Hold every kept archive by a manifest so the store's collector keeps it (hq issue 253)

The store's garbage-collect marks only from manifests, and archives were
published as bare blobs, so the first real collection would delete every
archive the mesh keeps. PublishArchive now puts a deterministic OCI holder
manifest (empty config, one layer) beside each archive; the sweep holds every
kept archive before it lets anything go, which backfills existing bare blobs,
and lets go of an archive holder-first. A forgotten module no longer keeps its
five recent builds (ADR 0189). `collection [--json]` reports kept archives
held/unheld and what may be let go, so the dry run can be lifted on evidence.

internal/artifacts/store.go

@@ -1,7 +1,8 @@
 // Package artifacts speaks to the mesh's artifact store over its own door.
 //
 // Only what the mesh needs that nothing else does: letting go of something it put there
-// (novox/hq ADR 0189, issue 108). Pushing is the builder's, through the container runtime; reading
+// (novox/hq ADR 0189, issue 108), and holding every archive it keeps by a manifest so the store's
+// own collector does not take it (novox/hq issue 253). Pushing is the builder's, through the container runtime; reading
 // is every machine's, through its runtime. This is the one operation that belongs to the thing
 // holding the records, because it is the only one that is a decision rather than a transfer.
 package artifacts
@@ -12,7 +13,6 @@ import (
 	"fmt"
 	"net/http"
 	"strings"
-	"time"
 
 	"github.com/novox/mesh-controller/internal/catalogue"
 )
@@ -43,6 +43,9 @@ var ErrNotOurs = errors.New("not a reference into the mesh's artifact store")
 // an image, `…/blobs/sha256:…` for an archive — because that is the identity every record uses,
 // and composes the address here at the moment of use.
 //
+// An archive is let go of in two deletes, its holder manifest and then the blob's link (novox/hq
+// issue 253); an image in one.
+//
 // Returns Gone when the store answers that it does not have it. That is not a failure: the sweep
 // wants the artifact absent, and it is. It is distinguished from success only so a caller can say
 // which of the two happened.
@@ -66,17 +69,24 @@ func (s Store) LetGo(ctx context.Context, reference string) error {
 	if err != nil {
 		return err
 	}
-	url := "http://" + s.Address + "/v2/" + repository + "/" + kind + "/" + digest
+	if kind == "blobs" {
+		// **An archive's holder goes before the archive** (novox/hq issue 253): a manifest left
+		// naming the blob would keep its bytes through every collection while the record said
+		// collected. Gone here means the store has no such blob, so there is nothing to let go.
+		if err := s.letGoOfHolder(ctx, repository, digest); err != nil {
+			return err
+		}
+	}
+	return s.remove(ctx, s.url(repository, kind, digest), reference)
+}
 
+// 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)
 	if err != nil {
 		return err
 	}
-	client := s.HTTP
-	if client == nil {
-		client = &http.Client{Timeout: 30 * time.Second}
-	}
-	response, err := client.Do(request)
+	response, err := s.client().Do(request)
 	if err != nil {
 		return err
 	}
@@ -92,9 +102,9 @@ func (s Store) LetGo(ctx context.Context, reference string) error {
 		return fmt.Errorf(
 			"the artifact store refuses deletion: its server was started without it enabled "+
 				"(REGISTRY_STORAGE_DELETE_ENABLED), so nothing can be collected until the store "+
-				"module is applied again (novox/hq ADR 0189). Asking about %s", reference)
+				"module is applied again (novox/hq ADR 0189). Asking about %s", what)
 	default:
-		return fmt.Errorf("the artifact store answered %s for %s", response.Status, reference)
+		return fmt.Errorf("the artifact store answered %s for %s", response.Status, what)
 	}
 }