package builder

import (
	"bufio"
	"bytes"
	"context"
	"encoding/json"
	"errors"
	"fmt"
	"io"
	"net"
	"os"
	"os/exec"
	"path/filepath"
	"regexp"
	"strings"
	"time"

	"github.com/novox/mesh-controller/internal/artifacts"
	"github.com/novox/mesh-controller/internal/facts"
)

// A pull request's merge check, run on the build seat (novox/hq to-be 45 §9, ADR 0237).
//
// **The build machine already has what a check needs**: the repositories, a container runtime, the
// artifact store where the controller keeps the facts snapshot, and a Go toolchain. So a check is one
// more kind of work on the build seat's queue rather than a CI the mesh would have to run beside itself.
//
// **Two layers, each its own status on the pull request** (ADR 0237 as amended, 2026-10-06):
//
//   - **the gate** (`mesh/merge-gate`) runs when the change touches a module of the mesh's graph — the
//     controller, which holds the graph, says which (Modules) and which directories it adds a module in
//     (New). The touched manifests through `module check`; every machine of the facts snapshot composed
//     with the change and validated by the node-engine's own validator; then mesh-lab's replays. The
//     judge is the controller the mesh runs — or, for a change to the controller, the change's own
//     controller, and for a change to the node-engine, the running controller with the change's validator
//     in place of the one it vendors. The graph decides whether this runs, never the repository.
//   - **the repository's own check** (`mesh/repo-check`): its merge-check.sh, its unit tests and code
//     quality, run when present in the toolchain it declares (`# mesh-check-toolchain: go|typescript`
//     among its first lines; go when it declares none). A repository that reaches the build seat with
//     none is said as a warning: it is the mesh's, and nothing of its own is tested before it merges.
//
// One check:
//
//  1. clones the repository at the pull request's head, and beside it the repositories its check
//     reads — the controller the mesh runs, the catalogue, the host, the lab — each at the ref asked;
//  2. reads the facts snapshot the controller keeps, and with it **the versions the mesh runs**: the
//     store a check's tests stand on is the store's own release, and the bus the bus's;
//  3. raises a throwaway PostgreSQL and a throwaway bus of those versions, labelled with the ask so a
//     kill or a crash leaves nothing behind;
//  4. runs the gate, then the repository's script — **each in a toolchain container of its own**, never
//     in the build machine: a pull request is code nobody has approved yet, and the build machine holds
//     the container runtime's socket; the check's container holds none, and reaches only the throwaway
//     store and bus on loopback. Only the replays — mesh-lab's main, reviewed code — get the socket;
//  5. answers each layer pass, warning or fail from what ran, and error — never pass — when it could
//     not run.

// CheckSpec is one check, as the controller asks it.
type CheckSpec struct {
	ID         string
	Repository string
	Ref        string
	Owner      string
	Repo       string
	Number     int
	Paths      []string
	// Beside are the repositories cloned next to it, by the directory they are found under.
	Beside map[string]Beside
	// Modules are the modules of the mesh's graph the change touches, New the directories it adds a
	// module in, and Manifests the manifests among them in the change's tree: the gate runs when Modules
	// or New is not empty.
	Modules   []string
	New       []string
	Manifests []string
	// Base is the branch the pull request merges into: what the gate compares the change against.
	Base string
	// Judge is who judges the gate: "" the controller the mesh runs, "self" the change's own, "validator"
	// the running one with the change's validator.
	Judge string
	// Toolchain is the image a check's Go runs in: the mesh's own Go toolchain, as it holds it.
	// Toolchains is every toolchain the mesh holds, by language, for a script that declares another.
	Toolchain  string
	Toolchains map[string]string
	// Group are a delivery group's other heads (novox/hq ADR 0239), each cloned beside this one at its head
	// and composed with it by the gate as one future state. The repository's own check is not run for a
	// group: each member's pull request runs its own.
	Group []GroupHead
}

// GroupHead is one other head of a delivery group's composed check.
type GroupHead struct {
	Owner      string
	Repo       string
	Repository string
	Ref        string
	Paths      []string
}

// Gated is whether the change touches the mesh's graph, and so whether the gate runs.
func (s CheckSpec) Gated() bool { return len(s.Modules)+len(s.New) > 0 }

// ToolchainOf is the Go toolchain image among what the mesh holds, empty when it holds none.
func ToolchainOf(held map[string]string) string {
	return ToolchainsOf(held)["go"]
}

// ToolchainsOf is every toolchain image the mesh holds, by language.
func ToolchainsOf(held map[string]string) map[string]string {
	out := map[string]string{}
	for _, chain := range toolchains {
		if image := held[chain.Base+"/"+chain.Artifact]; image != "" {
			out[chain.Language] = image
		}
	}
	return out
}

// Beside is one repository cloned next to the one checked.
type Beside struct {
	Repository string
	Ref        string
}

// CheckVerdict is what came of one check. Verdict and Summary are the gate's; Gate and Repo each layer.
type CheckVerdict struct {
	Verdict string
	Summary string
	Report  string
	Took    time.Duration
	Gate    *Layer
	Repo    *Layer
}

// Layer is one layer of a check, judged.
type Layer struct {
	Verdict string
	Summary string
	Modules []string
}

// CheckScript is what a repository declares its own merge check as: run from its root, with the
// environment below.
const CheckScript = "merge-check.sh"

// Where a check finds what the builder raised and read for it.
const (
	EnvFacts      = "MESH_FACTS"
	EnvGate       = "MESH_GATE"
	EnvGateStore  = "MESH_GATE_POSTGRES"
	EnvTestStore  = "MESH_TEST_POSTGRES"
	EnvTestBus    = "MESH_TEST_NATS"
	EnvRepository = "MESH_CHECK_REPOSITORY"
	EnvChanged    = "MESH_CHECK_CHANGED"
	EnvVerdict    = "MESH_CHECK_VERDICT"
	EnvBeside     = "MESH_CHECK_BESIDE"
	EnvModules    = "MESH_CHECK_MODULES"
	// EnvGroup is a delivery group's other heads, as JSON, for the gate (novox/hq ADR 0239); empty for a
	// pull request checked alone.
	EnvGroup = "MESH_CHECK_GROUP"
)

// CheckTimeout bounds one check; a check that runs past it is an error, not a pass.
var CheckTimeout = 45 * time.Minute

// reportLines is how much of what a check printed travels in its verdict.
const reportLines = 200

// noModule is the gate's word for a change that touches nothing of the mesh's graph: a fact, not a
// missing check, so a pass.
const noModule = "the change touches no module of the mesh's graph"

// noScript is the repository layer's word for a repository with no merge-check.sh of its own.
const noScript = "the repository declares no " + CheckScript + ": none of its own tests run before it merges"

// toolchainLine is how a merge-check.sh declares the toolchain it runs in.
var toolchainLine = regexp.MustCompile(`^#\s*mesh-check-toolchain:\s*([a-z0-9-]+)\s*$`)

// ScriptToolchain is the language a merge-check.sh declares it runs in, among its first twenty lines;
// go when it declares none.
func ScriptToolchain(script []byte) string {
	lines := bufio.NewScanner(bytes.NewReader(script))
	for i := 0; i < 20 && lines.Scan(); i++ {
		if m := toolchainLine.FindStringSubmatch(strings.TrimSpace(lines.Text())); m != nil {
			return m[1]
		}
	}
	return "go"
}

// Check runs one merge check. An error is that it could not run; the verdict is then "error".
func Check(ctx context.Context, run Runner, spec CheckSpec, workspace, registry string, forge GitCredential,
	log Log) (CheckVerdict, error) {
	say := logging(log)
	began := time.Now()
	ctx, stop := context.WithTimeout(ctx, CheckTimeout)
	defer stop()

	root := filepath.Join(workspace, "check")
	if err := os.RemoveAll(root); err != nil {
		return CheckVerdict{}, err
	}
	if err := os.MkdirAll(root, 0o755); err != nil {
		return CheckVerdict{}, err
	}
	defer os.RemoveAll(root)
	credentials := ""
	if forge.URL != "" {
		credentials = filepath.Join(workspace, "git-credentials")
		if err := os.WriteFile(credentials, []byte(forge.URL+"\n"), 0o600); err != nil {
			return CheckVerdict{}, err
		}
	}
	clone := func(repository, ref, dir string) error {
		if _, err := run(ctx, root, "git", cloneWith(credentials, "clone", "--quiet", repository, dir)...); err != nil {
			return fmt.Errorf("cannot clone %s: %w", repository, err)
		}
		if ref != "" {
			if _, err := run(ctx, filepath.Join(root, dir), "git", "checkout", "--quiet", ref); err != nil {
				return fmt.Errorf("%s has no %s: %w", repository, ref, err)
			}
		}
		return nil
	}
	name := spec.Repo
	if name == "" || !safeName.MatchString(name) {
		name = "checked"
	}
	say("check", "%s/%s#%d at %s", spec.Owner, spec.Repo, spec.Number, short(spec.Ref))
	if err := clone(spec.Repository, spec.Ref, name); err != nil {
		return CheckVerdict{}, err
	}
	tree := filepath.Join(root, name)
	script, scriptErr := os.ReadFile(filepath.Join(tree, CheckScript))
	hasScript := scriptErr == nil
	gated := spec.Gated()
	if !gated && !hasScript {
		// Nothing to run: said, never passed silently.
		v := CheckVerdict{Verdict: "pass", Summary: noModule, Took: time.Since(began),
			Gate: &Layer{Verdict: "pass", Summary: noModule}, Repo: &Layer{Verdict: "warning", Summary: noScript}}
		say("check", "%s; %s", noModule, noScript)
		return v, nil
	}
	for dir, b := range spec.Beside {
		if dir == name || !safeName.MatchString(dir) {
			continue
		}
		if err := clone(b.Repository, b.Ref, dir); err != nil {
			return CheckVerdict{}, fmt.Errorf("beside it, %w", err)
		}
		say("check", "beside it %s at %s", dir, short(b.Ref))
	}

	// A delivery group's other heads (novox/hq ADR 0239), each at its head, for the gate to compose with this.
	groupFile := ""
	if len(spec.Group) > 0 {
		var heads []map[string]any
		for i, g := range spec.Group {
			dir := fmt.Sprintf("group-%d", i)
			if g.Repo != "" && safeName.MatchString(g.Repo) {
				dir = "group-" + g.Repo
			}
			if err := clone(g.Repository, g.Ref, dir); err != nil {
				return CheckVerdict{}, fmt.Errorf("a head of the group, %w", err)
			}
			say("check", "with it, of its group, %s/%s at %s", g.Owner, g.Repo, short(g.Ref))
			heads = append(heads, map[string]any{"repository": g.Owner + "/" + g.Repo,
				"tree": filepath.Join(root, dir), "changed": g.Paths})
		}
		raw, err := json.Marshal(heads)
		if err != nil {
			return CheckVerdict{}, err
		}
		groupFile = filepath.Join(root, "group.json")
		if err := os.WriteFile(groupFile, raw, 0o644); err != nil {
			return CheckVerdict{}, err
		}
	}

	// The facts, and the versions they say the mesh runs.
	if registry == "" {
		return CheckVerdict{}, errors.New("no artifact store to read the facts snapshot from")
	}
	body, digest, err := (artifacts.Store{Address: registry}).GetTagged(ctx, facts.Repository, facts.Tag)
	if err != nil {
		return CheckVerdict{}, fmt.Errorf("the facts snapshot cannot be read, and a check without it judges nothing: %w", err)
	}
	f, err := facts.Decode(body)
	if err != nil {
		return CheckVerdict{}, err
	}
	factsFile := filepath.Join(root, "facts.json")
	if err := os.WriteFile(factsFile, body, 0o644); err != nil {
		return CheckVerdict{}, err
	}
	say("check", "the facts of %s (%s): %d machine(s), the bus at %s, the store at %s", f.Taken.Format(time.RFC3339),
		short(strings.TrimPrefix(digest, "sha256:")), len(f.Machines), f.Versions.Bus, f.Versions.Store)

	// The throwaway store and bus, of the versions the mesh runs, removed whatever happens.
	defer func() {
		removing, done := context.WithTimeout(context.Background(), time.Minute)
		defer done()
		if n, err := RemoveContainersOf(removing, run, spec.ID); err == nil && n > 0 {
			say("check", "removed %d throwaway container(s)", n)
		}
	}()
	labelled := Labelled(run, spec.ID)
	store, err := throwaway(ctx, labelled, run, spec.ID+"-store", StoreImage(f.Versions.Store), 5432,
		[]string{"-e", "POSTGRES_PASSWORD=check"}, nil)
	if err != nil {
		return CheckVerdict{}, err
	}
	storeURL := "postgres://postgres:check@" + store + "/postgres?sslmode=disable"
	if err := waitFor(ctx, run, spec.ID+"-store", []string{"pg_isready", "-U", "postgres"}); err != nil {
		return CheckVerdict{}, err
	}
	bus, err := throwaway(ctx, labelled, run, spec.ID+"-bus", BusImage(f.Versions.Bus), 4222, nil, []string{"-js"})
	if err != nil {
		return CheckVerdict{}, err
	}
	if err := dialable(ctx, bus); err != nil {
		return CheckVerdict{}, err
	}
	say("check", "a throwaway store (%s) and bus (%s) of the versions the mesh runs", StoreImage(f.Versions.Store),
		BusImage(f.Versions.Bus))

	if spec.Toolchain == "" {
		return CheckVerdict{}, errors.New("the mesh holds no Go toolchain to run a check in")
	}
	in := func(image, dir string, env []string, command ...string) []string {
		// As the builder itself: what a check writes into the workspace is the builder's to remove.
		args := []string{"run", "--rm", "--network", "host", "--volume", workspace + ":" + workspace, "--workdir", dir,
			"--user", fmt.Sprintf("%d:%d", os.Getuid(), os.Getgid()), "--env", "HOME=" + workspace}
		for _, e := range env {
			args = append(args, "--env", e)
		}
		return append(append(args, image), command...)
	}
	inToolchain := func(dir string, env []string, command ...string) []string {
		return in(spec.Toolchain, dir, env, command...)
	}
	var out tail
	// running runs one container of the check, its output into the report, in a process group of its own.
	running := func(args []string) error {
		cmd := exec.CommandContext(ctx, "docker", LabelledArgs("docker", args, spec.ID)...)
		inItsOwnGroup(cmd)
		cmd.Stdout, cmd.Stderr = &out, &out
		return cmd.Run()
	}

	// The judge. Its failing to build is the change's fault when the change is the judge or its validator,
	// and the check's when it is the controller the mesh runs.
	gate, judgeFault, err := judgeFor(ctx, labelled, run, spec, root, tree, inToolchain, say)
	if err != nil {
		return CheckVerdict{}, err
	}
	verdictFile := filepath.Join(root, "verdict.json")
	env := []string{EnvFacts + "=" + factsFile, EnvGate + "=" + gate, EnvGateStore + "=" + storeURL,
		EnvTestStore + "=" + storeURL, EnvTestBus + "=nats://" + bus, EnvRepository + "=" + spec.Owner + "/" + spec.Repo,
		EnvChanged + "=" + strings.Join(spec.Paths, ","), EnvBeside + "=" + root,
		EnvModules + "=" + strings.Join(append(append([]string{}, spec.Modules...), spec.New...), ","),
		EnvGroup + "=" + groupFile,
		"GOCACHE=" + filepath.Join(workspace, "go-cache"), "GOMODCACHE=" + filepath.Join(workspace, "go-modules")}

	v := CheckVerdict{}
	modules := append(append([]string{}, spec.Modules...), prefixed("new:", spec.New)...)
	timedOut := func() bool { return errors.Is(ctx.Err(), context.DeadlineExceeded) }

	// **The gate**, when the graph says the change touches it.
	if !gated {
		v.Gate = &Layer{Verdict: "pass", Summary: noModule}
	} else {
		v.Gate = &Layer{Modules: modules}
		switch {
		case judgeFault != "":
			v.Gate.Verdict, v.Gate.Summary = "fail", judgeFault
		default:
			say("check", "the gate: %d module(s) of the graph touched — %s", len(modules), strings.Join(modules, ", "))
			fmt.Fprintf(&out, "--- the gate: %s\n", strings.Join(modules, ", "))
			v.Gate.Verdict, v.Gate.Summary = gateLayer(ctx, spec, tree, root, gate, verdictFile, env, inToolchain,
				running, &out, bus, workspace, name, say)
		}
		if timedOut() {
			v.Gate.Verdict, v.Gate.Summary = "error", fmt.Sprintf("the check ran past %s and was ended", CheckTimeout)
		}
	}
	if ctx.Err() != nil && !timedOut() {
		return v, ctx.Err()
	}

	// **The repository's own check**, in the toolchain it declares.
	switch {
	case len(spec.Group) > 0:
		// A group's composed check judges the heads together; each member's own tests are its pull request's.
		v.Repo = &Layer{Verdict: "pass", Summary: "a group's composed check: each member's own " + CheckScript +
			" runs on its pull request"}
	case !hasScript:
		v.Repo = &Layer{Verdict: "warning", Summary: noScript}
	case timedOut():
		v.Repo = &Layer{Verdict: "error", Summary: fmt.Sprintf("the check ran past %s before its %s ran", CheckTimeout, CheckScript)}
	default:
		language := ScriptToolchain(script)
		image := spec.Toolchains[language]
		if language == "go" && image == "" {
			image = spec.Toolchain
		}
		if image == "" {
			v.Repo = &Layer{Verdict: "error", Summary: fmt.Sprintf("its %s runs in the %s toolchain, which the mesh does "+
				"not hold", CheckScript, language)}
			break
		}
		say("check", "running its %s in the mesh's %s toolchain", CheckScript, language)
		fmt.Fprintf(&out, "--- its %s (%s toolchain)\n", CheckScript, language)
		var own tail
		cmd := exec.CommandContext(ctx, "docker", LabelledArgs("docker", in(image, tree, env, "sh", CheckScript), spec.ID)...)
		inItsOwnGroup(cmd)
		w := io.MultiWriter(&out, &own)
		cmd.Stdout, cmd.Stderr = w, w
		switch err := cmd.Run(); {
		case timedOut():
			v.Repo = &Layer{Verdict: "error", Summary: fmt.Sprintf("its %s ran past %s and was ended", CheckScript, CheckTimeout)}
		case ctx.Err() != nil:
			return v, ctx.Err()
		case err != nil:
			v.Repo = &Layer{Verdict: "fail", Summary: "its " + CheckScript + " failed: " + lastLine(own.String())}
		default:
			v.Repo = &Layer{Verdict: "pass", Summary: "its " + CheckScript + " passed"}
		}
	}

	v.Verdict, v.Summary = v.Gate.Verdict, v.Gate.Summary
	v.Report, v.Took = out.String(), time.Since(began)
	say("check", "gate %s — %s; repository %s — %s (%s)", strings.ToUpper(v.Gate.Verdict), v.Gate.Summary,
		strings.ToUpper(v.Repo.Verdict), v.Repo.Summary, v.Took.Round(time.Second))
	return v, nil
}

// gateLayer runs the gate: the touched manifests through `module check`, every machine composed with the
// change, and the replays of what the mesh runs. It answers the gate's verdict and summary.
func gateLayer(ctx context.Context, spec CheckSpec, tree, root, gate, verdictFile string, env []string,
	inToolchain func(string, []string, ...string) []string, running func([]string) error, out *tail, bus, workspace,
	name string, say func(step, format string, args ...any)) (string, string) {
	// 1. The manifests the change touches, as the judge reads them: a manifest it cannot read, or one with a
	// problem the change brings, fails here. **What was already so on the base branch is said and fails
	// nothing** — the gate's own rule: a module whose manifest the running controller already finds fault
	// with would otherwise fail every pull request that touches it, for a fault none of them made.
	var manifests []string
	for _, m := range spec.Manifests {
		if _, err := os.Stat(filepath.Join(tree, m)); err == nil {
			manifests = append(manifests, m)
		}
	}
	if len(manifests) > 0 {
		checked := func(dir string) (string, error) {
			var own tail
			cmd := exec.CommandContext(ctx, "docker", LabelledArgs("docker",
				inToolchain(dir, env, append([]string{gate, "module", "check"}, manifests...)...), spec.ID)...)
			inItsOwnGroup(cmd)
			w := io.MultiWriter(out, &own)
			cmd.Stdout, cmd.Stderr = w, w
			err := cmd.Run()
			return own.String(), err
		}
		said, err := checked(tree)
		if err != nil {
			if ctx.Err() != nil {
				return "error", "the check was ended during the module check"
			}
			// The same manifests as the base branch has them, beside the change.
			was := ""
			if spec.Base != "" {
				base := filepath.Join(root, "base-manifests")
				for _, m := range manifests {
					body, err := gitShow(ctx, tree, "origin/"+spec.Base, m)
					if err != nil {
						continue // new on this branch: nothing was so before it
					}
					if err := os.MkdirAll(filepath.Dir(filepath.Join(base, m)), 0o755); err == nil {
						_ = os.WriteFile(filepath.Join(base, m), body, 0o644)
					}
				}
				fmt.Fprintf(out, "--- the same manifests on %s\n", spec.Base)
				if _, err := os.Stat(base); err == nil {
					was, _ = checked(base)
				}
			}
			brought := newProblems(said, was)
			if len(brought) > 0 {
				return "fail", "a manifest the change touches fails the module check: " + brought[0]
			}
			fmt.Fprintf(out, "the module check's problems were all so on %s already: said, not the change's\n", spec.Base)
		}
	}

	// 2. Every machine composed with the change.
	if err := running(inToolchain(tree, append(env, EnvVerdict+"="+verdictFile), "sh", "-c",
		`"$MESH_GATE" merge-gate --facts "$MESH_FACTS" --store "$MESH_GATE_POSTGRES" `+
			`--repository "$MESH_CHECK_REPOSITORY" --tree . --changed "$MESH_CHECK_CHANGED" `+
			`${MESH_CHECK_GROUP:+--group "$MESH_CHECK_GROUP"} --json > "$MESH_CHECK_VERDICT"`)); err != nil {
		if ctx.Err() != nil {
			return "error", "the check was ended during the merge gate"
		}
		var said struct {
			Verdict string `json:"verdict"`
			Summary string `json:"summary"`
		}
		if raw, err := os.ReadFile(verdictFile); err == nil && json.Unmarshal(raw, &said) == nil && said.Verdict == "fail" {
			return "fail", said.Summary
		}
		// The gate could not judge: not the change's fault, and never a pass.
		return "error", "the merge gate could not judge the change: " + lastLine(out.String())
	}
	var said struct {
		Verdict string `json:"verdict"`
		Summary string `json:"summary"`
	}
	raw, err := os.ReadFile(verdictFile)
	if err != nil || json.Unmarshal(raw, &said) != nil || said.Verdict == "" {
		return "error", "the merge gate said no verdict"
	}

	// 3. **The replays of what the mesh runs** (to-be 45 §9, M9): mesh-lab's, from its main — reviewed code,
	// so given the container runtime the resolver replay raises containers with — against the bus of the
	// release the mesh runs and the change's own catalogue when the change is to the catalogue.
	if lab := filepath.Join(root, "mesh-lab", "replays"); ctx.Err() == nil {
		if _, err := os.Stat(lab); err == nil {
			catalogue := filepath.Join(root, "mesh-catalog")
			if name == "mesh-catalog" {
				catalogue = tree
			}
			say("check", "the replays of what the mesh runs, from mesh-lab")
			fmt.Fprintln(out, "--- the replays (mesh-lab replays/)")
			args := inToolchain(lab, []string{EnvTestBus + "=nats://" + bus, "MESH_REPLAY_CATALOGUE=" + catalogue,
				"GOCACHE=" + filepath.Join(workspace, "go-cache"), "GOMODCACHE=" + filepath.Join(workspace, "go-modules")},
				"go", "test", "-count=1", "./...")
			// The socket goes to the replays alone, never to the change's own code above.
			args = append([]string{args[0], "--volume", "/var/run/docker.sock:/var/run/docker.sock"}, args[1:]...)
			if err := running(args); err != nil {
				if ctx.Err() != nil {
					return "error", "the check was ended during the replays"
				}
				return "fail", "a replay of a core incident fails with this change: " + lastLine(out.String())
			}
		}
	}
	if said.Verdict == "pass" || said.Verdict == "warning" || said.Verdict == "fail" {
		return said.Verdict, said.Summary
	}
	return "error", "the merge gate said " + said.Verdict
}

// judgeFor builds the judge of a check: the change's own controller (a change to the controller), the
// running controller built with the change's validator (a change to the node-engine), or the controller
// the mesh runs. It answers the judge's path; or, when the change makes its own judge unbuildable, why —
// the change's fault, a failing gate; or an error when the check cannot build a judge at all. A check
// whose gate does not run builds a judge only to hand its scripts one, and goes on without when it cannot.
func judgeFor(ctx context.Context, labelled, run Runner, spec CheckSpec, root, tree string,
	inToolchain func(string, []string, ...string) []string, say func(step, format string, args ...any)) (string, string, error) {
	gated := spec.Gated()
	switch spec.Judge {
	case "self":
		bin := filepath.Join(root, "bin", "judge-of-itself")
		if _, err := labelled(ctx, root, "docker", inToolchain(tree,
			[]string{"CGO_ENABLED=0", "GOFLAGS=-mod=vendor", "GOPROXY=off", "GOCACHE=" + filepath.Join(filepath.Dir(root), "go-cache")},
			"go", "build", "-o", bin, "./cmd/mesh-controller")...); err != nil {
			return "", "the change's controller, which judges itself, does not build: " + firstLine(err.Error()), nil
		}
		say("check", "judged by the change's own controller")
		return bin, "", nil
	}
	bin, judgeTree, err := judge(ctx, labelled, run, root, inToolchain, say)
	if err != nil {
		if !gated {
			say("check", "no judge for its script: %v", err)
			return "", "", nil
		}
		return "", "", err
	}
	if spec.Judge != "validator" {
		return bin, "", nil
	}
	// The node-engine's change: its validator in place of the one the judge vendors.
	vendored := filepath.Join(root, judgeTree, "vendor", "github.com", "novox", "mesh-host")
	for _, pkg := range []string{"validate", filepath.Join("internal", "declaration")} {
		if err := replaceGoFiles(filepath.Join(tree, pkg), filepath.Join(vendored, pkg)); err != nil {
			return "", "", fmt.Errorf("the change's validator could not be put in the judge: %w", err)
		}
	}
	withValidator := filepath.Join(root, "bin", "judge-with-this-validator")
	if _, err := labelled(ctx, root, "docker", inToolchain(filepath.Join(root, judgeTree),
		[]string{"CGO_ENABLED=0", "GOFLAGS=-mod=vendor", "GOPROXY=off", "GOCACHE=" + filepath.Join(filepath.Dir(root), "go-cache")},
		"go", "build", "-o", withValidator, "./cmd/mesh-controller")...); err != nil {
		return "", "the controller the mesh runs does not build with this change's validator: " + firstLine(err.Error()), nil
	}
	say("check", "judged by the controller the mesh runs, with this change's validator")
	return withValidator, "", nil
}

// replaceGoFiles puts a package's Go files — never its tests — in place of another copy's.
func replaceGoFiles(from, into string) error {
	old, err := filepath.Glob(filepath.Join(into, "*.go"))
	if err != nil {
		return err
	}
	for _, f := range old {
		if err := os.Remove(f); err != nil {
			return err
		}
	}
	if err := os.MkdirAll(into, 0o755); err != nil {
		return err
	}
	files, err := filepath.Glob(filepath.Join(from, "*.go"))
	if err != nil {
		return err
	}
	if len(files) == 0 {
		return fmt.Errorf("%s holds no Go files", from)
	}
	for _, f := range files {
		if strings.HasSuffix(f, "_test.go") {
			continue
		}
		body, err := os.ReadFile(f)
		if err != nil {
			return err
		}
		if err := os.WriteFile(filepath.Join(into, filepath.Base(f)), body, 0o644); err != nil {
			return err
		}
	}
	return nil
}

// problemLines are the lines of a module check's output that name a problem: not a manifest found ok, not
// the count, not the closing note.
func problemLines(s string) []string {
	var out []string
	for _, line := range strings.Split(s, "\n") {
		line = strings.TrimSpace(line)
		switch {
		case line == "", strings.Contains(line, ": ok"), strings.Contains(line, "problem(s) in"),
			strings.Contains(line, "manifest(s) checked"):
			continue
		}
		out = append(out, line)
	}
	return out
}

// newProblems are the problems a module check said of the change that it did not say of the base.
func newProblems(change, base string) []string {
	was := map[string]bool{}
	for _, p := range problemLines(base) {
		was[p] = true
	}
	var out []string
	for _, p := range problemLines(change) {
		if !was[p] {
			out = append(out, p)
		}
	}
	if len(out) == 0 && len(problemLines(change)) == 0 {
		// It failed and named nothing: not readable as already so.
		out = append(out, lastLine(change))
	}
	return out
}

// gitShow is a file as a ref has it.
func gitShow(ctx context.Context, dir, ref, file string) ([]byte, error) {
	cmd := exec.CommandContext(ctx, "git", "show", ref+":"+filepath.ToSlash(file))
	cmd.Dir = dir
	return cmd.Output()
}

func prefixed(prefix string, items []string) []string {
	out := make([]string, 0, len(items))
	for _, i := range items {
		out = append(out, prefix+i)
	}
	return out
}

func firstLine(s string) string {
	line, _, _ := strings.Cut(strings.TrimSpace(s), "\n")
	return line
}

// safeName is a directory a repository beside a check may be cloned under.
var safeName = regexp.MustCompile(`^[a-z0-9][a-z0-9-]*$`)

// StoreImage is the store a check stands on: the major release the mesh's store runs (`17.11` → 17),
// on Alpine as the store module runs it.
func StoreImage(version string) string {
	major, _, _ := strings.Cut(strings.TrimSpace(version), ".")
	if major == "" || strings.ContainsAny(major, " (") {
		major = "17"
	}
	return "postgres:" + major + "-alpine"
}

// BusImage is the bus a check stands on: the release the mesh's bus server runs.
func BusImage(version string) string {
	v := strings.TrimPrefix(strings.TrimSpace(version), "v")
	if v == "" {
		v = "2.11"
	}
	return "nats:" + v + "-alpine"
}

// throwaway starts a container publishing one port on loopback, and answers where it is reached.
func throwaway(ctx context.Context, run, plain Runner, name, image string, port int, opts, args []string) (string, error) {
	invocation := []string{"run", "-d", "--rm", "--name", name, "-p", fmt.Sprintf("127.0.0.1::%d", port)}
	invocation = append(invocation, opts...)
	invocation = append(invocation, image)
	invocation = append(invocation, args...)
	if _, err := run(ctx, "", "docker", invocation...); err != nil {
		return "", fmt.Errorf("a throwaway %s could not be raised: %w", image, err)
	}
	out, err := plain(ctx, "", "docker", "port", name, fmt.Sprintf("%d/tcp", port))
	if err != nil {
		return "", err
	}
	for _, line := range strings.Split(strings.TrimSpace(out), "\n") {
		if strings.HasPrefix(line, "127.0.0.1:") {
			return strings.TrimSpace(line), nil
		}
	}
	return "", fmt.Errorf("%s published %d nowhere on loopback: %q", name, port, out)
}

// waitFor runs a readiness command in a container until it answers, for a minute.
func waitFor(ctx context.Context, run Runner, name string, ready []string) error {
	for i := 0; i < 60; i++ {
		if _, err := run(ctx, "", "docker", append([]string{"exec", name}, ready...)...); err == nil {
			return nil
		}
		select {
		case <-ctx.Done():
			return ctx.Err()
		case <-time.After(time.Second):
		}
	}
	return fmt.Errorf("%s never became ready", name)
}

// dialable waits for an address to take a connection, for a minute.
func dialable(ctx context.Context, address string) error {
	for i := 0; i < 60; i++ {
		if c, err := net.DialTimeout("tcp", address, time.Second); err == nil {
			c.Close()
			return nil
		}
		select {
		case <-ctx.Done():
			return ctx.Err()
		case <-time.After(time.Second):
		}
	}
	return fmt.Errorf("nothing took a connection at %s", address)
}

// judge builds the controller that judges a change: the one the mesh runs, beside the check as
// mesh-controller — or, when that one predates the merge gate, the controller's main, said. It answers
// the binary and the directory it was built from.
func judge(ctx context.Context, run, plain Runner, root string, inToolchain func(string, []string, ...string) []string,
	say func(step, format string, args ...any)) (string, string, error) {
	bin := filepath.Join(root, "bin", "mesh-controller")
	build := func(dir string) error {
		_, err := run(ctx, root, "docker", inToolchain(filepath.Join(root, dir),
			[]string{"CGO_ENABLED=0", "GOFLAGS=-mod=vendor", "GOPROXY=off", "GOCACHE=" + filepath.Join(filepath.Dir(root), "go-cache")},
			"go", "build", "-o", bin, "./cmd/mesh-controller")...)
		return err
	}
	// Static, so it runs where the builder does.
	hasGate := func() bool {
		out, _ := plain(ctx, root, bin, "merge-gate")
		return strings.Contains(out, "merge-gate --facts")
	}
	if _, err := os.Stat(filepath.Join(root, "mesh-controller")); err == nil {
		if err := build("mesh-controller"); err != nil {
			return "", "", fmt.Errorf("the controller the mesh runs does not build: %w", err)
		}
		if hasGate() {
			say("check", "judged by the controller the mesh runs")
			return bin, "mesh-controller", nil
		}
	}
	if _, err := os.Stat(filepath.Join(root, "mesh-controller-main")); err != nil {
		return "", "", errors.New("no controller beside the check to judge it with")
	}
	if err := build("mesh-controller-main"); err != nil {
		return "", "", fmt.Errorf("the controller's main does not build: %w", err)
	}
	say("check", "judged by the controller's main: the one the mesh runs predates the merge gate")
	return bin, "mesh-controller-main", nil
}

// tail keeps the last lines written to it.
type tail struct{ buf bytes.Buffer }

func (t *tail) Write(p []byte) (int, error) {
	t.buf.Write(p)
	if t.buf.Len() > 1<<20 {
		keep := t.buf.Bytes()[t.buf.Len()-(512<<10):]
		t.buf = *bytes.NewBuffer(append([]byte(nil), keep...))
	}
	return len(p), nil
}

func (t *tail) String() string {
	lines := strings.Split(strings.TrimRight(t.buf.String(), "\n"), "\n")
	if len(lines) > reportLines {
		lines = lines[len(lines)-reportLines:]
	}
	return strings.Join(lines, "\n")
}

func lastLine(s string) string {
	lines := strings.Split(strings.TrimSpace(s), "\n")
	return strings.TrimSpace(lines[len(lines)-1])
}