Build a change once: the check builds what its merge moves, the walk takes it by key

A merge built every moved module a second time although its check had judged the same tree (issue 649). The check now builds into the scratch namespace under a reuse key (source fingerprint plus the builder's own digest), and the walk of the trunk commit copies a build of the same key by digest instead of building, saying per module why when it cannot (novox/hq ADR 0350).
2026-10-13 23:04:40 +02:00
parent bff5b7f0e4
commit cd49b50c40
23 changed files with 2195 additions and 13 deletions
@@ -0,0 +1,541 @@
+package builder
+
+import (
+	"context"
+	"encoding/json"
+	"errors"
+	"net/http"
+	"net/http/httptest"
+	"slices"
+	"strconv"
+	"strings"
+	"testing"
+	"time"
+
+	"github.com/novox/mesh-controller/internal/catalogue"
+)
+
+// One build per change (novox/hq ADR 0350): the merge check builds what the merge would move into the scratch
+// namespace under its reuse key, and the walk of the trunk commit takes that build by digest instead of building.
+
+// aScratch is the scratch namespace in memory: check builds by module and key, and what the walk took.
+type aScratch struct {
+	builds   map[string]CheckBuild
+	took     []string
+	failTake bool
+}
+
+func newScratch() *aScratch { return &aScratch{builds: map[string]CheckBuild{}} }
+
+func (s *aScratch) Keep(_ context.Context, _ PullRef, _ string, _ time.Time, builds []CheckBuild) error {
+	for _, b := range builds {
+		s.builds[b.Module+" "+b.Key] = b
+	}
+	return nil
+}
+
+func (s *aScratch) Find(_ context.Context, module, key string) (CheckBuild, bool, error) {
+	b, ok := s.builds[module+" "+key]
+	return b, ok, nil
+}
+
+func (s *aScratch) OfPulls(_ context.Context, pulls []PullRef) ([]CheckBuild, error) {
+	var out []CheckBuild
+	for _, b := range s.builds {
+		if slices.Contains(pulls, b.Pull) {
+			out = append(out, b)
+		}
+	}
+	return out, nil
+}
+
+func (s *aScratch) Take(_ context.Context, module string, made catalogue.Built) (catalogue.Built, error) {
+	if s.failTake {
+		return catalogue.Built{}, errors.New("the store refused the mount")
+	}
+	s.took = append(s.took, made.Reference)
+	made.Reference = strings.Replace(made.Reference, ScratchPrefix, "", 1)
+	return made, nil
+}
+
+func (s *aScratch) Forget(context.Context, PullRef) (int, error)                  { return 0, nil }
+func (s *aScratch) Expire(context.Context, time.Duration, time.Time) (int, error) { return 0, nil }
+func (s *aScratch) Discard(context.Context, []CheckBuild) error                   { return nil }
+
+const reusedModule = `{"module":"meshboard","version":"1",
+  "build":{"artifacts":[
+    {"name":"server","kind":"image","from":"Dockerfile"},
+    {"name":"look","kind":"archive","from":"files"}]},
+  "resources":[
+    {"id":"svc","type":"container","name":"meshboard","artifact":"server"},
+    {"id":"theme","type":"archive","path":"/opt/meshboard","artifact":"look"}]}`
+
+// aModuleAt is a repository holding the module under mod/, whose directory git names as tree, on its trunk.
+func aModuleAt(t *testing.T, tree string) (*recorded, Runner, string) {
+	t.Helper()
+	r := &recorded{tree: tree, contents: map[string]string{"mod/" + ManifestName: reusedModule,
+		"mod/Dockerfile": "FROM scratch", "mod/files/theme.conf": "dark"}}
+	run := func(ctx context.Context, dir, name string, args ...string) (string, error) {
+		if name == "git" && len(args) > 0 && args[0] == "symbolic-ref" {
+			r.ran = append(r.ran, name+" "+strings.Join(args, " "))
+			return "origin/main\n", nil
+		}
+		return r.run(ctx, dir, name, args...)
+	}
+	return r, run, t.TempDir()
+}
+
+func dockerBuilt(r *recorded) bool {
+	for _, line := range r.ran {
+		if strings.HasPrefix(line, "docker build") {
+			return true
+		}
+	}
+	return false
+}
+
+var aPull = PullRef{Owner: "novox", Repo: "meshboard", Number: 7}
+
+// aCheckBuilt is the check of aPull building the module from a tree, into the scratch namespace.
+func aCheckBuilt(t *testing.T, scratch *aScratch, tree string) CheckBuilt {
+	t.Helper()
+	r, run, workspace := aModuleAt(t, tree)
+	spec := CheckSpec{ID: "build-1", Owner: aPull.Owner, Repo: aPull.Repo, Number: aPull.Number,
+		Builds: []CheckBuildSpec{{Module: "meshboard", Repository: "https://forge.invalid/meshboard.git", Path: "mod",
+			Ref: "main"}}}
+	built := buildForTheMerge(t.Context(), run, spec, CheckBuilding{Publish: r, Scratch: scratch}, workspace, "", "",
+		CheckVerdict{Head: "abcdefabcdef", Trunk: "123456123456"}, GitCredential{}, nil, logging(nil))
+	if len(built) != 1 || built[0].NotBuilt != "" || built[0].Key == "" {
+		t.Fatalf("the check did not build the module for its merge: %+v", built)
+	}
+	if !dockerBuilt(r) {
+		t.Fatal("the check built no image")
+	}
+	for repository := range r.images {
+		if !strings.HasPrefix(repository, ScratchPrefix) {
+			t.Fatalf("the check published %s outside the scratch namespace", repository)
+		}
+	}
+	for repository := range r.archives {
+		if !strings.HasPrefix(repository, ScratchPrefix) {
+			t.Fatalf("the check published %s outside the scratch namespace", repository)
+		}
+	}
+	return built[0]
+}
+
+func withVersion(t *testing.T, v string) {
+	t.Helper()
+	was := Version
+	Version = v
+	t.Cleanup(func() { Version = was })
+}
+
+// A merge whose reuse key equals its check's builds nothing again: the walk copies the check's build by digest
+// and registers that digest.
+func TestAWalkWhoseKeyIsItsChecksBuildsNothingAndRegistersTheChecksDigest(t *testing.T) {
+	withVersion(t, "builder-1")
+	scratch := newScratch()
+	checked := aCheckBuilt(t, scratch, "tree-one")
+
+	r, run, workspace := aModuleAt(t, "tree-one")
+	got, err := BuildReusing(t.Context(), run, r, Reuse{Scratch: scratch, Pulls: []PullRef{aPull}},
+		"https://forge.invalid/meshboard.git", "mod", "main", workspace, nil, Npmrc{}, GitCredential{}, nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if dockerBuilt(r) || len(r.images) > 0 || len(r.archives) > 0 {
+		t.Fatalf("the walk built again: ran %v, published %v %v", r.ran, r.images, r.archives)
+	}
+	if got.Reused == nil || got.Why != "" {
+		t.Fatalf("the walk did not say it reused the check's build: reused %+v, why %q", got.Reused, got.Why)
+	}
+	if got.ReuseKey != checked.Key {
+		t.Fatalf("the walk's key %s is not the check's %s", got.ReuseKey, checked.Key)
+	}
+	if len(scratch.took) != 2 {
+		t.Fatalf("the walk copied %v, not the check's two artifacts", scratch.took)
+	}
+	for _, made := range checked.Made {
+		registered := strings.Replace(made.Reference, ScratchPrefix, "", 1)
+		found := false
+		for _, b := range got.Built {
+			found = found || (b.Reference == registered && b.Digest == made.Digest)
+		}
+		if !found {
+			t.Fatalf("the walk registers %+v, not the check's %s copied", got.Built, made.Reference)
+		}
+	}
+	if image, _ := got.Manifest.Resources[0]["image"].(string); image != "registry.invalid/meshboard/server@sha256:"+strings.Repeat("a", 64) {
+		t.Fatalf("the manifest pins %q, not the check's digest in the module's namespace", image)
+	}
+	words := got.Reused.Words()
+	if !strings.HasPrefix(words, "reused from the check of novox/meshboard#7 at head abcdefabcdef (merged into trunk 123456123456)") ||
+		!strings.Contains(words, "sha256:") {
+		t.Fatalf("the walk says %q", words)
+	}
+}
+
+// A differing key builds, and says which part differs.
+func TestAWalkWhoseKeyDiffersBuildsAndSaysWhy(t *testing.T) {
+	withVersion(t, "builder-1")
+	scratch := newScratch()
+	aCheckBuilt(t, scratch, "tree-one")
+
+	r, run, workspace := aModuleAt(t, "tree-two")
+	got, err := BuildReusing(t.Context(), run, r, Reuse{Scratch: scratch, Pulls: []PullRef{aPull}},
+		"https://forge.invalid/meshboard.git", "mod", "main", workspace, nil, Npmrc{}, GitCredential{}, nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !dockerBuilt(r) || got.Reused != nil {
+		t.Fatalf("a walk whose source differs did not build: reused %+v", got.Reused)
+	}
+	if !strings.Contains(got.Why, "the key differs") || !strings.Contains(got.Why, partTree) {
+		t.Fatalf("the walk says %q, not which part of the key differs", got.Why)
+	}
+
+	// And a builder of another version builds too, saying so.
+	withVersion(t, "builder-2")
+	r, run, workspace = aModuleAt(t, "tree-one")
+	got, err = BuildReusing(t.Context(), run, r, Reuse{Scratch: scratch, Pulls: []PullRef{aPull}},
+		"https://forge.invalid/meshboard.git", "mod", "main", workspace, nil, Npmrc{}, GitCredential{}, nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !dockerBuilt(r) || !strings.Contains(got.Why, partBuilder) {
+		t.Fatalf("a newer builder took the older one's build, or did not say why not: %q", got.Why)
+	}
+
+	// No check of the walk's pull requests built it: said.
+	r, run, workspace = aModuleAt(t, "tree-one")
+	got, _ = BuildReusing(t.Context(), run, r, Reuse{Scratch: newScratch(), Pulls: []PullRef{aPull}},
+		"https://forge.invalid/meshboard.git", "mod", "main", workspace, nil, Npmrc{}, GitCredential{}, nil)
+	if !dockerBuilt(r) || !strings.Contains(got.Why, "no check of novox/meshboard#7 built it") {
+		t.Fatalf("the walk says %q", got.Why)
+	}
+}
+
+// A copy that fails is said and falls back to a build; it never fails the walk.
+func TestAFailedCopyOfAChecksBuildFallsBackToBuilding(t *testing.T) {
+	withVersion(t, "builder-1")
+	scratch := newScratch()
+	aCheckBuilt(t, scratch, "tree-one")
+	scratch.failTake = true
+
+	r, run, workspace := aModuleAt(t, "tree-one")
+	got, err := BuildReusing(t.Context(), run, r, Reuse{Scratch: scratch, Pulls: []PullRef{aPull}},
+		"https://forge.invalid/meshboard.git", "mod", "main", workspace, nil, Npmrc{}, GitCredential{}, nil)
+	if err != nil {
+		t.Fatalf("a failed copy failed the walk: %v", err)
+	}
+	if !dockerBuilt(r) || got.Reused != nil || len(r.images) == 0 {
+		t.Fatal("a failed copy did not fall back to a build")
+	}
+	if !strings.Contains(got.Why, "copying meshboard's") || !strings.Contains(got.Why, "refused the mount") {
+		t.Fatalf("the walk says %q", got.Why)
+	}
+	for repository := range r.images {
+		if strings.HasPrefix(repository, ScratchPrefix) {
+			t.Fatalf("the walk's own build went into the scratch namespace: %s", repository)
+		}
+	}
+}
+
+// Only a walk takes a check's build: a plain build — a check's, a dry run's, `build --ref`'s — builds, and a commit
+// off the trunk never takes one.
+func TestOnlyAWalkOfATrunkCommitTakesAChecksBuild(t *testing.T) {
+	withVersion(t, "builder-1")
+	scratch := newScratch()
+	aCheckBuilt(t, scratch, "tree-one")
+
+	r, run, workspace := aModuleAt(t, "tree-one")
+	if _, err := Build(t.Context(), run, r, "https://forge.invalid/meshboard.git", "mod", "main", workspace, nil,
+		Npmrc{}, GitCredential{}, nil); err != nil {
+		t.Fatal(err)
+	}
+	if !dockerBuilt(r) || len(scratch.took) > 0 {
+		t.Fatal("a build not asked to reuse took a check's build")
+	}
+
+	r, _, workspace = aModuleAt(t, "tree-one") // no trunk said: off it
+	got, err := BuildReusing(t.Context(), r.run, r, Reuse{Scratch: scratch}, "https://forge.invalid/meshboard.git",
+		"mod", "main", workspace, nil, Npmrc{}, GitCredential{}, nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !dockerBuilt(r) || got.Reused != nil || !strings.Contains(got.Why, "not on its trunk") {
+		t.Fatalf("a commit off the trunk took a check's build: %q", got.Why)
+	}
+}
+
+// A check that did not pass builds nothing for its merge, and one asked to build nothing builds nothing.
+func TestACheckThatFailedBuildsNothingForItsMerge(t *testing.T) {
+	if skipsTheBuilds(CheckVerdict{Gate: &Layer{Verdict: "pass"}, Repo: &Layer{Verdict: "fail"}}) == "" {
+		t.Fatal("a check whose repository layer failed builds for its merge")
+	}
+	if skipsTheBuilds(CheckVerdict{Gate: &Layer{Verdict: "pass"}, Repo: &Layer{Verdict: "warning"}}) != "" {
+		t.Fatal("a check that passed with a warning builds nothing for its merge")
+	}
+	r, run, workspace := aModuleAt(t, "tree-one")
+	if built := buildForTheMerge(t.Context(), run, CheckSpec{ID: "build-1"}, CheckBuilding{Publish: r, Scratch: newScratch()},
+		workspace, "", "", CheckVerdict{}, GitCredential{}, nil, logging(nil)); built != nil || len(r.ran) > 0 {
+		t.Fatalf("a check moving no module built %v, ran %v", built, r.ran)
+	}
+}
+
+// A module whose source does not pin its build has no key and is not built by the check; what stands after it
+// is not built either.
+func TestACheckBuildsNoModuleWithoutAKeyNorWhatStandsAfterIt(t *testing.T) {
+	withVersion(t, "builder-1")
+	r, run, workspace := aModuleAt(t, "tree-one")
+	r.contents["mod/"+ManifestName] = `{"module":"meshboard","version":"1","build":{"artifacts":[
+		{"name":"mirror","kind":"upstream","from":"docker.io/library/alpine:3"}]}}`
+	spec := CheckSpec{ID: "build-1", Owner: "novox", Repo: "meshboard", Number: 7, Builds: []CheckBuildSpec{
+		{Module: "meshboard", Repository: "https://forge.invalid/meshboard.git", Path: "mod", Tier: 0},
+		{Module: "after", Repository: "https://forge.invalid/after.git", Tier: 1}}}
+	built := buildForTheMerge(t.Context(), run, spec, CheckBuilding{Publish: r, Scratch: newScratch()}, workspace, "", "",
+		CheckVerdict{}, GitCredential{}, nil, logging(nil))
+	if len(built) != 2 || !strings.Contains(built[0].NotBuilt, "named by tag") ||
+		!strings.Contains(built[1].NotBuilt, "stands after meshboard") {
+		t.Fatalf("built %+v", built)
+	}
+	if len(r.images) > 0 {
+		t.Fatalf("a module without a key was published: %v", r.images)
+	}
+}
+
+// What the key names: every input a fresh build could differ by.
+func TestTheReuseKeyNamesEveryInputOfTheBuild(t *testing.T) {
+	src := newSourceInputs("m")
+	src.tree = "t1"
+	src.bases = []string{"store:5000/b/x@sha256:" + strings.Repeat("b", 64)}
+	src.contexts["server"] = "ctx1"
+	src.toolchains["tools"] = "go sha256:c 1234"
+	manifest := catalogue.Manifest{Module: "m"}
+	base := keyOf(src, manifest, "v1")
+	if base.Key == "" {
+		t.Fatalf("no key: %s", base.Why)
+	}
+	for name, change := range map[string]func(*sourceInputs) string{
+		partTree: func(s *sourceInputs) string { s.tree = "t2"; return "v1" },
+		partBases: func(s *sourceInputs) string {
+			s.bases = []string{"other@sha256:" + strings.Repeat("c", 64)}
+			return "v1"
+		},
+		partContext: func(s *sourceInputs) string { s.contexts["server"] = "ctx2"; return "v1" },
+		partTool:    func(s *sourceInputs) string { s.toolchains["tools"] = "go sha256:d 1234"; return "v1" },
+		partBuilder: func(*sourceInputs) string { return "v2" },
+	} {
+		s := src.copy()
+		version := change(s)
+		if k := keyOf(s, manifest, version); k.Key == base.Key {
+			t.Errorf("a change of %s left the key the same", name)
+		}
+	}
+	// A base copied to another address is the same base: the key names it by digest.
+	moved := src.copy()
+	moved.bases = []string{"elsewhere:5000/scratch/b/x@sha256:" + strings.Repeat("b", 64)}
+	if keyOf(moved, manifest, "v1").Key != base.Key {
+		t.Error("the same base at another address changed the key")
+	}
+	if k := keyOf(src, manifest, ""); k.Key != "" {
+		t.Error("a builder whose version cannot be said keyed its build")
+	}
+	unpinned := src.copy()
+	unpinned.notPinned("it resolves packages from the mesh's registry at build time")
+	if k := keyOf(unpinned, manifest, "v1"); k.Key != "" || !strings.Contains(k.Why, "registry") {
+		t.Errorf("a build its source does not pin was keyed: %+v", k)
+	}
+}
+
+// aTaggingRegistry is the in-memory registry with tags and their listing, as the store's scratch namespace needs.
+type aTaggingRegistry struct {
+	*aRegistryOfRepositories
+	tags map[string]map[string]string // repository → tag → manifest digest
+}
+
+func (m *aTaggingRegistry) serve() http.Handler {
+	under := m.handler()
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		if repository, ok := strings.CutSuffix(strings.TrimPrefix(r.URL.Path, "/v2/"), "/tags/list"); ok {
+			m.mu.Lock()
+			var tags []string
+			for tag := range m.tags[repository] {
+				tags = append(tags, tag)
+			}
+			m.mu.Unlock()
+			if tags == nil {
+				w.WriteHeader(http.StatusNotFound)
+				return
+			}
+			_ = json.NewEncoder(w).Encode(map[string]any{"name": repository, "tags": tags})
+			return
+		}
+		repository, kind, rest := splitPath(r.URL.Path)
+		if kind == "blobs" && r.Method == http.MethodHead {
+			m.mu.Lock()
+			body, linked := m.blobs[rest], m.links[repository][rest]
+			m.mu.Unlock()
+			if !linked {
+				w.WriteHeader(http.StatusNotFound)
+				return
+			}
+			w.Header().Set("Content-Length", strconv.Itoa(len(body)))
+			w.WriteHeader(http.StatusOK)
+			return
+		}
+		if kind != "manifests" {
+			under.ServeHTTP(w, r)
+			return
+		}
+		if !strings.HasPrefix(rest, "sha256:") {
+			m.mu.Lock()
+			switch r.Method {
+			case http.MethodPut:
+				body, _ := readAll(r)
+				if m.tags[repository] == nil {
+					m.tags[repository] = map[string]string{}
+				}
+				m.tags[repository][rest] = digestOf(body)
+				m.manifests[repository+"@"+digestOf(body)] = body
+				m.mu.Unlock()
+				w.WriteHeader(http.StatusCreated)
+				return
+			default:
+				digest, ok := m.tags[repository][rest]
+				body, held := m.manifests[repository+"@"+digest]
+				m.mu.Unlock()
+				if !ok || !held {
+					w.WriteHeader(http.StatusNotFound)
+					return
+				}
+				w.Header().Set("Docker-Content-Digest", digest)
+				w.Header().Set("Content-Type", mediaTypeOf(body))
+				if r.Method == http.MethodGet {
+					_, _ = w.Write(body)
+					return
+				}
+				w.WriteHeader(http.StatusOK)
+				return
+			}
+		}
+		if r.Method == http.MethodDelete {
+			m.mu.Lock()
+			for _, tags := range m.tags {
+				for tag, digest := range tags {
+					if digest == rest {
+						delete(tags, tag)
+					}
+				}
+			}
+			m.mu.Unlock()
+		}
+		under.ServeHTTP(w, r)
+	})
+}
+
+// A check's builds go after seven days, and when its pull request closes or is merged; the walk's copy in the
+// module's own namespace stays.
+func TestCheckBuildsExpireAfterSevenDaysAndGoWhenTheirPullRequestCloses(t *testing.T) {
+	m := &aTaggingRegistry{aRegistryOfRepositories: newRegistryOfRepositories(), tags: map[string]map[string]string{}}
+	server := httptest.NewServer(m.serve())
+	defer server.Close()
+	address := strings.TrimPrefix(server.URL, "http://")
+	registry := Registry{Address: address}
+	ctx := t.Context()
+	now := time.Date(2026, 10, 13, 20, 0, 0, 0, time.UTC)
+
+	image := func(repository, body string) string {
+		digest := digestOf([]byte(body))
+		m.manifests[repository+"@"+digest] = []byte(body)
+		return address + "/" + repository + "@" + digest
+	}
+	check := func(pull PullRef, id string, at time.Time, key string) string {
+		reference := image("scratch/meshboard/server", `{"mediaType":"`+mediaManifestOCI+`","layers":[],"n":"`+id+`"}`)
+		build := CheckBuild{Key: key, Module: "meshboard", Made: []catalogue.Built{{Name: "server", Kind: "image",
+			Reference: reference}}, Pull: pull, Check: id, At: at}
+		if err := registry.Keep(ctx, pull, id, at, []CheckBuild{build}); err != nil {
+			t.Fatal(err)
+		}
+		return reference
+	}
+	held := func(reference string) bool {
+		_, digest, _ := strings.Cut(reference, "@")
+		_, ok := m.manifests["scratch/meshboard/server@"+digest]
+		return ok
+	}
+	other := PullRef{Owner: "novox", Repo: "meshboard", Number: 8}
+	old := check(aPull, "build-1", now.Add(-8*24*time.Hour), "r1:"+strings.Repeat("1", 64))
+	fresh := check(aPull, "build-2", now.Add(-time.Hour), "r1:"+strings.Repeat("2", 64))
+	others := check(other, "build-3", now.Add(-time.Hour), "r1:"+strings.Repeat("3", 64))
+	walked := image("meshboard/server", `{"mediaType":"`+mediaManifestOCI+`","layers":[],"n":"walk"}`)
+
+	if _, found, err := registry.Find(ctx, "meshboard", "r1:"+strings.Repeat("2", 64)); err != nil || !found {
+		t.Fatalf("a check build kept is not found by its key: %v %v", found, err)
+	}
+	n, err := registry.Expire(ctx, CheckBuildsKept, now)
+	if err != nil || n != 1 {
+		t.Fatalf("expired %d checks (%v), want the one eight days old", n, err)
+	}
+	if held(old) || !held(fresh) || !held(others) {
+		t.Fatalf("after the week's sweep: old %v, fresh %v, another's %v", held(old), held(fresh), held(others))
+	}
+	if _, found, _ := registry.Find(ctx, "meshboard", "r1:"+strings.Repeat("1", 64)); found {
+		t.Fatal("the expired check build's record is still found")
+	}
+
+	n, err = registry.Forget(ctx, aPull)
+	if err != nil || n != 1 {
+		t.Fatalf("forgot %d checks of %s (%v)", n, aPull, err)
+	}
+	if held(fresh) || !held(others) {
+		t.Fatalf("after %s closed: its build held %v, another pull request's %v", aPull, held(fresh), held(others))
+	}
+	if _, ok := m.manifests["meshboard/server@"+strings.SplitN(walked, "@", 2)[1]]; !ok {
+		t.Fatal("letting go of check builds took the walk's copy in the module's namespace")
+	}
+	if left, _ := registry.OfPulls(ctx, []PullRef{aPull}); len(left) != 0 {
+		t.Fatalf("%s still has check builds: %+v", aPull, left)
+	}
+}
+
+// The walk copies a check build by digest: the digest registered is the digest the check built, and the copy is
+// in the module's own namespace.
+func TestAChecksBuildIsCopiedByDigestIntoTheModulesNamespace(t *testing.T) {
+	m := &aTaggingRegistry{aRegistryOfRepositories: newRegistryOfRepositories(), tags: map[string]map[string]string{}}
+	server := httptest.NewServer(m.serve())
+	defer server.Close()
+	address := strings.TrimPrefix(server.URL, "http://")
+	registry := Registry{Address: address}
+
+	layer := []byte("layer")
+	m.blobs[digestOf(layer)] = layer
+	m.link("scratch/meshboard/server", digestOf(layer))
+	manifest := []byte(`{"mediaType":"` + mediaManifestOCI + `","layers":[{"digest":"` + digestOf(layer) + `","size":5}]}`)
+	m.manifests["scratch/meshboard/server@"+digestOf(manifest)] = manifest
+	archive := []byte("archive")
+	m.blobs[digestOf(archive)] = archive
+	m.link("scratch/meshboard/look", digestOf(archive))
+
+	got, err := registry.Take(t.Context(), "meshboard", catalogue.Built{Name: "server", Kind: "image",
+		Reference: address + "/scratch/meshboard/server@" + digestOf(manifest)})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if got.Reference != address+"/meshboard/server@"+digestOf(manifest) {
+		t.Fatalf("copied as %s", got.Reference)
+	}
+	if m.uploads != 0 || m.mounts != 1 {
+		t.Fatalf("the image's layer was moved (%d uploads, %d mounts), not mounted", m.uploads, m.mounts)
+	}
+	got, err = registry.Take(t.Context(), "meshboard", catalogue.Built{Name: "look", Kind: "archive",
+		Digest: digestOf(archive), Reference: "http://" + address + "/v2/scratch/meshboard/look/blobs/" + digestOf(archive)})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if got.Digest != digestOf(archive) || got.Reference != "http://"+address+"/v2/meshboard/look/blobs/"+digestOf(archive) ||
+		!m.links["meshboard/look"][digestOf(archive)] {
+		t.Fatalf("the archive was copied as %+v", got)
+	}
+}