Let platform manifests go only on a confirmed collect, and copy again what a sweep took

An unrecorded index or a copy in progress can name a platform the records do not see, so
only a person's collect, after its dry run, takes an index's platforms, and only once every
kept index of each repository it touches was read. A copy missing a platform is copied
again, and a copy a build holds again is no longer recorded as collected (review of #144).

cmd/mesh-builder/main.go

@@ -264,6 +264,8 @@ func answer(ctx context.Context, publisher builder.Publisher, on, workspace stri
 			request.Repository, request.Path, request.Ref, workspace, request.Held, npmrc,
 			forgeFrom(), say, request.Seats)
 	}
+	// What it copied into the store, whatever became of the build (novox/hq ADR 0257).
+	result.Mirrored = built.Mirrored
 	// Only a build the kill ended: the kill came before its work did. One that finished — built, or
 	// failed on its own — in the moment the kill arrived says what it did, and the kill is refused.
 	killed := false

cmd/mesh-controller/build.go

@@ -160,6 +160,9 @@ func buildFrom(result link.BuildResult) inventory.Build {
 	for _, ref := range result.Against {
 		kept.Against = append(kept.Against, catalogue.Recorded(ref))
 	}
+	for _, ref := range result.Mirrored {
+		kept.Mirrored = append(kept.Mirrored, catalogue.Recorded(ref))
+	}
 	for _, r := range result.Read {
 		kept.Read = append(kept.Read, inventory.ReadRepository{Repository: r.Repository, Ref: r.Ref})
 	}

cmd/mesh-controller/collect.go

@@ -31,6 +31,11 @@ type sweepBounds struct {
 	most int
 	// budget is the longest it keeps its caller waiting.
 	budget time.Duration
+	// platforms is whether an index is let go of with its own platform manifests (novox/hq ADR 0257).
+	// Only a sweep a person confirmed, after its dry run listed what stays and names a platform: the
+	// records cannot see an unrecorded index, or a copy in progress, naming the same platform, and the
+	// store's tools can.
+	platforms bool
 }
 
 // afterBuild are the bounds of the sweep inside somebody's build.
@@ -76,6 +81,29 @@ func sweep(ctx context.Context, inv *inventory.Inventory, store artifacts.Store,
 	// as builds come, and is two HEADs each once done. A kept archive that could not be held stops
 	// the sweep before it deletes anything: "everything kept is held" is the precondition the
 	// collector's safety rests on, and a store refusing a hold would refuse the deletes too.
+	// **An index goes with its platform manifests only when a person confirmed it** (novox/hq ADR
+	// 0257), and a kept index keeps its own: which platform manifests the kept indexes name is read
+	// from the store for every repository the sweep will touch, before the first delete. A single
+	// read that fails stops the sweep before it deletes anything, as a kept archive that cannot be
+	// held does. The sweep after a build lets an index go alone, as it always has.
+	store.Spare = nil
+	if bounds.platforms {
+		if inv == nil {
+			r.Stopped = "no records to read which platform manifests a kept index names, so nothing was let go"
+			r.Left = len(references)
+			return r
+		}
+		images, err := inv.KeptImages(ctx)
+		if err == nil {
+			store.Spare, err = store.SpareKeptIndexes(within, images, references)
+		}
+		if err != nil {
+			r.Stopped = fmt.Sprintf("which platform manifests a kept index names could not be read, so nothing was let go: %v", err)
+			r.Left = len(references)
+			return r
+		}
+	}
+
 	wrote, missing, err := holdKept(within, store, kept)
 	r.HoldersWritten, r.Missing = wrote, missing
 	if err != nil {

cmd/mesh-controller/handacts.go

@@ -83,6 +83,10 @@ var handActVerbs = []handActVerb{
 	// a moment the person chose (ADR 0251) — never a repair.
 	{Verb: "collect", Decision: "letting the store go of what the records keep for no reason, now rather " +
 		"than at the next build, is a person's word (ADR 0251)"},
+	// Recording a copy no record names as the mesh's: a person's reading of the store, never a repair
+	// (ADR 0257).
+	{Verb: "mirrors", Decision: "which copies in the store no record names are the mesh's is a person's " +
+		"word (ADR 0257)"},
 	{Verb: "bus upgrade", Decision: "the bus is never rolled by the mesh: replacing it is a planned step a " +
 		"person starts (ADR 0236)"},
 	{Verb: "upgrade release-backlog", Decision: "after a release plan failed, the next opens only when a " +

cmd/mesh-controller/main.go

@@ -100,6 +100,8 @@ func run() error {
 		return collectCommand(ctx, args[1:])
 	case "images":
 		return imagesCommand(ctx, args[1:])
+	case "mirrors":
+		return mirrorsCommand(ctx, args[1:])
 	case "plans":
 		return plansCommand(ctx, args[1:])
 	case "delivery":

cmd/mesh-controller/mirrors.go

@@ -0,0 +1,237 @@
+package main
+
+import (
+	"context"
+	"encoding/json"
+	"errors"
+	"flag"
+	"fmt"
+	"os"
+	"regexp"
+	"strings"
+
+	"github.com/novox/mesh-controller/internal/artifacts"
+	"github.com/novox/mesh-controller/internal/catalogue"
+	"github.com/novox/mesh-controller/internal/inventory"
+)
+
+// The bases the mesh copied into its artifact store, and the copies no record names (novox/hq ADR 0257).
+//
+// A build that stands on an image published elsewhere copies it in first (ADR 0096, 0097). Since ADR 0257
+// each copy is recorded where it is made, and a copy is kept while a kept build stood on it. Copies made
+// before then by a build that failed — or before the records kept what a build stood on — are in the store
+// with no record naming them, and ADR 0189 §3 keeps the sweep away from anything no record names. This
+// verb is how a person says such a copy is the mesh's: it records it, and the sweep then decides as it
+// does for every other copy. It deletes nothing.
+
+// mirrorRepository is a repository only the mirror writes: one image's repository, or a module's own
+// repository of a base from before ADR 0257.
+var mirrorRepository = regexp.MustCompile(`^(upstream/[a-z0-9][a-z0-9._/-]*|[a-z0-9][a-z0-9._-]*/on-[a-z0-9_]+)$`)
+
+// mirrorReference reads a reference a person gave into its recorded form, refusing anything that is not
+// a manifest in a repository the mirror writes.
+func mirrorReference(given string) (string, error) {
+	reference := catalogue.Recorded(strings.TrimSpace(given))
+	path, ours := catalogue.InArtifactStore(reference)
+	if !ours {
+		return "", fmt.Errorf("%q is not a reference into the artifact store (artifact-store://<repository>@sha256:<hex>)", given)
+	}
+	repository, digest, ok := strings.Cut(path, "@sha256:")
+	if !ok || len(digest) != 64 || strings.Trim(digest, "0123456789abcdef") != "" {
+		return "", fmt.Errorf("%q names no manifest by digest", given)
+	}
+	if !mirrorRepository.MatchString(repository) {
+		return "", fmt.Errorf("%q is in %s, which is not a repository the mirror writes "+
+			"(upstream/<host>/<path>, or <module>/on-<argument>)", given, repository)
+	}
+	return reference, nil
+}
+
+type mirrorEntry struct {
+	Reference string   `json:"reference"`
+	State     string   `json:"state"`
+	Why       []string `json:"why,omitempty"`
+}
+
+type mirrorsAnswer struct {
+	DryRun bool `json:"dry_run,omitempty"`
+	// Mirrors is every copy the records hold that is not yet let go of.
+	Mirrors []mirrorEntry `json:"mirrors"`
+	Counts  struct {
+		Kept      int `json:"kept"`
+		Eligible  int `json:"eligible"`
+		Collected int `json:"collected"`
+	} `json:"counts"`
+	// Recorded, AlreadyRecorded and Refused answer a record: what was (or, a dry run, would be)
+	// recorded, what the records already held, and what was refused, with why.
+	Recorded []string `json:"recorded,omitempty"`
+	// Then says what recording does: a recorded copy is eligible, and the next sweep lets it go.
+	Then            string            `json:"then,omitempty"`
+	AlreadyRecorded []string          `json:"already_recorded,omitempty"`
+	Refused         map[string]string `json:"refused,omitempty"`
+}
+
+// mirrorsOf is the copies among the recorded states.
+func mirrorsOf(states []inventory.ArtifactState, mirrored map[string]bool) mirrorsAnswer {
+	a := mirrorsAnswer{Mirrors: []mirrorEntry{}}
+	for _, s := range states {
+		if !mirrored[s.Reference] {
+			continue
+		}
+		switch s.State {
+		case inventory.ArtifactKept:
+			a.Counts.Kept++
+		case inventory.ArtifactEligible:
+			a.Counts.Eligible++
+		case inventory.ArtifactCollected:
+			a.Counts.Collected++
+			continue
+		}
+		a.Mirrors = append(a.Mirrors, mirrorEntry{Reference: s.Reference, State: s.State, Why: s.Why})
+	}
+	return a
+}
+
+// splitReferences reads references separated by spaces or commas.
+func splitReferences(given string) []string {
+	return strings.FieldsFunc(given, func(r rune) bool { return r == ',' || r == ' ' || r == '\n' || r == '\t' })
+}
+
+// recordMirrors decides, for each reference given, whether it may be recorded: in a repository the
+// mirror writes, not already recorded, and held by the store. A real run records those.
+func recordMirrors(ctx context.Context, inv *inventory.Inventory, store artifacts.Store, given []string,
+	known map[string]bool, why string, real bool) (mirrorsAnswer, error) {
+	a := mirrorsAnswer{DryRun: !real, Mirrors: []mirrorEntry{}, Refused: map[string]string{}}
+	var record []string
+	seen := map[string]bool{}
+	for _, g := range given {
+		reference, err := mirrorReference(g)
+		if err != nil {
+			a.Refused[g] = err.Error()
+			continue
+		}
+		if seen[reference] {
+			continue
+		}
+		seen[reference] = true
+		if known[reference] {
+			a.AlreadyRecorded = append(a.AlreadyRecorded, reference)
+			continue
+		}
+		if store.Address == "" {
+			a.Refused[g] = "this mesh has no artifact store on its network to ask whether it holds this"
+			continue
+		}
+		held, err := store.HoldsManifest(ctx, reference)
+		switch {
+		case err != nil:
+			a.Refused[g] = err.Error()
+			continue
+		case !held:
+			a.Refused[g] = "the artifact store does not hold it"
+			continue
+		}
+		record = append(record, reference)
+	}
+	a.Recorded = record
+	if len(record) > 0 {
+		verb := "would become"
+		if real {
+			verb = "became"
+		}
+		a.Then = fmt.Sprintf("%d cop%s %s eligible: the next sweep (after any build, or collect) lets each go "+
+			"unless one of the five kept builds of a module the mesh holds stood on it", len(record),
+			map[bool]string{true: "y", false: "ies"}[len(record) == 1], verb)
+	}
+	if real && len(record) > 0 {
+		if err := inv.RecordMirrored(ctx, "", why, record); err != nil {
+			return a, fmt.Errorf("recording %d copies: %w", len(record), err)
+		}
+	}
+	return a, nil
+}
+
+func mirrorsCommand(ctx context.Context, args []string) error {
+	set := flag.NewFlagSet("mirrors", flag.ContinueOnError)
+	asJSON := set.Bool("json", false, "answer as JSON")
+	record := set.String("record", "", "references of copies no record names, separated by spaces or commas, to record as the mesh's")
+	confirm := set.Bool("confirm", false, "record them, rather than only say what would be recorded")
+	f := addHandActFlags(set)
+	positionals, err := parseAround(set, args)
+	if err != nil {
+		return err
+	}
+	if len(positionals) != 0 {
+		return errors.New("mirrors [--json] [--record <references> [--confirm --why <text>]]")
+	}
+	if *confirm {
+		if strings.TrimSpace(*record) == "" {
+			return errors.New("mirrors: --confirm records what --record names, and it names nothing. Nothing was done")
+		}
+		if err := f.require("mirrors"); err != nil {
+			return err
+		}
+	}
+	open, err := openStores(ctx)
+	if err != nil {
+		return err
+	}
+	defer open.Close()
+	inv := open.inventory
+	states, err := inv.Artifacts(ctx)
+	if err != nil {
+		return err
+	}
+	known, err := inv.Mirrored(ctx)
+	if err != nil {
+		return err
+	}
+
+	var answer mirrorsAnswer
+	if strings.TrimSpace(*record) == "" {
+		answer = mirrorsOf(states, known)
+	} else {
+		shelf, err := inv.Catalogue(ctx)
+		if err != nil {
+			return err
+		}
+		address, err := artifactStoreAddress(ctx, inv, shelf, "")
+		if err != nil {
+			return err
+		}
+		answer, err = recordMirrors(ctx, inv, artifacts.Store{Address: address}, splitReferences(*record), known,
+			strings.TrimSpace(*f.why), *confirm)
+		if err != nil {
+			return err
+		}
+		// The hand act is logged once the records say what it did, never before: an act that
+		// failed half-way is not logged as done.
+		if *confirm && len(answer.Recorded) > 0 {
+			f.record(ctx, "mirrors", append([]string{"--record"}, answer.Recorded...))
+		}
+	}
+	if *asJSON {
+		encoder := json.NewEncoder(os.Stdout)
+		encoder.SetIndent("", " ")
+		return encoder.Encode(answer)
+	}
+	if answer.DryRun && len(answer.Recorded) > 0 {
+		fmt.Println("a dry run: nothing was recorded (--confirm --why <text> to record)")
+	}
+	if answer.Then != "" {
+		fmt.Println(answer.Then)
+	}
+	for _, r := range answer.Recorded {
+		fmt.Printf("  record   %s\n", r)
+	}
+	for _, r := range answer.AlreadyRecorded {
+		fmt.Printf("  already  %s\n", r)
+	}
+	for g, why := range answer.Refused {
+		fmt.Printf("  refused  %s: %s\n", g, why)
+	}
+	for _, m := range answer.Mirrors {
+		fmt.Printf("  %-9s %s %s\n", m.State, m.Reference, strings.Join(m.Why, ", "))
+	}
+	return nil
+}

cmd/mesh-controller/mirrors_test.go

@@ -0,0 +1,280 @@
+package main
+
+import (
+	"fmt"
+	"net/http"
+	"net/http/httptest"
+	"slices"
+	"strings"
+	"sync"
+	"testing"
+	"time"
+
+	"github.com/novox/mesh-controller/internal/artifacts"
+	"github.com/novox/mesh-controller/internal/catalogue"
+	"github.com/novox/mesh-controller/internal/inventory"
+	"github.com/novox/mesh-controller/internal/link"
+)
+
+// The bases the mesh copied in, and recording a copy no record names (novox/hq ADR 0257).
+
+func TestTheMirrorsVerbComposesItsCommandAndRefusesWhatItDoesNotTake(t *testing.T) {
+	r := ref("route-proxy", "on-go_base", 1)
+	for _, c := range []struct {
+		args map[string]any
+		want string
+	}{
+		{map[string]any{}, "mirrors --json"},
+		{map[string]any{"record": r}, "mirrors --json --record " + r},
+		{map[string]any{"record": r, "confirm": "true", "why": "copied before copies were recorded"},
+			"mirrors --json --record " + r + " --confirm --why copied before copies were recorded"},
+	} {
+		argv, err := argvFor("mirrors", c.args)
+		if err != nil || strings.Join(argv, " ") != c.want {
+			t.Errorf("mirrors %v: %q %v, want %q", c.args, argv, err, c.want)
+		}
+	}
+	for _, args := range []map[string]any{
+		{"record": r, "confirm": "true"},            // recorded without a why
+		{"record": r, "why": "x"},                   // a why for a dry run
+		{"confirm": "true", "why": "x"},             // nothing to record
+		{"record": r, "confirm": "yes", "why": "x"}, // a switch is true or false
+		{"delete": "true"},
+	} {
+		if argv, err := argvFor("mirrors", args); err == nil {
+			t.Errorf("mirrors %v was composed as %q", args, argv)
+		}
+	}
+	if repairingCommand([]string{"mirrors", "--json", "--record", r, "--confirm", "--why", "x"}) != "mirrors" {
+		t.Error("recording a copy through the generic verb would go unrecorded")
+	}
+	if repairingCommand([]string{"mirrors", "--json", "--record", r}) != "" {
+		t.Error("a dry run was taken for an act by hand")
+	}
+	if !personsDecision(link.HandAct{Verb: "mirrors"}) {
+		t.Error("recording a copy would count toward a healer the mesh lacks")
+	}
+}
+
+func TestOnlyACopyInARepositoryTheMirrorWritesIsTaken(t *testing.T) {
+	for given, ok := range map[string]bool{
+		ref("route-proxy", "on-go_base", 1): true,
+		catalogue.ArtifactStoreScheme + "upstream/docker.io/library/golang@sha256:" + strings.Repeat("a", 64): true,
+		ref("route-proxy", "server", 1): false, // a module's own image
+		catalogue.ArtifactStoreScheme + "novox/invoicing-api@sha256:" + strings.Repeat("a", 64): false,
+		archiveRef("web", "on-tools", 1):                             false, // a blob
+		catalogue.ArtifactStoreScheme + "web/on-x@sha256:abc":        false, // not a whole digest
+		"docker.io/library/golang@sha256:" + strings.Repeat("a", 64): false,
+	} {
+		if _, err := mirrorReference(given); (err == nil) != ok {
+			t.Errorf("%s: taken %v, want %v (%v)", given, err == nil, ok, err)
+		}
+	}
+}
+
+// heldStore answers a manifest HEAD with 200 for the digests it holds, and records every request.
+func heldStore(t *testing.T, holds ...string) (artifacts.Store, *[]string) {
+	t.Helper()
+	var asked []string
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		asked = append(asked, r.Method+" "+r.URL.Path)
+		if r.Method == http.MethodHead && slices.ContainsFunc(holds, func(d string) bool { return strings.HasSuffix(r.URL.Path, d) }) {
+			w.WriteHeader(http.StatusOK)
+			return
+		}
+		w.WriteHeader(http.StatusNotFound)
+	}))
+	t.Cleanup(server.Close)
+	return artifacts.Store{Address: strings.TrimPrefix(server.URL, "http://")}, &asked
+}
+
+func TestRecordingACopyTakesWhatTheStoreHoldsAndDeletesNothing(t *testing.T) {
+	inv := inventory.ForTest(t)
+	held := ref("route-proxy", "on-go_base", 1)
+	absent := ref("route-proxy", "on-go_base", 2)
+	known := ref("nats", "on-nats_base", 3)
+	notACopy := ref("route-proxy", "server", 4)
+	if err := inv.RecordMirrored(t.Context(), "b1", "", []string{known}); err != nil {
+		t.Fatal(err)
+	}
+	store, asked := heldStore(t, fmt.Sprintf("%064x", 1))
+	given := []string{held, absent, known, notACopy}
+	mirrored, err := inv.Mirrored(t.Context())
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// A dry run says, and records nothing.
+	dry, err := recordMirrors(t.Context(), inv, store, given, mirrored, "", false)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !dry.DryRun || !slices.Equal(dry.Recorded, []string{held}) || !slices.Equal(dry.AlreadyRecorded, []string{known}) ||
+		len(dry.Refused) != 2 || dry.Refused[absent] == "" || dry.Refused[notACopy] == "" {
+		t.Fatalf("a dry run answered %+v", dry)
+	}
+	if !strings.Contains(dry.Then, "would become eligible") || !strings.Contains(dry.Then, "next sweep") {
+		t.Fatalf("a dry run did not say what recording does: %q", dry.Then)
+	}
+	if again, _ := inv.Mirrored(t.Context()); again[held] {
+		t.Fatal("a dry run recorded a copy")
+	}
+
+	// A real one records what the store holds, with the person's why, and the sweep may now decide.
+	real, err := recordMirrors(t.Context(), inv, store, given, mirrored, "copied before copies were recorded", true)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !slices.Equal(real.Recorded, []string{held}) {
+		t.Fatalf("recorded %v", real.Recorded)
+	}
+	left, err := inv.ToCollect(t.Context())
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !slices.Contains(left, held) {
+		t.Fatalf("a recorded copy no kept build stood on is not offered to collect: %v", left)
+	}
+	for _, r := range *asked {
+		if !strings.HasPrefix(r, "HEAD ") {
+			t.Fatalf("recording a copy asked the store %s", r)
+		}
+	}
+}
+
+// indexRegistry holds indexes and platforms of one image's repository, answers GETs with their
+// documents, refuses GETs for the digests in fail, and records every delete.
+type indexRegistry struct {
+	mu      sync.Mutex
+	indexes map[string][]string
+	held    map[string]bool
+	fail    map[string]bool
+	deleted []string
+}
+
+func (f *indexRegistry) serve(t *testing.T) artifacts.Store {
+	t.Helper()
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		f.mu.Lock()
+		defer f.mu.Unlock()
+		digest := r.URL.Path[strings.LastIndex(r.URL.Path, "/")+1:]
+		switch r.Method {
+		case http.MethodGet:
+			if f.fail[digest] {
+				w.WriteHeader(http.StatusInternalServerError)
+				return
+			}
+			if children, ok := f.indexes[digest]; ok && f.held[digest] {
+				var named []string
+				for _, c := range children {
+					named = append(named, `{"mediaType":"application/vnd.oci.image.manifest.v1+json","digest":"`+c+`"}`)
+				}
+				_, _ = w.Write([]byte(`{"schemaVersion":2,"mediaType":"application/vnd.oci.image.index.v1+json","manifests":[` +
+					strings.Join(named, ",") + `]}`))
+				return
+			}
+			if f.held[digest] {
+				_, _ = w.Write([]byte(`{"schemaVersion":2,"layers":[]}`))
+				return
+			}
+			w.WriteHeader(http.StatusNotFound)
+		case http.MethodHead:
+			if f.held[digest] {
+				w.WriteHeader(http.StatusOK)
+				return
+			}
+			w.WriteHeader(http.StatusNotFound)
+		case http.MethodDelete:
+			if !f.held[digest] {
+				w.WriteHeader(http.StatusNotFound)
+				return
+			}
+			delete(f.held, digest)
+			f.deleted = append(f.deleted, digest)
+			w.WriteHeader(http.StatusAccepted)
+		default:
+			w.WriteHeader(http.StatusBadRequest)
+		}
+	}))
+	t.Cleanup(server.Close)
+	return artifacts.Store{Address: strings.TrimPrefix(server.URL, "http://")}
+}
+
+func copyDigest(n int) string { return fmt.Sprintf("sha256:%064x", n) }
+
+// twoCopies records, in one image's repository, a kept copy (stood on by a held module's build) naming
+// platforms 11 and 12, and an eligible one (a failed build's) naming 12 and 13.
+func twoCopies(t *testing.T, inv *inventory.Inventory) (kept, eligible string, reg *indexRegistry) {
+	t.Helper()
+	const repository = "upstream/docker.io/library/golang"
+	kept = catalogue.ArtifactStoreScheme + repository + "@" + copyDigest(1)
+	eligible = catalogue.ArtifactStoreScheme + repository + "@" + copyDigest(2)
+	if err := inv.RegisterModule(t.Context(), catalogue.Manifest{Module: "proxy", Version: "1"},
+		inventory.Source{Repository: "https://forge.invalid/proxy.git"}); err != nil {
+		t.Fatal(err)
+	}
+	failed := inventory.Build{ID: "f1", Repository: "https://forge.invalid/proxy.git", On: "a-build-machine",
+		Failed: "a recipe refused", Mirrored: []string{eligible}}
+	if err := inv.RecordBuild(t.Context(), failed); err != nil {
+		t.Fatal(err)
+	}
+	ok := inventory.Build{ID: "b1", Repository: "https://forge.invalid/proxy.git", Module: "proxy", On: "a-build-machine",
+		Commit: "c0ffee", Made: []inventory.Artifact{{Name: "app", Kind: "image", Reference: ref("proxy", "app", 7)}},
+		Against: []string{kept}, Mirrored: []string{kept}}
+	if err := inv.RecordBuild(t.Context(), ok); err != nil {
+		t.Fatal(err)
+	}
+	reg = &indexRegistry{
+		indexes: map[string][]string{copyDigest(1): {copyDigest(11), copyDigest(12)}, copyDigest(2): {copyDigest(12), copyDigest(13)}},
+		held:    map[string]bool{copyDigest(1): true, copyDigest(2): true, copyDigest(11): true, copyDigest(12): true, copyDigest(13): true},
+		fail:    map[string]bool{},
+	}
+	return kept, eligible, reg
+}
+
+// Through the records: a confirmed collect lets the eligible index go with the platform only it names,
+// and leaves the platform the kept index names.
+func TestAConfirmedCollectLetsAnIndexGoWithItsOwnPlatformsOnly(t *testing.T) {
+	inv := inventory.ForTest(t)
+	_, eligible, reg := twoCopies(t, inv)
+	store := reg.serve(t)
+	references, err := inv.ToCollect(t.Context())
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !slices.Equal(references, []string{eligible}) {
+		t.Fatalf("offered %v, want the failed build's copy", references)
+	}
+	a := runCollect(t.Context(), inv, store, references, nil, true,
+		sweepBounds{most: 10, budget: 5 * time.Second, platforms: true})
+	if !slices.Equal(a.LetGo, []string{eligible}) || a.Stopped != "" {
+		t.Fatalf("let go %v, stopped %q", a.LetGo, a.Stopped)
+	}
+	if !slices.Equal(reg.deleted, []string{copyDigest(13), copyDigest(2)}) {
+		t.Fatalf("deleted %v; want its own platform, then the index", reg.deleted)
+	}
+}
+
+// The sweep after a build lets an index go alone: its platforms wait for a person's collect.
+func TestTheSweepAfterABuildLetsAnIndexGoAlone(t *testing.T) {
+	inv := inventory.ForTest(t)
+	_, eligible, reg := twoCopies(t, inv)
+	store := reg.serve(t)
+	r := sweep(t.Context(), inv, store, []string{eligible}, nil, afterBuild)
+	if !slices.Equal(r.LetGo, []string{eligible}) || !slices.Equal(reg.deleted, []string{copyDigest(2)}) {
+		t.Fatalf("after a build: let go %v, deleted %v; want the index alone", r.LetGo, reg.deleted)
+	}
+}
+
+// A kept index the store will not answer for stops a confirmed collect before its first delete.
+func TestASpareListThatCannotBeReadStopsTheSweepBeforeAnyDelete(t *testing.T) {
+	inv := inventory.ForTest(t)
+	_, eligible, reg := twoCopies(t, inv)
+	reg.fail[copyDigest(1)] = true
+	store := reg.serve(t)
+	a := runCollect(t.Context(), inv, store, []string{eligible}, nil, true,
+		sweepBounds{most: 10, budget: 5 * time.Second, platforms: true})
+	if len(a.LetGo) != 0 || len(reg.deleted) != 0 || !strings.Contains(a.Stopped, "nothing was let go") {
+		t.Fatalf("let go %v, deleted %v, stopped %q", a.LetGo, reg.deleted, a.Stopped)
+	}
+}

cmd/mesh-controller/registry_verbs.go

@@ -209,7 +209,7 @@ func collectCommand(ctx context.Context, args []string) error {
 	if *most < 1 {
 		return fmt.Errorf("collect: --most is at least 1, not %d", *most)
 	}
-	bounds := sweepBounds{most: min(*most, collectMostAtMost), budget: collectBudget}
+	bounds := sweepBounds{most: min(*most, collectMostAtMost), budget: collectBudget, platforms: true}
 
 	open, err := openStores(ctx)
 	if err != nil {

cmd/mesh-controller/registry_verbs_test.go

@@ -133,6 +133,10 @@ func (f *fakeStore) serve(t *testing.T) artifacts.Store {
 			w.WriteHeader(http.StatusOK)
 		case r.Method == http.MethodHead:
 			w.WriteHeader(http.StatusNotFound)
+		case r.Method == http.MethodGet && strings.Contains(r.URL.Path, "/manifests/"):
+			// An image, not an index: it names no platform manifests.
+			w.Header().Set("Content-Type", "application/vnd.oci.image.manifest.v1+json")
+			_, _ = w.Write([]byte(`{"schemaVersion":2,"mediaType":"application/vnd.oci.image.manifest.v1+json","layers":[]}`))
 		case r.Method == http.MethodPost:
 			w.Header().Set("Location", "/upload/x?state=1")
 			w.WriteHeader(http.StatusAccepted)

cmd/mesh-controller/seatverbs.go

@@ -605,6 +605,30 @@ func (a *verbArguments) commandLine() ([]string, error) {
 			return nil, err
 		}
 		return []string{"images", str("node"), "--json"}, nil
+	case "mirrors":
+		// novox/hq ADR 0257.
+		argv := []string{"mirrors", "--json"}
+		record := str("record")
+		why := str("why")
+		if record == "" {
+			if on("confirm") || why != "" {
+				return nil, errors.New("mirrors takes confirm and why only with record: there is nothing " +
+					"else it records. Nothing was done")
+			}
+			return argv, nil
+		}
+		argv = append(argv, "--record", record)
+		if !on("confirm") {
+			if why != "" {
+				return nil, errors.New("mirrors takes why only with confirm: without confirm it is a dry run, " +
+					"and a reason for nothing would be recorded nowhere. Nothing was done")
+			}
+			return argv, nil
+		}
+		if err := need("why"); err != nil {
+			return nil, fmt.Errorf("%w: recording a copy as the mesh's is a hand act, which says why. Nothing was done", err)
+		}
+		return append(argv, "--confirm", "--why", why), nil
 	case "data":
 		argv := []string{"data", "--json"}
 		if m := str("machine"); m != "" {
@@ -804,7 +828,7 @@ func (a *verbArguments) commandLine() ([]string, error) {
 var jsonVerbs = map[string]bool{"status": true, "seats": true, "plan": true, "collection": true,
 	"hand-acts": true, "durations": true, "conditions": true, "doctor": true, "retire": true, "cleanup": true, "data": true,
 	// What the records say the registries may keep (novox/hq ADR 0251).
-	"artifacts": true, "collect": true, "images": true,
+	"artifacts": true, "collect": true, "images": true, "mirrors": true,
 	// The delivery's owner's verbs answer JSON where they read (plan, order, check, walks) — novox/hq ADR 0239.
 	"delivery": true}
 
@@ -835,6 +859,8 @@ func repairingCommand(argv []string) string {
 		return "cleanup delete"
 	case argv[0] == "collect" && slices.Contains(argv, "--confirm"):
 		return "collect"
+	case argv[0] == "mirrors" && slices.Contains(argv, "--confirm"):
+		return "mirrors"
 	}
 	return ""
 }

cmd/mesh-controller/seatverbs_schema_test.go

@@ -284,6 +284,7 @@ var accountedFlags = map[string]map[string]string{
 	"artifacts":          {"json": "set by the verb: the answer is data"},
 	"collect":            {"json": "set by the verb: the answer is data"},
 	"images":             {"json": "set by the verb: the answer is data"},
+	"mirrors":            {"json": "set by the verb: the answer is data"},
 	"conditions history": {"json": "set by the verb: the answer is data"},
 	"conditions show":    {"json": "set by the verb: the answer is data"},
 	"healers":            {"json": "set by the verb: the answer is data"},

internal/artifacts/platforms_test.go

@@ -0,0 +1,176 @@
+package artifacts
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"net/http"
+	"net/http/httptest"
+	"slices"
+	"strings"
+	"sync"
+	"testing"
+
+	"github.com/novox/mesh-controller/internal/catalogue"
+)
+
+// An index is let go of with the platform manifests it names, except those a kept index of the same
+// repository names too (novox/hq ADR 0257). Each platform manifest is a manifest of the repository in
+// its own right, and the store's collector keeps every manifest a repository holds — so an index let go
+// of alone frees nothing of the images it names.
+
+func digestN(n int) string { return fmt.Sprintf("sha256:%064x", n) }
+
+// indexStore holds indexes and images in one repository, answers GETs with their documents, and records
+// every delete.
+type indexStore struct {
+	mu      sync.Mutex
+	indexes map[string][]string // digest → the platform manifests it names
+	images  map[string]bool
+	deleted []string
+}
+
+func (s *indexStore) serve(t *testing.T) Store {
+	t.Helper()
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		s.mu.Lock()
+		defer s.mu.Unlock()
+		digest := r.URL.Path[strings.LastIndex(r.URL.Path, "/")+1:]
+		switch r.Method {
+		case http.MethodGet:
+			if children, ok := s.indexes[digest]; ok {
+				var named []string
+				for _, c := range children {
+					named = append(named, `{"mediaType":"application/vnd.oci.image.manifest.v1+json","digest":"`+c+`","size":1}`)
+				}
+				w.Header().Set("Content-Type", "application/vnd.oci.image.index.v1+json")
+				_, _ = w.Write([]byte(`{"schemaVersion":2,"mediaType":"application/vnd.oci.image.index.v1+json","manifests":[` +
+					strings.Join(named, ",") + `]}`))
+				return
+			}
+			if s.images[digest] {
+				w.Header().Set("Content-Type", "application/vnd.oci.image.manifest.v1+json")
+				_, _ = w.Write([]byte(`{"schemaVersion":2,"mediaType":"application/vnd.oci.image.manifest.v1+json","layers":[]}`))
+				return
+			}
+			w.WriteHeader(http.StatusNotFound)
+		case http.MethodHead:
+			if _, ok := s.indexes[digest]; ok || s.images[digest] {
+				w.WriteHeader(http.StatusOK)
+				return
+			}
+			w.WriteHeader(http.StatusNotFound)
+		case http.MethodDelete:
+			if _, ok := s.indexes[digest]; !ok && !s.images[digest] {
+				w.WriteHeader(http.StatusNotFound)
+				return
+			}
+			s.deleted = append(s.deleted, digest)
+			delete(s.indexes, digest)
+			delete(s.images, digest)
+			w.WriteHeader(http.StatusAccepted)
+		default:
+			w.WriteHeader(http.StatusBadRequest)
+		}
+	}))
+	t.Cleanup(server.Close)
+	return Store{Address: strings.TrimPrefix(server.URL, "http://")}
+}
+
+func TestAnIndexGoesWithItsPlatformsAndAKeptIndexKeepsItsOwn(t *testing.T) {
+	// Two indexes of one image's repository: the old one names platforms 11 and 12, the kept one 12 and 13.
+	s := &indexStore{
+		indexes: map[string][]string{digestN(1): {digestN(11), digestN(12)}, digestN(2): {digestN(12), digestN(13)}},
+		images:  map[string]bool{digestN(11): true, digestN(12): true, digestN(13): true},
+	}
+	store := s.serve(t)
+	repository := "upstream/docker.io/library/golang"
+	old := catalogue.ArtifactStoreScheme + repository + "@" + digestN(1)
+	kept := catalogue.ArtifactStoreScheme + repository + "@" + digestN(2)
+	spare, err := store.SpareKeptIndexes(context.Background(), []string{kept}, []string{old})
+	if err != nil {
+		t.Fatal(err)
+	}
+	store.Spare = spare
+
+	if err := store.LetGo(context.Background(), old); err != nil {
+		t.Fatal(err)
+	}
+	// Its own platform first, the index last; the platform the kept index names is left in place.
+	if !slices.Equal(s.deleted, []string{digestN(11), digestN(1)}) {
+		t.Fatalf("deleted %v; want the old index's own platform, then the index", s.deleted)
+	}
+	if !s.images[digestN(12)] || !s.images[digestN(13)] {
+		t.Fatal("a platform a kept index names was let go of")
+	}
+}
+
+func TestWithoutSparingAnIndexGoesAloneAndAnImageNamesNoPlatforms(t *testing.T) {
+	s := &indexStore{indexes: map[string][]string{digestN(1): {digestN(11)}}, images: map[string]bool{digestN(11): true, digestN(5): true}}
+	store := s.serve(t)
+	// No Spare: as before ADR 0257, the index alone.
+	if err := store.LetGo(context.Background(), catalogue.ArtifactStoreScheme+"web/base@"+digestN(1)); err != nil {
+		t.Fatal(err)
+	}
+	if !slices.Equal(s.deleted, []string{digestN(1)}) {
+		t.Fatalf("without a Spare, deleted %v", s.deleted)
+	}
+	// An image is one delete, with a Spare or without.
+	s.deleted = nil
+	spare, err := store.SpareKeptIndexes(context.Background(), nil, []string{catalogue.ArtifactStoreScheme + "web/app@" + digestN(5)})
+	if err != nil {
+		t.Fatal(err)
+	}
+	store.Spare = spare
+	if err := store.LetGo(context.Background(), catalogue.ArtifactStoreScheme+"web/app@"+digestN(5)); err != nil {
+		t.Fatal(err)
+	}
+	if !slices.Equal(s.deleted, []string{digestN(5)}) {
+		t.Fatalf("an image was let go of as %v", s.deleted)
+	}
+	// An index the store no longer holds is Gone, and nothing is deleted on its account.
+	s.deleted = nil
+	err = store.LetGo(context.Background(), catalogue.ArtifactStoreScheme+"web/base@"+digestN(9))
+	if !errors.Is(err, Gone) || len(s.deleted) != 0 {
+		t.Fatalf("an index the store does not hold: %v, deleted %v", err, s.deleted)
+	}
+}
+
+func TestASpareThatCannotBeReadKeepsTheIndex(t *testing.T) {
+	s := &indexStore{indexes: map[string][]string{digestN(1): {digestN(11)}}, images: map[string]bool{digestN(11): true}}
+	store := s.serve(t)
+	store.Spare = func(context.Context, string) (map[string]bool, error) { return nil, fmt.Errorf("the store went away") }
+	err := store.LetGo(context.Background(), catalogue.ArtifactStoreScheme+"web/base@"+digestN(1))
+	if err == nil || !strings.Contains(err.Error(), "was kept") {
+		t.Fatalf("a spare that could not be read: %v", err)
+	}
+	if len(s.deleted) != 0 {
+		t.Fatalf("deleted %v without knowing what a kept index names", s.deleted)
+	}
+}
+
+// The spare list is read whole before the sweep: a kept index that cannot be read is an error before
+// anything is deleted, and a repository not read before keeps its index.
+func TestASpareListIsReadWholeBeforeAnythingGoes(t *testing.T) {
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusInternalServerError)
+	}))
+	t.Cleanup(server.Close)
+	store := Store{Address: strings.TrimPrefix(server.URL, "http://")}
+	kept := catalogue.ArtifactStoreScheme + "web/base@" + digestN(2)
+	old := catalogue.ArtifactStoreScheme + "web/base@" + digestN(1)
+	if _, err := store.SpareKeptIndexes(context.Background(), []string{kept}, []string{old}); err == nil {
+		t.Fatal("a kept index the store would not answer for was taken as naming nothing")
+	}
+
+	s := &indexStore{indexes: map[string][]string{digestN(1): {digestN(11)}}, images: map[string]bool{digestN(11): true}}
+	good := s.serve(t)
+	spare, err := good.SpareKeptIndexes(context.Background(), nil, []string{catalogue.ArtifactStoreScheme + "other/repo@" + digestN(3)})
+	if err != nil {
+		t.Fatal(err)
+	}
+	good.Spare = spare
+	if err := good.LetGo(context.Background(), old); err == nil || len(s.deleted) != 0 {
+		t.Fatalf("an index in a repository not read before the sweep was let go of: %v, deleted %v", err, s.deleted)
+	}
+}

internal/artifacts/store.go

@@ -9,8 +9,10 @@ package artifacts
 
 import (
 	"context"
+	"encoding/json"
 	"errors"
 	"fmt"
+	"io"
 	"net/http"
 	"strings"
 
@@ -24,6 +26,10 @@ type Store struct {
 	Address string
 	// HTTP is the client used; nil is a client with a modest timeout.
 	HTTP *http.Client
+	// Spare says, for one repository, which manifests a kept index there names: the platform
+	// manifests an index being let go of must leave in place (novox/hq ADR 0257). Nil spares every
+	// platform manifest — an index is then let go of alone, as it was before.
+	Spare func(ctx context.Context, repository string) (map[string]bool, error)
 }
 
 // Gone is the answer when the store does not hold it: the outcome wanted, already true.
@@ -77,9 +83,142 @@ func (s Store) LetGo(ctx context.Context, reference string) error {
 			return err
 		}
 	}
+	if kind == "manifests" && s.Spare != nil {
+		// **An index's platform manifests go before the index** (novox/hq ADR 0257). Each was put
+		// under its own digest, so each is a manifest of the repository in its own right, and the
+		// store's collector keeps every manifest a repository holds: an index let go of alone frees
+		// no byte of the images it names. Before, not after, so a sweep cut short leaves the index
+		// to be asked about again, and with it the list of what is still to go.
+		if err := s.letGoOfPlatforms(ctx, repository, digest, reference); err != nil {
+			return err
+		}
+	}
 	return s.remove(ctx, s.url(repository, kind, digest), reference)
 }
 
+// letGoOfPlatforms deletes the manifests an index names, except those a kept index of the same
+// repository also names. A manifest that is not an index names none.
+func (s Store) letGoOfPlatforms(ctx context.Context, repository, digest, reference string) error {
+	children, err := s.Platforms(ctx, repository, digest)
+	if err != nil || len(children) == 0 {
+		// Gone: there is no index to read, and the delete that follows says so.
+		if errors.Is(err, Gone) {
+			return nil
+		}
+		return err
+	}
+	spared, err := s.Spare(ctx, repository)
+	if err != nil {
+		return fmt.Errorf("cannot say which platform manifests of %s a kept index names, so %s was kept: %w",
+			repository, reference, err)
+	}
+	for _, child := range children {
+		if spared[child] {
+			continue
+		}
+		if err := s.remove(ctx, s.url(repository, "manifests", child), reference+" (its "+child+")"); err != nil && !errors.Is(err, Gone) {
+			return err
+		}
+	}
+	return nil
+}
+
+// anyManifest is every manifest the store may hold, indexes included, so it answers with the
+// document as it is.
+var anyManifest = []string{
+	"application/vnd.oci.image.index.v1+json",
+	"application/vnd.docker.distribution.manifest.list.v2+json",
+	"application/vnd.oci.image.manifest.v1+json",
+	"application/vnd.docker.distribution.manifest.v2+json",
+}
+
+// Platforms is the manifests an index names, by digest; none for a manifest that is not an index.
+// Gone when the store does not hold it.
+func (s Store) Platforms(ctx context.Context, repository, digest string) ([]string, error) {
+	request, err := http.NewRequestWithContext(ctx, http.MethodGet, s.url(repository, "manifests", digest), nil)
+	if err != nil {
+		return nil, err
+	}
+	request.Header.Set("Accept", strings.Join(anyManifest, ", "))
+	response, err := s.client().Do(request)
+	if err != nil {
+		return nil, err
+	}
+	defer response.Body.Close()
+	switch response.StatusCode {
+	case http.StatusOK:
+	case http.StatusNotFound:
+		return nil, Gone
+	default:
+		return nil, fmt.Errorf("the artifact store answered %s when asked for %s@%s", response.Status, repository, digest)
+	}
+	var document struct {
+		Manifests []struct {
+			Digest string `json:"digest"`
+		} `json:"manifests"`
+	}
+	if err := json.NewDecoder(io.LimitReader(response.Body, 4<<20)).Decode(&document); err != nil {
+		return nil, fmt.Errorf("%s@%s is not a manifest: %w", repository, digest, err)
+	}
+	var out []string
+	for _, m := range document.Manifests {
+		if strings.HasPrefix(m.Digest, "sha256:") {
+			out = append(out, m.Digest)
+		}
+	}
+	return out, nil
+}
+
+// SpareKeptIndexes is a Spare for a sweep, read whole before it starts: for every repository an
+// image reference of `touching` is in, each kept manifest there and every platform manifest a kept
+// index there names. Any read that fails is an error, and the sweep lets nothing go. A repository not
+// read here is answered with an error, so an index in it is kept. A kept index the store no longer
+// holds names nothing.
+func (s Store) SpareKeptIndexes(ctx context.Context, kept, touching []string) (func(context.Context, string) (map[string]bool, error), error) {
+	imageIn := func(reference string) (repository, digest string, ok bool) {
+		path, ours := catalogue.InArtifactStore(reference)
+		if !ours {
+			return "", "", false
+		}
+		repository, kind, digest, err := split(path)
+		if err != nil || kind != "manifests" {
+			return "", "", false
+		}
+		return repository, digest, true
+	}
+	read := map[string]map[string]bool{}
+	for _, reference := range touching {
+		if repository, _, ok := imageIn(reference); ok {
+			read[repository] = map[string]bool{}
+		}
+	}
+	for _, reference := range kept {
+		repository, digest, ok := imageIn(reference)
+		spared, touched := read[repository]
+		if !ok || !touched {
+			continue
+		}
+		spared[digest] = true
+		children, err := s.Platforms(ctx, repository, digest)
+		if errors.Is(err, Gone) {
+			continue
+		}
+		if err != nil {
+			return nil, fmt.Errorf("reading the kept %s@%s: %w", repository, digest, err)
+		}
+		for _, c := range children {
+			spared[c] = true
+		}
+	}
+	return func(_ context.Context, repository string) (map[string]bool, error) {
+		spared, done := read[repository]
+		if !done {
+			return nil, fmt.Errorf("%s was not read before the sweep began", repository)
+		}
+		return spared, nil
+	}, nil
+}
+
 // remove asks the store to delete what is at url. Gone when it has no such thing.
 func (s Store) remove(ctx context.Context, url, what string) error {
 	request, err := http.NewRequestWithContext(ctx, http.MethodDelete, url, nil)
@@ -121,3 +260,19 @@ func split(path string) (repository, kind, digest string, err error) {
 	}
 	return "", "", "", fmt.Errorf("%w: %q names nothing the store holds by digest", ErrNotOurs, path)
 }
+
+// HoldsManifest is whether the store holds the manifest a recorded image reference names.
+func (s Store) HoldsManifest(ctx context.Context, reference string) (bool, error) {
+	path, ours := catalogue.InArtifactStore(reference)
+	if !ours {
+		return false, fmt.Errorf("%w: %s", ErrNotOurs, reference)
+	}
+	repository, kind, digest, err := split(path)
+	if err != nil {
+		return false, err
+	}
+	if kind != "manifests" {
+		return false, fmt.Errorf("%w: %s names a blob, not a manifest", ErrNotOurs, reference)
+	}
+	return s.has(ctx, s.url(repository, kind, digest), anyManifest...)
+}

internal/builder/builder.go

@@ -77,6 +77,11 @@ type Result struct {
 	// the default, and that branch is its trunk.
 	Branches []string
 
+	// Mirrored is every base this build copied into the artifact store, as pinned (novox/hq ADR 0257):
+	// said whether the build worked or not, because the copy is in the store either way, and the
+	// records are what decide whether it stays.
+	Mirrored []string
+
 	// Source is the build's source fingerprint (source.go): what it was made from — the module's tree,
 	// the contexts' trees, the bases and toolchains by digest — hashed. Empty where the source does not
 	// pin the build. Two builds with one fingerprint are one build, whatever digests they made
@@ -106,6 +111,19 @@ type GitCredential struct {
 func Build(ctx context.Context, run Runner, publish Publisher,
 	repository, path, ref, workspace string, held map[string]string, npmrc Npmrc,
 	forge GitCredential, log Log, seats ...map[string]string) (Result, error) {
+	// **What was copied is said whether the build worked or not** (novox/hq ADR 0257): a base is in
+	// the store from the moment it is copied, and a build that failed after copying it is the
+	// one record that it is there.
+	var mirrored []string
+	result, err := build(ctx, run, publish, repository, path, ref, workspace, held, npmrc, forge, log,
+		&mirrored, seats...)
+	result.Mirrored = mirrored
+	return result, err
+}
+
+func build(ctx context.Context, run Runner, publish Publisher,
+	repository, path, ref, workspace string, held map[string]string, npmrc Npmrc,
+	forge GitCredential, log Log, mirrored *[]string, seats ...map[string]string) (Result, error) {
 	// The clone base of each seat a context may name (novox/hq ADR 0155); variadic so the callers
 	// that hand none — tests of everything but contexts — read as they did.
 	var seatBases map[string]string
@@ -222,10 +240,22 @@ func Build(ctx context.Context, run Runner, publish Publisher,
 		// An image published elsewhere that the build stands on is copied into the mesh's own
 		// registry first, like an upstream artifact (ADR 0096), and the recipe is handed the copy.
 		// Genesis has nowhere to copy to and pulls it into this machine's store instead.
-		mirror := func(ctx context.Context, from, repository string) (string, error) {
+		mirror := func(ctx context.Context, from, repository, former string) (string, error) {
+			if m, can := publish.(BaseMirrorer); can {
+				say("bases", "copying %s into the mesh's registry as %s", from, repository)
+				reference, err := m.MirrorBase(ctx, from, repository, former)
+				if err == nil {
+					*mirrored = append(*mirrored, reference)
+				}
+				return reference, err
+			}
 			if m, can := publish.(Mirrorer); can {
-				say("bases", "copying %s into the mesh's registry", from)
+				say("bases", "copying %s into the mesh's registry as %s", from, repository)
-				return m.MirrorImage(ctx, from, repository)
+				reference, err := m.MirrorImage(ctx, from, repository)
+				if err == nil {
+					*mirrored = append(*mirrored, reference)
+				}
+				return reference, err
 			}
 			if _, err := run(ctx, tree, "docker", "pull", from); err != nil {
 				return "", fmt.Errorf("cannot fetch %s: %w", from, err)
@@ -901,7 +931,7 @@ var _ io.Writer = (*stringWriter)(nil)
 // The order is fixed so two builds of one commit invoke the same command. Returned alongside the
 // arguments is every reference they resolved to, which is what the build stood on.
 func standingOn(ctx context.Context, manifest catalogue.Manifest, held map[string]string,
-	mirror func(ctx context.Context, from, repository string) (string, error)) ([]string, []string, error) {
+	mirror func(ctx context.Context, from, repository, former string) (string, error)) ([]string, []string, error) {
 	if manifest.Build == nil || len(manifest.Build.On) == 0 {
 		return nil, nil, nil
 	}
@@ -924,7 +954,14 @@ func standingOn(ctx context.Context, manifest catalogue.Manifest, held map[strin
 					"%s stands on the image %q, which is not pinned by digest. A tag is what "+
 						"somebody else can move; name it as <image>@sha256:…", manifest.Module, base.Image)
 			}
-			reference, err := mirror(ctx, base.Image, manifest.Module+"/on-"+strings.ToLower(base.Arg))
+			// **One copy per upstream image, whichever modules stand on it** (novox/hq ADR 0257). Its
+			// repository is named for the image, not the module; the module's own repository of
+			// before is offered as a source, so the move to one copy asks upstream for nothing.
+			repository, err := MirrorRepository(base.Image)
+			if err != nil {
+				return nil, nil, fmt.Errorf("%s stands on %s: %w", manifest.Module, base.Image, err)
+			}
+			reference, err := mirror(ctx, base.Image, repository, FormerMirrorRepository(manifest.Module, base.Arg))
 			if err != nil {
 				return nil, nil, fmt.Errorf("%s stands on %s: %w", manifest.Module, base.Image, err)
 			}

internal/builder/mirror.go

@@ -27,6 +27,40 @@ type Mirrorer interface {
 	MirrorImage(ctx context.Context, from, repository string) (string, error)
 }
 
+// BaseMirrorer copies a base a build stands on into the one repository the mesh keeps for that
+// upstream image, taking what an earlier copy under `former` already holds rather than asking
+// upstream for it again (novox/hq ADR 0257).
+type BaseMirrorer interface {
+	MirrorBase(ctx context.Context, from, repository, former string) (string, error)
+}
+
+// MirrorPrefix is the namespace of the repositories a base is mirrored into: `upstream/<host>/<path>`,
+// one repository per upstream image, whichever modules stand on it (novox/hq ADR 0257).
+const MirrorPrefix = "upstream/"
+
+// MirrorRepository is the repository the mesh keeps an upstream image's copy in: the image's own
+// host and path under MirrorPrefix, so two modules standing on one image stand on one copy, and two
+// images that share a path on different hosts are never confused. Lower case and without a port's
+// colon, because a repository name allows neither.
+func MirrorRepository(from string) (string, error) {
+	where, err := parseReference(from)
+	if err != nil {
+		return "", err
+	}
+	host := strings.TrimPrefix(strings.TrimPrefix(where.base, "https://"), "http://")
+	if host == "registry-1.docker.io" {
+		host = "docker.io"
+	}
+	host = strings.ReplaceAll(host, ":", "-")
+	return strings.ToLower(MirrorPrefix + host + "/" + where.repository), nil
+}
+
+// FormerMirrorRepository is where a module's base was copied before ADR 0257: under the module's own
+// repository, one copy per module (ADR 0097). Read only, as a source of what is already held.
+func FormerMirrorRepository(module, arg string) string {
+	return module + "/on-" + strings.ToLower(arg)
+}
+
 const (
 	mediaIndexOCI       = "application/vnd.oci.image.index.v1+json"
 	mediaIndexDocker    = "application/vnd.docker.distribution.manifest.list.v2+json"
@@ -86,6 +120,9 @@ func parseReference(ref string) (upstream, error) {
 type source struct {
 	client *http.Client
 	token  string
+	// mountFrom is a repository of the mesh's own registry the source is, so a blob is mounted from
+	// it rather than read and written again.
+	mountFrom string
 }
 
 // get fetches a registry URL, answering a bearer challenge once with an anonymous token — which is
@@ -176,6 +213,15 @@ type descriptor struct {
 // under `repository`, and returns the reference the mesh will pin: this registry, the repository,
 // and the digest of the document that was put last, which is the index where there is one.
 func (r Registry) MirrorImage(ctx context.Context, from, repository string) (string, error) {
+	return r.MirrorBase(ctx, from, repository, "")
+}
+
+// MirrorBase is MirrorImage, and where `former` — a repository of this registry — already holds the
+// image by its digest, the copy is made from there: manifests read from this registry, blobs mounted
+// across rather than moved (novox/hq ADR 0257). Upstream is asked only for what no repository here
+// holds, which is what kept the public hub's anonymous pull limit out of reach when every module's
+// base moved into one shared repository.
+func (r Registry) MirrorBase(ctx context.Context, from, repository, former string) (string, error) {
 	where, err := parseReference(from)
 	if err != nil {
 		return "", err
@@ -186,7 +232,11 @@ func (r Registry) MirrorImage(ctx context.Context, from, repository string) (str
 	// on the first merge that rebuilt a whole catalogue (2026-09-28), and every module whose base
 	// lives there failed on a copy it did not need.
 	if strings.HasPrefix(where.reference, "sha256:") {
-		held, err := r.has(ctx, "http://"+r.Address+"/v2/"+repository+"/manifests/"+where.reference, manifestAccept)
+		// **Held whole, not only its index** (novox/hq ADR 0257): every platform manifest the index
+		// names is asked about too. A sweep that stopped half-way, or that ran while this build was
+		// copying, may have left an index naming a platform the store no longer holds; such a copy
+		// is made again, and the copy puts back what is missing.
+		held, err := r.holdsWhole(ctx, repository, where.reference)
 		if err != nil {
 			return "", fmt.Errorf("asking %s whether it holds %s: %w", r.Address, from, err)
 		}
@@ -195,6 +245,17 @@ func (r Registry) MirrorImage(ctx context.Context, from, repository string) (str
 		}
 	}
 	src := &source{client: r.client()}
+	if former != "" && former != repository && strings.HasPrefix(where.reference, "sha256:") {
+		held, err := r.holdsWhole(ctx, former, where.reference)
+		if err != nil {
+			return "", fmt.Errorf("asking %s whether %s holds %s: %w", r.Address, former, from, err)
+		}
+		if held {
+			// The same bytes, already here: copied from this registry, each blob mounted.
+			where = upstream{base: "http://" + r.Address, repository: former, reference: where.reference}
+			src.mountFrom = former
+		}
+	}
 	digest, err := r.copyManifest(ctx, src, where, where.reference, repository)
 	if err != nil {
 		return "", fmt.Errorf("copying %s into %s/%s: %w", from, r.Address, repository, err)
@@ -202,6 +263,42 @@ func (r Registry) MirrorImage(ctx context.Context, from, repository string) (str
 	return r.Address + "/" + repository + "@" + digest, nil
 }
 
+// holdsWhole is whether this registry holds the manifest under the repository and, for an index, every
+// manifest it names.
+func (r Registry) holdsWhole(ctx context.Context, repository, digest string) (bool, error) {
+	url := "http://" + r.Address + "/v2/" + repository + "/manifests/"
+	held, err := r.has(ctx, url+digest, manifestAccept)
+	if err != nil || !held {
+		return false, err
+	}
+	request, err := http.NewRequestWithContext(ctx, http.MethodGet, url+digest, nil)
+	if err != nil {
+		return false, err
+	}
+	request.Header.Set("Accept", manifestAccept)
+	response, err := r.client().Do(request)
+	if err != nil {
+		return false, fmt.Errorf("cannot reach the registry at %s: %w", r.Address, err)
+	}
+	defer response.Body.Close()
+	if response.StatusCode != http.StatusOK {
+		return false, nil
+	}
+	var document struct {
+		Manifests []descriptor `json:"manifests"`
+	}
+	if err := json.NewDecoder(io.LimitReader(response.Body, 4<<20)).Decode(&document); err != nil {
+		return false, fmt.Errorf("%s@%s is not a manifest: %w", repository, digest, err)
+	}
+	for _, m := range document.Manifests {
+		held, err := r.has(ctx, url+m.Digest, manifestAccept)
+		if err != nil || !held {
+			return false, err
+		}
+	}
+	return true, nil
+}
+
 // copyManifest copies one manifest document and everything it names, and returns its digest. An
 // index is copied by copying each manifest it names first, so the index never points at something
 // the registry does not hold yet.
@@ -286,16 +383,14 @@ func (r Registry) copyBlob(ctx context.Context, src *source, where upstream, dig
 	} else if there {
 		return nil
 	}
-	response, err := src.get(ctx, where.base+"/v2/"+where.repository+"/blobs/"+digest, "")
+	// **Mounted where this registry already holds it** (novox/hq ADR 0257): a blob is stored once
-	if err != nil {
+	// whichever repositories link it, so a mount moves no bytes. A registry that cannot mount answers
-		return err
+	// with an ordinary upload's location, and the blob is moved as before.
+	uploads := base + "/blobs/uploads/"
+	if src.mountFrom != "" {
+		uploads += "?mount=" + digest + "&from=" + src.mountFrom
 	}
-	defer response.Body.Close()
+	start, err := http.NewRequestWithContext(ctx, http.MethodPost, uploads, nil)
-	if response.StatusCode != http.StatusOK {
-		return fmt.Errorf("%s/%s: blob %s: %s", where.base, where.repository, digest, response.Status)
-	}
-
-	start, err := http.NewRequestWithContext(ctx, http.MethodPost, base+"/blobs/uploads/", nil)
 	if err != nil {
 		return err
 	}
@@ -304,9 +399,21 @@ func (r Registry) copyBlob(ctx context.Context, src *source, where upstream, dig
 		return fmt.Errorf("cannot start an upload to %s: %w", base, err)
 	}
 	begun.Body.Close()
+	if begun.StatusCode == http.StatusCreated && src.mountFrom != "" {
+		return nil
+	}
 	if begun.StatusCode != http.StatusAccepted {
 		return fmt.Errorf("%s answered %s when asked where to put a blob", base, begun.Status)
 	}
+
+	response, err := src.get(ctx, where.base+"/v2/"+where.repository+"/blobs/"+digest, "")
+	if err != nil {
+		return err
+	}
+	defer response.Body.Close()
+	if response.StatusCode != http.StatusOK {
+		return fmt.Errorf("%s/%s: blob %s: %s", where.base, where.repository, digest, response.Status)
+	}
 	location := begun.Header.Get("Location")
 	if location == "" {
 		return fmt.Errorf("%s accepted an upload and said nowhere to put it", base)

internal/builder/mirror_shared_test.go

@@ -0,0 +1,375 @@
+package builder
+
+import (
+	"context"
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"sync"
+	"testing"
+)
+
+// One copy per upstream image, whichever modules stand on it (novox/hq ADR 0257).
+
+func TestAnUpstreamImageHasOneRepositoryNamedForIt(t *testing.T) {
+	for from, want := range map[string]string{
+		"golang@sha256:abc":                       "upstream/docker.io/library/golang",
+		"n8nio/n8n@sha256:abc":                    "upstream/docker.io/n8nio/n8n",
+		"quay.io/minio/mc@sha256:abc":             "upstream/quay.io/minio/mc",
+		"ghcr.io/Mailu/Admin@sha256:abc":          "upstream/ghcr.io/mailu/admin",
+		"localhost:5000/x/y@sha256:abc":           "upstream/localhost-5000/x/y",
+		"docker.io/library/alpine:3.20@sha256:ab": "upstream/docker.io/library/alpine",
+	} {
+		got, err := MirrorRepository(from)
+		if err != nil {
+			t.Fatalf("%s: %v", from, err)
+		}
+		if got != want {
+			t.Errorf("%s: got %q want %q", from, got, want)
+		}
+	}
+	if got := FormerMirrorRepository("route-proxy", "GO_BASE"); got != "route-proxy/on-go_base" {
+		t.Fatalf("the former repository is %q", got)
+	}
+}
+
+// aRegistryOfRepositories is a registry the way the real one is: blobs stored once, and each
+// repository linking the blobs and manifests it holds. It mounts a blob across repositories.
+type aRegistryOfRepositories struct {
+	mu        sync.Mutex
+	blobs     map[string][]byte          // digest → bytes, stored once
+	links     map[string]map[string]bool // repository → blob digests it links
+	manifests map[string][]byte          // repository@digest → document
+	uploads   int
+	mounts    int
+	gets      int
+	// noMount answers every mount with an ordinary upload's location, as a registry that cannot
+	// mount across repositories does.
+	noMount bool
+}
+
+func newRegistryOfRepositories() *aRegistryOfRepositories {
+	return &aRegistryOfRepositories{blobs: map[string][]byte{}, links: map[string]map[string]bool{},
+		manifests: map[string][]byte{}}
+}
+
+func (m *aRegistryOfRepositories) link(repository, digest string) {
+	if m.links[repository] == nil {
+		m.links[repository] = map[string]bool{}
+	}
+	m.links[repository][digest] = true
+}
+
+// split reads /v2/<repository>/<kind>/<rest> with a repository of any depth.
+func splitPath(path string) (repository, kind, rest string) {
+	path = strings.TrimPrefix(path, "/v2/")
+	for _, k := range []string{"/manifests/", "/blobs/uploads/", "/blobs/"} {
+		if i := strings.Index(path, k); i >= 0 {
+			return path[:i], strings.Trim(k, "/"), path[i+len(k):]
+		}
+	}
+	return "", "", ""
+}
+
+func (m *aRegistryOfRepositories) handler() http.Handler {
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		m.mu.Lock()
+		defer m.mu.Unlock()
+		repository, kind, rest := splitPath(r.URL.Path)
+		switch {
+		case kind == "manifests" && (r.Method == http.MethodHead || r.Method == http.MethodGet):
+			body, ok := m.manifests[repository+"@"+rest]
+			if !ok {
+				w.WriteHeader(http.StatusNotFound)
+				return
+			}
+			if r.Method == http.MethodGet {
+				m.gets++
+				w.Header().Set("Content-Type", mediaTypeOf(body))
+				_, _ = w.Write(body)
+				return
+			}
+			w.WriteHeader(http.StatusOK)
+		case kind == "manifests" && r.Method == http.MethodPut:
+			body, _ := readAll(r)
+			m.manifests[repository+"@"+digestOf(body)] = body
+			w.WriteHeader(http.StatusCreated)
+		case kind == "blobs" && (r.Method == http.MethodHead || r.Method == http.MethodGet):
+			if !m.links[repository][rest] {
+				w.WriteHeader(http.StatusNotFound)
+				return
+			}
+			if r.Method == http.MethodGet {
+				m.gets++
+				_, _ = w.Write(m.blobs[rest])
+				return
+			}
+			w.WriteHeader(http.StatusOK)
+		case kind == "manifests" && r.Method == http.MethodDelete:
+			delete(m.manifests, repository+"@"+rest)
+			w.WriteHeader(http.StatusAccepted)
+		case kind == "blobs/uploads" && r.Method == http.MethodPost:
+			if digest, from := r.URL.Query().Get("mount"), r.URL.Query().Get("from"); digest != "" && m.links[from][digest] && !m.noMount {
+				m.link(repository, digest)
+				m.mounts++
+				w.WriteHeader(http.StatusCreated)
+				return
+			}
+			w.Header().Set("Location", "/v2/"+repository+"/blobs/uploads/one")
+			w.WriteHeader(http.StatusAccepted)
+		case kind == "blobs/uploads" && r.Method == http.MethodPut:
+			body, _ := readAll(r)
+			digest := r.URL.Query().Get("digest")
+			if digestOf(body) != digest {
+				w.WriteHeader(http.StatusBadRequest)
+				return
+			}
+			m.blobs[digest] = body
+			m.link(repository, digest)
+			m.uploads++
+			w.WriteHeader(http.StatusCreated)
+		default:
+			w.WriteHeader(http.StatusNotFound)
+		}
+	})
+}
+
+func mediaTypeOf(body []byte) string {
+	switch {
+	case strings.Contains(string(body), mediaIndexOCI):
+		return mediaIndexOCI
+	default:
+		return mediaManifestOCI
+	}
+}
+
+// A base a module's own repository already holds is copied into the image's repository from there:
+// every blob mounted, nothing asked of upstream — which may be gone, or rationing anonymous pulls.
+func TestABaseHeldUnderTheModulesFormerRepositoryIsMountedNotFetchedAgain(t *testing.T) {
+	src, indexDigest, srcBlobs := anUpstreamRegistry(t)
+	dst := newRegistryOfRepositories()
+	dstServer := httptest.NewServer(dst.handler())
+	defer dstServer.Close()
+	address := strings.TrimPrefix(dstServer.URL, "http://")
+	r := Registry{Address: address, HTTP: src.Client()}
+	host := strings.TrimPrefix(src.URL, "http://")
+
+	// Before: copied the old way, under the module's repository.
+	if _, err := r.MirrorImage(context.Background(), host+"/library/thing:latest", "hello-web/on-thing_base"); err != nil {
+		t.Fatal(err)
+	}
+	uploaded := dst.uploads
+	if uploaded != len(srcBlobs) {
+		t.Fatalf("the first copy uploaded %d of %d blobs", uploaded, len(srcBlobs))
+	}
+	src.Close()
+
+	from := host + "/library/thing@" + indexDigest
+	repository, err := MirrorRepository(from)
+	if err != nil {
+		t.Fatal(err)
+	}
+	reference, err := r.MirrorBase(context.Background(), from, repository, "hello-web/on-thing_base")
+	if err != nil {
+		t.Fatalf("a base the registry holds was asked of an upstream that is gone: %v", err)
+	}
+	if reference != address+"/"+repository+"@"+indexDigest {
+		t.Fatalf("pinned as %q", reference)
+	}
+	if dst.uploads != uploaded {
+		t.Fatalf("the move to one repository uploaded %d blobs again", dst.uploads-uploaded)
+	}
+	if dst.mounts != len(srcBlobs) {
+		t.Fatalf("%d of %d blobs mounted", dst.mounts, len(srcBlobs))
+	}
+	// The index and both platform manifests are in the image's repository, and every blob is linked.
+	for key := range dst.manifests {
+		if strings.HasPrefix(key, "hello-web/") {
+			continue
+		}
+		if !strings.HasPrefix(key, repository+"@") {
+			t.Fatalf("a manifest went to %s", key)
+		}
+	}
+	if n := countPrefix(dst.manifests, repository+"@"); n != 3 {
+		t.Fatalf("%d manifests in %s, want the index and two platforms", n, repository)
+	}
+	for digest := range srcBlobs {
+		if !dst.links[repository][digest] {
+			t.Fatalf("blob %s is not linked into %s", digest, repository)
+		}
+	}
+
+	// A second module standing on the same image: already held, nothing copied, the same reference.
+	again, err := r.MirrorBase(context.Background(), from, repository, "other-module/on-thing_base")
+	if err != nil || again != reference {
+		t.Fatalf("a second module's copy: %q %v", again, err)
+	}
+}
+
+func countPrefix(m map[string][]byte, prefix string) int {
+	n := 0
+	for k := range m {
+		if strings.HasPrefix(k, prefix) {
+			n++
+		}
+	}
+	return n
+}
+
+// What a build copied is said whether it worked or not: the copy is in the store either way, and a
+// build that failed after copying is the one record that it is there.
+func TestABuildSaysWhatItMirroredEvenWhenItFails(t *testing.T) {
+	// A recipe that reaches for an image nobody declared is refused — after the base was copied.
+	r, workspace := aRepository(t, anImage, map[string]string{
+		"modules/bus/Dockerfile": "ARG BASE\nFROM ${BASE}\nCOPY --from=vendor/other:1 /x /x"})
+	r.contents["modules/bus/module.json"] = anImage
+	r.tree = "aaaa"
+	m := &mirroring{recorded: r, at: "registry-a:5000"}
+	got, err := Build(context.Background(), r.run, m, "https://forge.invalid/catalogue.git", "modules/bus", "", workspace,
+		nil, Npmrc{}, GitCredential{}, nil)
+	if err == nil {
+		t.Fatal("a recipe fetching an undeclared image was built")
+	}
+	want := "registry-a:5000/upstream/docker.io/vendor/server@sha256:" + strings.Repeat("1", 64)
+	if len(got.Mirrored) != 1 || got.Mirrored[0] != want {
+		t.Fatalf("a failed build said it mirrored %v, want [%s]", got.Mirrored, want)
+	}
+
+	// And a build that works says it too.
+	r2, workspace2 := aRepository(t, anImage, map[string]string{"modules/bus/Dockerfile": "ARG BASE\nFROM ${BASE}"})
+	r2.contents["modules/bus/module.json"] = anImage
+	r2.tree = "aaaa"
+	ok, err := Build(context.Background(), r2.run, &mirroring{recorded: r2, at: "registry-a:5000"},
+		"https://forge.invalid/catalogue.git", "modules/bus", "", workspace2, nil, Npmrc{}, GitCredential{}, nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(ok.Mirrored) != 1 || ok.Mirrored[0] != want {
+		t.Fatalf("a build said it mirrored %v, want [%s]", ok.Mirrored, want)
+	}
+}
+
+// held puts one image in a repository the old way and answers its index digest and the registry.
+func heldUnderAModule(t *testing.T) (*aRegistryOfRepositories, Registry, string, string, map[string][]byte, func()) {
+	t.Helper()
+	src, indexDigest, srcBlobs := anUpstreamRegistry(t)
+	t.Cleanup(src.Close)
+	dst := newRegistryOfRepositories()
+	dstServer := httptest.NewServer(dst.handler())
+	t.Cleanup(dstServer.Close)
+	r := Registry{Address: strings.TrimPrefix(dstServer.URL, "http://"), HTTP: src.Client()}
+	host := strings.TrimPrefix(src.URL, "http://")
+	if _, err := r.MirrorImage(context.Background(), host+"/library/thing:latest", "hello-web/on-thing_base"); err != nil {
+		t.Fatal(err)
+	}
+	return dst, r, host + "/library/thing@" + indexDigest, indexDigest, srcBlobs, src.Close
+}
+
+// An index whose platform manifests are not all held is not "already held": it is copied again, and the
+// copy puts back what is missing — what a sweep that stopped half-way, or ran while a build was copying,
+// leaves behind.
+func TestAnIndexMissingAPlatformIsCopiedAgain(t *testing.T) {
+	dst, r, from, indexDigest, _, _ := heldUnderAModule(t)
+	repository, _ := MirrorRepository(from)
+	if _, err := r.MirrorBase(context.Background(), from, repository, ""); err != nil {
+		t.Fatal(err)
+	}
+	// A sweep lets one platform go, between this build's copy and the next.
+	var platform string
+	for key := range dst.manifests {
+		if strings.HasPrefix(key, repository+"@") && !strings.HasSuffix(key, indexDigest) {
+			platform = key
+			break
+		}
+	}
+	delete(dst.manifests, platform)
+	if _, err := r.MirrorBase(context.Background(), from, repository, ""); err != nil {
+		t.Fatal(err)
+	}
+	if _, back := dst.manifests[platform]; !back {
+		t.Fatalf("an index missing %s was taken as held", platform)
+	}
+}
+
+// The same, for the module's former copy: a former copy missing a platform is not a source; upstream
+// is asked instead.
+func TestAFormerCopyMissingAPlatformIsNotTheSource(t *testing.T) {
+	dst, r, from, indexDigest, _, _ := heldUnderAModule(t)
+	for key := range dst.manifests {
+		if strings.HasPrefix(key, "hello-web/on-thing_base@") && !strings.HasSuffix(key, indexDigest) {
+			delete(dst.manifests, key)
+			break
+		}
+	}
+	repository, _ := MirrorRepository(from)
+	if _, err := r.MirrorBase(context.Background(), from, repository, "hello-web/on-thing_base"); err != nil {
+		t.Fatal(err)
+	}
+	if dst.mounts != 0 {
+		t.Fatalf("a former copy missing a platform was mounted from (%d mounts)", dst.mounts)
+	}
+	if n := countPrefix(dst.manifests, repository+"@"); n != 3 {
+		t.Fatalf("%d manifests copied from upstream, want 3", n)
+	}
+}
+
+// A registry that answers a mount with an upload's location (202) gets the blob moved as before.
+func TestAMountRefusedFallsBackToAnUpload(t *testing.T) {
+	dst, r, from, _, srcBlobs, _ := heldUnderAModule(t)
+	dst.noMount = true
+	uploaded := dst.uploads
+	repository, _ := MirrorRepository(from)
+	reference, err := r.MirrorBase(context.Background(), from, repository, "hello-web/on-thing_base")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !strings.HasSuffix(reference, repository+"@"+strings.SplitN(from, "@", 2)[1]) {
+		t.Fatalf("pinned as %s", reference)
+	}
+	if dst.mounts != 0 || dst.uploads-uploaded != len(srcBlobs) {
+		t.Fatalf("%d mounts, %d uploads; want every blob uploaded from the former copy", dst.mounts, dst.uploads-uploaded)
+	}
+	for digest := range srcBlobs {
+		if !dst.links[repository][digest] {
+			t.Fatalf("blob %s is not linked into %s", digest, repository)
+		}
+	}
+}
+
+// A sweep and a build at once: while builds copy the base, platforms are let go of under them. Each
+// build that returns has the copy whole.
+func TestABuildCopyingWhileASweepLetsGoEndsWhole(t *testing.T) {
+	dst, r, from, indexDigest, _, _ := heldUnderAModule(t)
+	repository, _ := MirrorRepository(from)
+	if _, err := r.MirrorBase(context.Background(), from, repository, ""); err != nil {
+		t.Fatal(err)
+	}
+	done := make(chan struct{})
+	go func() {
+		defer close(done)
+		for i := 0; i < 50; i++ {
+			dst.mu.Lock()
+			for key := range dst.manifests {
+				if strings.HasPrefix(key, repository+"@") && !strings.HasSuffix(key, indexDigest) {
+					delete(dst.manifests, key)
+					break
+				}
+			}
+			dst.mu.Unlock()
+		}
+	}()
+	for i := 0; i < 20; i++ {
+		if _, err := r.MirrorBase(context.Background(), from, repository, ""); err != nil {
+			t.Fatal(err)
+		}
+	}
+	<-done
+	// The sweep is over; the next build finds what it left and makes the copy whole.
+	if _, err := r.MirrorBase(context.Background(), from, repository, ""); err != nil {
+		t.Fatal(err)
+	}
+	if n := countPrefix(dst.manifests, repository+"@"); n != 3 {
+		t.Fatalf("after the sweep and the builds, %d manifests in %s, want 3", n, repository)
+	}
+}

internal/builder/mirror_test.go

@@ -117,6 +117,13 @@ func (m *theMeshsRegistry) handler() http.Handler {
 			} else {
 				w.WriteHeader(http.StatusNotFound)
 			}
+		case r.Method == http.MethodGet && strings.Contains(r.URL.Path, "/manifests/"):
+			body, ok := m.manifests[r.URL.Path[strings.LastIndex(r.URL.Path, "/")+1:]]
+			if !ok {
+				w.WriteHeader(http.StatusNotFound)
+				return
+			}
+			_, _ = w.Write(body)
 		case r.Method == http.MethodHead && strings.Contains(r.URL.Path, "/blobs/"):
 			if _, ok := m.blobs[r.URL.Path[strings.LastIndex(r.URL.Path, "/")+1:]]; ok {
 				w.WriteHeader(http.StatusOK)

internal/builder/standing_on_test.go

@@ -72,7 +72,7 @@ func TestAnIncompleteBaseIsRefused(t *testing.T) {
 }
 
 // noMirror is a mirror for tests whose bases are all the mesh's own.
-func noMirror(context.Context, string, string) (string, error) {
+func noMirror(context.Context, string, string, string) (string, error) {
 	return "", fmt.Errorf("nothing to copy in this test")
 }
 
@@ -86,17 +86,20 @@ func TestADeclaredVendorImageIsCopiedInAndHandedToTheRecipe(t *testing.T) {
 		},
 	}
 	var asked []string
-	args, _, err := standingOn(context.Background(), manifest, nil, func(_ context.Context, from, repository string) (string, error) {
+	args, _, err := standingOn(context.Background(), manifest, nil, func(_ context.Context, from, repository, former string) (string, error) {
-		asked = append(asked, from+" -> "+repository)
+		asked = append(asked, from+" -> "+repository+" (from "+former+")")
 		return "127.0.0.1:5000/" + repository + "@sha256:" + strings.Repeat("d", 64), nil
 	})
 	if err != nil {
 		t.Fatal(err)
 	}
-	if len(asked) != 1 || asked[0] != "quay.io/minio/mc@sha256:"+strings.Repeat("c", 64)+" -> minio/on-mc_base" {
+	// One repository per upstream image, named for the image (novox/hq ADR 0257); the module's
-		t.Fatalf("the image was not copied under the module's repository: %v", asked)
+	// repository of before offered as where it may already be held.
+	if len(asked) != 1 || asked[0] != "quay.io/minio/mc@sha256:"+strings.Repeat("c", 64)+
+		" -> upstream/quay.io/minio/mc (from minio/on-mc_base)" {
+		t.Fatalf("the image was not copied into the image's own repository: %v", asked)
 	}
-	if strings.Join(args, " ") != "--build-arg MC_BASE=127.0.0.1:5000/minio/on-mc_base@sha256:"+strings.Repeat("d", 64) {
+	if strings.Join(args, " ") != "--build-arg MC_BASE=127.0.0.1:5000/upstream/quay.io/minio/mc@sha256:"+strings.Repeat("d", 64) {
 		t.Fatalf("the recipe was not handed the copy: %v", args)
 	}
 	// Unpinned, it is refused: a tag is what somebody else can move.

internal/catalogue/verbs.go

@@ -449,7 +449,9 @@ var ControllerVerbs = []Verb{
 	{Name: "collect", Description: "The artifact store's sweep, now: what the mesh made and keeps for no reason " +
 		"is let go of. Without confirm a dry run — every kept archive asked about with HEADs, what would be let go " +
 		"listed, nothing held or deleted. With confirm and why: every kept archive held first, then each eligible " +
-		"artifact let go of, oldest first, and recorded collected — a hand act, recorded with its why. Bounded by " +
+		"artifact let go of, oldest first, and recorded collected — a hand act, recorded with its why. A confirmed " +
+		"collect lets an index go with the platform manifests no kept index names (read first; any read that " +
+		"fails lets nothing go); the sweep after a build lets an index go alone (novox/hq ADR 0257). Bounded by " +
 		"most (500 by default, at most 5000) and 45 seconds; what is left is said. Bytes are reclaimed by the " +
 		"store's nightly collector. Never touches a digest the mesh did not record (novox/hq ADR 0189, ADR 0251).",
 		Input: schema(map[string]string{
@@ -457,6 +459,20 @@ var ControllerVerbs = []Verb{
 			"why":     "with confirm: why — required, and recorded in the hand-act log",
 			"most":    "let go of at most this many (500 by default, at most 5000)",
 		}, nil, "confirm")},
+	{Name: "mirrors", Description: "The bases the mesh copied into its artifact store for builds to stand on, as " +
+		"the records hold them: each with its state — kept because a kept build stood on it, or eligible — and " +
+		"which build or person recorded it. With record: references of copies no record names (a copy made " +
+		"before copies were recorded, or by a build that failed before then) to record as the mesh's, so the " +
+		"sweep keeps or lets go of them like any other; only a repository the mirror writes is taken " +
+		"(upstream/<host>/<path>, or a module's <module>/on-<argument>), and each must be in the store. Without " +
+		"confirm a dry run; with confirm and why, recorded — a hand act. A recorded copy is eligible: the next " +
+		"sweep lets it go unless a kept build stood on it. Records only: this verb deletes nothing " +
+		"(novox/hq ADR 0257).",
+		Input: schema(map[string]string{
+			"record":  "references to record, artifact-store://<repository>@sha256:<hex>, separated by spaces or commas",
+			"confirm": "\"true\": record them (needs why); without it nothing is recorded",
+			"why":     "with confirm: why — required, and recorded with each copy and in the hand-act log",
+		}, nil, "confirm")},
 	{Name: "images", Description: "Every container image one machine's declaration names — the declaration the " +
 		"mesh would send it now and the one it was last sent — each with the resources naming it: what the " +
 		"machine keeps when it prunes its images. sent_known is false when the last one sent is not kept with " +

internal/inventory/builds.go

@@ -39,6 +39,9 @@ type Build struct {
 	// Against is every artifact this build stood on, as references rather than module names —
 	// what makes a build edge derived rather than declared (ADR 0009).
 	Against []string
+	// Mirrored is every base this build copied into the artifact store (novox/hq ADR 0257), said
+	// whether the build worked or not: the copy is the mesh's, recorded in artifact_mirrored.
+	Mirrored []string
 	// Read is every repository this build read source from besides the module's own (novox/hq
 	// 04-ISSUES/131), at the ref it read.
 	Read []ReadRepository
@@ -120,7 +123,41 @@ func (i *Inventory) RecordBuild(ctx context.Context, b Build) error {
 		 on conflict (id) do nothing`,
 		b.ID, b.Repository, b.Ref, module, b.Commit, b.On, b.Failed, made,
 		b.Path, manifestOrNil(b.Manifest), against, read, asked, b.SourceFingerprint)
-	return err
+	if err != nil {
+		return err
+	}
+	return i.RecordMirrored(ctx, b.ID, "", b.Mirrored)
+}
+
+// RecordMirrored keeps that the mesh copied these bases into the artifact store (novox/hq ADR 0257):
+// by the build that said so, or by a person who says why. The first record of a copy stands.
+//
+// **A copy a build says it holds is held again.** The builder answers a copy only once the store holds
+// it whole, copying again what was let go of; so a copy the records say was collected, and a build now
+// says it copied, is no longer collected. Left collected, it would stay in the store and out of every
+// sweep for ever.
+func (i *Inventory) RecordMirrored(ctx context.Context, build, why string, references []string) error {
+	var by *string
+	if build != "" {
+		by = &build
+	}
+	for _, reference := range references {
+		if reference == "" {
+			continue
+		}
+		if _, err := i.store.Pool().Exec(ctx,
+			`insert into artifact_mirrored (reference, build, why) values ($1, $2, $3)
+			 on conflict (reference) do nothing`, reference, by, why); err != nil {
+			return err
+		}
+		if build != "" {
+			if _, err := i.store.Pool().Exec(ctx,
+				`delete from artifact_collected where reference = $1`, reference); err != nil {
+				return err
+			}
+		}
+	}
+	return nil
 }
 
 // Builds is what has happened lately, newest first.

internal/inventory/collection.go

@@ -37,6 +37,10 @@ const (
 	// KeptUnaddressable: it names no digest, so nothing here can speak for it, and it is kept rather
 	// than guessed about.
 	KeptUnaddressable = "unaddressable"
+	// KeptStoodOn: it is a base the mesh copied in, and one of the KeptBuilds most recent successful
+	// builds of a module the mesh still holds stood on it (novox/hq ADR 0257). A copy has no builds of
+	// its own; it is kept by the builds that used it.
+	KeptStoodOn = "stood-on"
 )
 
 // The states an artifact the mesh recorded making is in (novox/hq ADR 0251).
@@ -77,7 +81,7 @@ func (i *Inventory) Artifacts(ctx context.Context) ([]ArtifactState, error) {
 	if err != nil {
 		return nil, err
 	}
-	all, err := i.everyReferenceMade(ctx)
+	all, err := i.everyReferenceRecorded(ctx)
 	if err != nil {
 		return nil, err
 	}
@@ -156,8 +160,9 @@ func (i *Inventory) KeepSet(ctx context.Context) (map[string][]string, error) {
 	// join every module ever built kept five builds' artifacts for ever — and once the store's
 	// collector runs for real, what the keep set says is what the disk holds.
 	recent, err := i.store.Pool().Query(ctx,
-		`select made from (
+		`select made, built_against from (
-		    select b.made, row_number() over (partition by b.module order by b.at desc, b.id desc) as back
+		    select b.made, b.built_against,
+		           row_number() over (partition by b.module order by b.at desc, b.id desc) as back
 		      from build b
 		      join module m on m.name = b.module
 		     where b.failed = '' and b.module is not null and b.module <> ''
@@ -166,18 +171,26 @@ func (i *Inventory) KeepSet(ctx context.Context) (map[string][]string, error) {
 		return nil, err
 	}
 	defer recent.Close()
+	var stoodOn []string
 	for recent.Next() {
-		var raw []byte
+		var raw, against []byte
-		if err := recent.Scan(&raw); err != nil {
+		if err := recent.Scan(&raw, &against); err != nil {
 			return nil, err
 		}
 		var made []Artifact
-		if err := json.Unmarshal(raw, &made); err != nil {
+		if err := json.Unmarshal(raw, &made); err == nil {
-			continue
+			for _, a := range made {
+				if reference := asRecorded(a.Reference); reference != "" {
+					because(reference, KeptByRecentBuild)
+				}
+			}
 		}
-		for _, a := range made {
+		var bases []string
-			if reference := asRecorded(a.Reference); reference != "" {
+		if err := json.Unmarshal(against, &bases); err == nil {
-				because(reference, KeptByRecentBuild)
+			for _, b := range bases {
+				if reference := asRecorded(b); reference != "" {
+					stoodOn = append(stoodOn, reference)
+				}
 			}
 		}
 	}
@@ -185,6 +198,27 @@ func (i *Inventory) KeepSet(ctx context.Context) (map[string][]string, error) {
 		return nil, err
 	}
 
+	// **A copied base is kept by the kept builds that stood on it, and by nothing else** (novox/hq
+	// ADR 0257). It has no builds of its own to count five of, and no machine is ever handed a base,
+	// so a definition is never its reason: a definition names the image upstream, by the digest every
+	// copy of it shares, and read as a reason it would keep every module's copy for as long as any
+	// module stood on that image. A copy already let go of stays let go of.
+	mirrored, err := i.mirroredSet(ctx)
+	if err != nil {
+		return nil, err
+	}
+	if len(mirrored) > 0 {
+		collected, err := i.alreadyCollected(ctx)
+		if err != nil {
+			return nil, err
+		}
+		for _, reference := range stoodOn {
+			if mirrored[reference] && !collected[reference] {
+				because(reference, KeptStoodOn)
+			}
+		}
+	}
+
 	// And anything a manifest mentions — every reference ever made, the recent ones included, so a
 	// kept artifact carries each of its reasons and the operator reads why, not only that (novox/hq
 	// ADR 0251). A manifest holds a reference composed with the store's address or kept bare, so the
@@ -195,6 +229,9 @@ func (i *Inventory) KeepSet(ctx context.Context) (map[string][]string, error) {
 			return nil, err
 		}
 		for _, reference := range all {
+			if mirrored[reference] {
+				continue
+			}
 			digest := digestIn(reference)
 			if digest == "" {
 				// Not something the store holds by digest; nothing here can speak for it, so it
@@ -237,6 +274,72 @@ func (i *Inventory) KeptArchives(ctx context.Context) ([]string, error) {
 	return out, nil
 }
 
+// KeptImages is every image the mesh keeps, in a stated order: the manifests whose platform manifests
+// a sweep spares when it lets an index of the same repository go (novox/hq ADR 0257).
+func (i *Inventory) KeptImages(ctx context.Context) ([]string, error) {
+	keep, err := i.KeepSet(ctx)
+	if err != nil {
+		return nil, err
+	}
+	var out []string
+	for reference := range keep {
+		if path, ours := catalogue.InArtifactStore(reference); ours && strings.Contains(path, "@sha256:") {
+			out = append(out, reference)
+		}
+	}
+	sort.Strings(out)
+	return out, nil
+}
+
+// everyReferenceRecorded is every artifact the mesh recorded putting in its store: what each
+// successful build made, and every base a build — or a person — recorded copying in (novox/hq ADR
+// 0257). Oldest first, a copy placed at the moment it was recorded.
+func (i *Inventory) everyReferenceRecorded(ctx context.Context) ([]string, error) {
+	made, err := i.everyReferenceMade(ctx)
+	if err != nil {
+		return nil, err
+	}
+	rows, err := i.store.Pool().Query(ctx, `select reference from artifact_mirrored order by at asc, reference asc`)
+	if err != nil {
+		return nil, err
+	}
+	defer rows.Close()
+	seen := map[string]bool{}
+	for _, r := range made {
+		seen[r] = true
+	}
+	out := made
+	for rows.Next() {
+		var reference string
+		if err := rows.Scan(&reference); err != nil {
+			return nil, err
+		}
+		if reference = asRecorded(reference); reference != "" && !seen[reference] {
+			seen[reference] = true
+			out = append(out, reference)
+		}
+	}
+	return out, rows.Err()
+}
+
+// mirroredSet is every base the records say the mesh copied in, as recorded.
+func (i *Inventory) mirroredSet(ctx context.Context) (map[string]bool, error) {
+	rows, err := i.store.Pool().Query(ctx, `select reference from artifact_mirrored`)
+	if err != nil {
+		return nil, err
+	}
+	defer rows.Close()
+	out := map[string]bool{}
+	for rows.Next() {
+		var reference string
+		if err := rows.Scan(&reference); err != nil {
+			return nil, err
+		}
+		out[asRecorded(reference)] = true
+	}
+	return out, rows.Err()
+}
+
 // everyReferenceMade is every artifact reference any successful build recorded, oldest build first.
 func (i *Inventory) everyReferenceMade(ctx context.Context) ([]string, error) {
 	rows, err := i.store.Pool().Query(ctx,
@@ -331,3 +434,8 @@ func (i *Inventory) MarkCollected(ctx context.Context, references []string) erro
 	}
 	return nil
 }
+
+// Mirrored is every base the records say the mesh copied into its store (novox/hq ADR 0257).
+func (i *Inventory) Mirrored(ctx context.Context) (map[string]bool, error) {
+	return i.mirroredSet(ctx)
+}

internal/inventory/dependencies.go

@@ -115,6 +115,11 @@ func dependenciesOf(entries []Entry, against map[string][]string, read map[strin
 		}
 		for _, ref := range against[name] {
 			if rest, ok := strings.CutPrefix(ref, catalogue.ArtifactStoreScheme); ok {
+				// A copy of an upstream image is no module's artifact (novox/hq ADR 0257): its
+				// repository is named for the image, and its first segment names no module.
+				if strings.HasPrefix(rest, builder.MirrorPrefix) {
+					continue
+				}
 				if base, _, found := strings.Cut(rest, "/"); found {
 					add(name, base, EdgeStandsOn)
 				}

internal/inventory/dependencies_test.go

@@ -154,3 +154,20 @@ func TestNothingStandsOnTheBuildAgentOnlyIsBuiltByIt(t *testing.T) {
 		t.Errorf("everything source-built is built by the agent: %d edges", builtBy)
 	}
 }
+
+// A copy of an upstream image is no module's artifact (novox/hq ADR 0257), even when a module happens to
+// carry the name its repository starts with.
+func TestACopyOfAnUpstreamImageIsNoEdge(t *testing.T) {
+	entries := []Entry{
+		{Manifest: catalogue.Manifest{Module: "route-proxy"}},
+		{Manifest: catalogue.Manifest{Module: "upstream"}},
+	}
+	against := map[string][]string{
+		"route-proxy": {catalogue.ArtifactStoreScheme + "upstream/docker.io/library/golang@sha256:a"},
+	}
+	for _, e := range dependenciesOf(entries, against, nil) {
+		if e.From == "route-proxy" && e.To == "upstream" {
+			t.Fatalf("a copy of an upstream image was read as a module's artifact: %+v", e)
+		}
+	}
+}

internal/inventory/migrations/0081-a-mirrored-base-is-recorded.sql

@@ -0,0 +1,30 @@
+-- A base a build copies into the artifact store is recorded where it is copied (novox/hq ADR 0257).
+--
+-- A build that stands on an image published elsewhere copies it into the store first (ADR 0096, 0097),
+-- and until now nothing recorded the copy as something the mesh made: the build named it only in what
+-- it stood on, and a build that failed after copying named it nowhere. So on 2026-10-08 the store held
+-- 374 manifests of copied bases, in 27 repositories, that the sweep could never let go of, because no
+-- record said the mesh had put them there (ADR 0189 §3).
+--
+-- One row per copy, by the reference as the mesh records it. `build` is the build that first said it
+-- copied it, whether that build worked or not; `why` is a person's words when the copy was recorded by
+-- hand (the `mirrors` verb), and empty otherwise. Not a foreign key to build: a build's row may be
+-- written after its copy is, and a copy recorded by hand has no build.
+create table artifact_mirrored (
+    reference text        primary key,
+    build     text,
+    at        timestamptz not null default now(),
+    why       text        not null default ''
+);
+
+-- The copies the records already name. A build that worked kept what it stood on, and every reference
+-- into a module's own `on-<argument>` repository there is a copy that build's mirror made: only the
+-- mirror ever wrote a repository so named (ADR 0097). The earliest build to name each is its maker.
+insert into artifact_mirrored (reference, build, at)
+select distinct on (stood_on) stood_on, b.id, b.at
+  from build b,
+       jsonb_array_elements_text(case when jsonb_typeof(b.built_against) = 'array'
+                                      then b.built_against else '[]'::jsonb end) as stood_on
+ where stood_on ~ '^artifact-store://[a-z0-9][a-z0-9._-]*/on-[a-z0-9_]+@sha256:[0-9a-f]{64}$'
+ order by stood_on, b.at asc, b.id asc
+on conflict (reference) do nothing;

internal/inventory/mirrored_test.go

@@ -0,0 +1,211 @@
+package inventory
+
+import (
+	"context"
+	"fmt"
+	"os"
+	"slices"
+	"strings"
+	"testing"
+
+	"github.com/novox/mesh-controller/internal/catalogue"
+)
+
+// A base a build copies into the store is recorded where it is copied, and kept while a kept build stood
+// on it (novox/hq ADR 0257).
+
+func mirrorRef(repository string, n int) string {
+	return fmt.Sprintf("%s%s@sha256:%064x", catalogue.ArtifactStoreScheme, repository, n)
+}
+
+// builtOn records a successful build of a module that made one image and stood on these bases.
+func builtOn(t *testing.T, inv *Inventory, id, module string, n int, mirrored []string, against ...string) {
+	t.Helper()
+	b := aBuild(id, module, "")
+	b.Made = []Artifact{{Name: "app", Kind: "image", Reference: ref(module, "app", n)}}
+	b.Against = against
+	b.Mirrored = mirrored
+	if err := inv.RecordBuild(context.Background(), b); err != nil {
+		t.Fatal(err)
+	}
+}
+
+func stateOf(t *testing.T, inv *Inventory, reference string) ArtifactState {
+	t.Helper()
+	states, err := inv.Artifacts(context.Background())
+	if err != nil {
+		t.Fatal(err)
+	}
+	for _, s := range states {
+		if s.Reference == reference {
+			return s
+		}
+	}
+	return ArtifactState{Reference: reference, State: "unrecorded"}
+}
+
+func TestACopyAFailedBuildMadeIsRecordedAndNothingKeepsIt(t *testing.T) {
+	inv := fresh(t)
+	golang := mirrorRef("upstream/docker.io/library/golang", 1)
+	failed := aBuild("f1", "", "the recipe fetched an undeclared image")
+	failed.Mirrored = []string{golang}
+	if err := inv.RecordBuild(context.Background(), failed); err != nil {
+		t.Fatal(err)
+	}
+	if s := stateOf(t, inv, golang); s.State != ArtifactEligible {
+		t.Fatalf("a copy only a failed build made reads %+v; want eligible", s)
+	}
+}
+
+func TestACopyIsKeptByTheKeptBuildsThatStoodOnItAndByNothingElse(t *testing.T) {
+	inv := fresh(t)
+	ctx := context.Background()
+	holding(t, inv, "proxy")
+	old := mirrorRef("upstream/docker.io/library/golang", 1)
+	current := mirrorRef("upstream/docker.io/library/golang", 2)
+
+	// The first build stood on the old copy; six more on the current one. The old copy falls out of
+	// the five kept builds; the current one is stood on by all five.
+	builtOn(t, inv, "b01", "proxy", 1, []string{old}, old)
+	for i := 2; i <= 7; i++ {
+		builtOn(t, inv, fmt.Sprintf("b%02d", i), "proxy", i, []string{current}, current)
+	}
+	if s := stateOf(t, inv, old); s.State != ArtifactEligible {
+		t.Fatalf("a copy no kept build stood on reads %+v", s)
+	}
+	if s := stateOf(t, inv, current); s.State != ArtifactKept || !slices.Equal(s.Why, []string{KeptStoodOn}) {
+		t.Fatalf("a copy the kept builds stood on reads %+v", s)
+	}
+
+	// A definition naming the image upstream, by the digest every copy shares, keeps no copy.
+	m := catalogue.Manifest{Module: "proxy", Version: "2",
+		Build: &catalogue.Build{On: []catalogue.BuildsOn{{Arg: "GO_BASE", Image: "golang@" + strings.SplitN(old, "@", 2)[1]}}}}
+	if err := inv.RegisterModule(ctx, m, Source{Repository: "https://forge.invalid/proxy.git"}); err != nil {
+		t.Fatal(err)
+	}
+	if s := stateOf(t, inv, old); s.State != ArtifactEligible {
+		t.Fatalf("a definition naming the upstream image kept the old copy: %+v", s)
+	}
+	go_, err := inv.ToCollect(ctx)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !slices.Contains(go_, old) || slices.Contains(go_, current) {
+		t.Fatalf("offered %v to collect", go_)
+	}
+
+	// Once let go of, a copy stays let go of, even should a kept build still name it.
+	if err := inv.MarkCollected(ctx, []string{current}); err != nil {
+		t.Fatal(err)
+	}
+	if s := stateOf(t, inv, current); s.State != ArtifactCollected {
+		t.Fatalf("a collected copy reads %+v", s)
+	}
+}
+
+func TestAModulesBaseIsNotKeptAsACopy(t *testing.T) {
+	// A base that is another module's artifact is kept by that module's own builds, as before: the
+	// stood-on reason is a copy's alone.
+	inv := fresh(t)
+	holding(t, inv, "app")
+	base := ref("go-base", "build", 1)
+	b := aBuild("g1", "go-base", "")
+	b.Made = []Artifact{{Name: "build", Kind: "image", Reference: base}}
+	if err := inv.RecordBuild(context.Background(), b); err != nil {
+		t.Fatal(err)
+	}
+	builtOn(t, inv, "a1", "app", 1, nil, base)
+	if s := stateOf(t, inv, base); s.State != ArtifactEligible {
+		t.Fatalf("a forgotten module's artifact read %+v because a build stood on it", s)
+	}
+}
+
+func TestAPersonRecordsACopyAndTheFirstRecordStands(t *testing.T) {
+	inv := fresh(t)
+	ctx := context.Background()
+	copyRef := mirrorRef("route-proxy/on-go_base", 3)
+	if err := inv.RecordMirrored(ctx, "", "copied before copies were recorded", []string{copyRef}); err != nil {
+		t.Fatal(err)
+	}
+	if err := inv.RecordMirrored(ctx, "b9", "", []string{copyRef}); err != nil {
+		t.Fatal(err)
+	}
+	known, err := inv.Mirrored(ctx)
+	if err != nil || !known[copyRef] {
+		t.Fatalf("recorded copies %v %v", known, err)
+	}
+	var why string
+	if err := inv.store.Pool().QueryRow(ctx, `select why from artifact_mirrored where reference = $1`, copyRef).Scan(&why); err != nil {
+		t.Fatal(err)
+	}
+	if why != "copied before copies were recorded" {
+		t.Fatalf("the first record did not stand: %q", why)
+	}
+	if s := stateOf(t, inv, copyRef); s.State != ArtifactEligible {
+		t.Fatalf("a copy a person recorded reads %+v", s)
+	}
+}
+
+// The migration records the copies the build records already name: only a module's own on-<argument>
+// repository, and only once, by the earliest build naming it.
+func TestTheCopiesTheRecordsAlreadyNameAreRecorded(t *testing.T) {
+	inv := fresh(t)
+	ctx := context.Background()
+	copyRef := mirrorRef("route-proxy/on-go_base", 1)
+	notACopy := ref("go-base", "build", 2)
+	builtOn(t, inv, "b1", "route-proxy", 1, nil, copyRef, notACopy)
+	builtOn(t, inv, "b2", "route-proxy", 2, nil, copyRef)
+	// The rows a mesh had before the table: written by builds that did not say what they copied.
+	if _, err := inv.store.Pool().Exec(ctx, `truncate artifact_mirrored`); err != nil {
+		t.Fatal(err)
+	}
+	sql, err := os.ReadFile("migrations/0081-a-mirrored-base-is-recorded.sql")
+	if err != nil {
+		t.Fatal(err)
+	}
+	_, backfill, _ := strings.Cut(string(sql), "insert into artifact_mirrored")
+	if _, err := inv.store.Pool().Exec(ctx, "insert into artifact_mirrored"+backfill); err != nil {
+		t.Fatal(err)
+	}
+	var reference, build string
+	var n int
+	if err := inv.store.Pool().QueryRow(ctx, `select count(*) from artifact_mirrored`).Scan(&n); err != nil || n != 1 {
+		t.Fatalf("%d copies recorded (%v); want the one", n, err)
+	}
+	if err := inv.store.Pool().QueryRow(ctx, `select reference, build from artifact_mirrored`).Scan(&reference, &build); err != nil {
+		t.Fatal(err)
+	}
+	if reference != copyRef || build != "b1" {
+		t.Fatalf("recorded %s by %s", reference, build)
+	}
+}
+
+// A copy let go of and copied again by a later build is held again: its record is back, and the sweep
+// decides about it as before.
+func TestACopyLetGoOfAndCopiedAgainIsHeldAgain(t *testing.T) {
+	inv := fresh(t)
+	ctx := context.Background()
+	holding(t, inv, "proxy")
+	golang := mirrorRef("upstream/docker.io/library/golang", 1)
+	builtOn(t, inv, "b01", "proxy", 1, []string{golang}, golang)
+	if err := inv.MarkCollected(ctx, []string{golang}); err != nil {
+		t.Fatal(err)
+	}
+	if s := stateOf(t, inv, golang); s.State != ArtifactCollected {
+		t.Fatalf("a let-go copy reads %+v", s)
+	}
+	builtOn(t, inv, "b02", "proxy", 2, []string{golang}, golang)
+	if s := stateOf(t, inv, golang); s.State != ArtifactKept || !slices.Equal(s.Why, []string{KeptStoodOn}) {
+		t.Fatalf("a copy copied again reads %+v; want kept again", s)
+	}
+	// A person's record never undoes a collection: only a build that copied it says it is there.
+	if err := inv.MarkCollected(ctx, []string{golang}); err != nil {
+		t.Fatal(err)
+	}
+	if err := inv.RecordMirrored(ctx, "", "again", []string{golang}); err != nil {
+		t.Fatal(err)
+	}
+	if s := stateOf(t, inv, golang); s.State != ArtifactCollected {
+		t.Fatalf("a person's record brought a collected copy back: %+v", s)
+	}
+}

internal/link/build.go

@@ -215,6 +215,10 @@ type BuildResult struct {
 	// (novox/hq ADR 0009). The catalogue turns these into edges; nothing else need care.
 	Against []string `json:"against,omitempty"`
 
+	// Mirrored is every base this build copied into the artifact store, as pinned (novox/hq ADR
+	// 0257). Carried on a failed build too: the copy is in the store whether the build worked or not.
+	Mirrored []string `json:"mirrored,omitempty"`
+
 	// Read is every repository this build read source from besides the module's own. A module whose
 	// recipe packages source that lives elsewhere is affected when that repository moves, and the
 	// manifest the mesh keeps says nothing about it (novox/hq 04-ISSUES/131).

module.json

@@ -75,7 +75,8 @@
     "build",
     "artifacts",
     "collect",
-    "images"
+    "images",
+    "mirrors"
   ],
   "resources": [
     {