same two fields npm publishes under.
func publishPackage(ctx context.Context, run Runner, module, dir string, a catalogue.Artifact,
	npmrc string, say func(step, format string, args ...any)) (string, error) {

	recipe, ok := packageRecipes[a.Language]
	if !ok {
		return "", fmt.Errorf(
			"a package written in %q cannot be built: no public toolchain is known for it", a.Language)
	}
	if npmrc == "" {
		// A package with nowhere to be published is not built. Said here rather than failing inside
		// npm publish with a message about a registry that is simply absent.
		return "", fmt.Errorf(
			"%s is a package and this build was given no package registry to publish it to", a.Name)
	}

	raw, err := os.ReadFile(filepath.Join(dir, "package.json"))
	if err != nil {
		return "", fmt.Errorf("a package is published by name and version, and %s has no package.json: %w", module, err)
	}
	var pkg struct {
		Name    string `json:"name"`
		Version string `json:"version"`
	}
	if err := json.Unmarshal(raw, &pkg); err != nil {
		return "", fmt.Errorf("%s's package.json is not readable: %w", module, err)
	}
	if pkg.Name == "" || pkg.Version == "" {
		return "", fmt.Errorf("%s's package.json names no %s to publish under",
			module, either(pkg.Name == "", "name", "version"))
	}

	const within = "/app/module"
	invocation := []string{
		"run", "--rm",
		"--volume", dir + ":" + within,
		// Read-only, so a build cannot alter the credential, and at /root where npm reads it.
		"--volume", npmrc + ":/root/.npmrc:ro",
		"--workdir", within,
		recipe.Base,
		"sh", "-c", recipe.Script,
	}
	if _, err := run(ctx, dir, "docker", invocation...); err != nil {
		return "", err
	}
	return pkg.Name + "@" + pkg.Version, nil
}

// either names whichever of two fields is the missing one, for a message that says which.
func either(first bool, a, b string) string {
	if first {
		return a
	}
	return b
}

func compile(ctx context.Context, run Runner, tree string, chain Toolchain,
	base string, a catalogue.Artifact) (string, error) {

	// Where inside the toolchain the module's source is mounted, and where its output lands. Fixed
	// rather than configurable: a module that could move this would be describing its own build.
	const within = "/app/modules/module"

	// **Its own output directory, because a module may be several languages at once.** One module
	// is one piece of software and can still carry a daemon in one language, tools in another and
	// a package in a third (ADR 0040). Compiling them all into one place would have them overwrite
	// each other and then be packed together, so each bundle compiles and packs alone.
	out := Out(a.Name)

	invocation := []string{
		"run", "--rm",
		"--volume", tree + ":" + within,
		"--workdir", within,
		base,
	}
	invocation = append(invocation, chain.Compile...)
	if chain.OutputFlag != "" {
		invocation = append(invocation, chain.OutputFlag, out)
	}
	// What to compile. Named by the module rather than discovered, so adding a file does not
	// silently change what a build produces.
	if len(a.Entrypoints) > 0 {
		invocation = append(invocation, sourcesFor(a.Entrypoints, out)...)
	}
	if _, err := run(ctx, tree, "docker", invocation...); err != nil {
		return "", err
	}
	return filepath.Join(tree, out), nil
}

// sourcesFor turns compiled entrypoints back into what to compile.
//
// A module names what a tool host should LOAD — compiled paths under the bundle's root — because
// that is the thing anything else needs to know. What to compile is the same list with the
// language's own extension, which is the toolchain's business rather than the module's.
func sourcesFor(entrypoints []string, out string) []string {
	sources := make([]string, 0, len(entrypoints))
	for _, e := range entrypoints {
		// An entrypoint is named as it will be FOUND — a path inside the unpacked bundle — so the
		// source is the same path with the output directory taken off the front and the language's
		// own extension on the end.
		at := strings.TrimPrefix(strings.TrimPrefix(e, out), "/")
		sources = append(sources, strings.TrimSuffix(at, filepath.Ext(at))+".ts")
	}
	return sources
}

func timeNow() time.Time       { return time.Now() }
func since(t time.Time) string { return time.Since(t).Round(time.Millisecond).String() }