The store keeps what the records name (hq ADR 0189)

The mesh names what may go from its own build records — a digest it did not
record making is never named, which is what keeps the sweep away from the
images genesis pushed. An artifact stays because a definition the mesh holds
names it, or because it belongs to one of the five most recent successful
builds of its module.

internal/artifacts asks the store to let go of one; internal/inventory
decides and remembers (migration 0055); the sweep runs after a build the mesh
recorded, which is when both the bytes and the keep set moved. Never fatal to
a build.

And the manifest side of while-stopped, refused from the definition alone:
no schedule, run-once, a container the module does not declare, itself.

cmd/mesh-controller/build.go

@@ -558,6 +558,10 @@ func takeIn(ctx context.Context, inv *inventory.Inventory, result link.BuildResu
 		}
 		return manifest, kept, err
 	}
+	// The keep set just moved, and new bytes just landed (novox/hq ADR 0189). Asked here rather
+	// than on a timer of its own: this is the only moment either is true. Never fatal — the build
+	// worked and the module is registered.
+	collect(ctx, inv)
 	return manifest, kept, nil
 }
 

cmd/mesh-controller/collect.go

@@ -0,0 +1,84 @@
+package main
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"os"
+
+	"github.com/novox/mesh-controller/internal/artifacts"
+	"github.com/novox/mesh-controller/internal/inventory"
+)
+
+// Letting the artifact store go of what the mesh no longer keeps (novox/hq ADR 0189, issue 108).
+//
+// **Run where the records change.** A build is the moment new bytes landed in the store and the
+// moment the keep set moved, so it is the moment to say what may go — and it needs no timer of
+// its own. Reclaiming the bytes is the store's own nightly step; this only decides.
+//
+// Never fatal to a build. The build succeeded, the module is registered, and a store that could
+// not be reached is a thing to say rather than a reason to undo any of that. The next build asks
+// again, and the references it could not collect are still uncollected, so nothing is lost by
+// having failed.
+
+// collect asks the store to let go of everything the mesh made and no longer keeps, and records
+// what it let go of. Says what it did and what it could not; returns nothing, because nothing
+// upstream should branch on it.
+func collect(ctx context.Context, inv *inventory.Inventory) {
+	references, err := inv.ToCollect(ctx)
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "could not work out what the artifact store may let go of: %v\n", err)
+		return
+	}
+	if len(references) == 0 {
+		return
+	}
+	shelf, err := inv.Catalogue(ctx)
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "could not read the catalogue to find the artifact store: %v\n", err)
+		return
+	}
+	// As the mesh reaches it from the network. Empty means the store is not on the network — on a
+	// mesh being raised it is not yet, and there the store holds one build of anything and has
+	// nothing to collect.
+	address, err := artifactStoreAddress(ctx, inv, shelf, "")
+	if err != nil || address == "" {
+		if err != nil {
+			fmt.Fprintf(os.Stderr, "could not find the artifact store to collect from: %v\n", err)
+		}
+		return
+	}
+
+	store := artifacts.Store{Address: address}
+	var done []string
+	var refused int
+	for _, reference := range references {
+		switch err := store.LetGo(ctx, reference); {
+		case err == nil, errors.Is(err, artifacts.Gone):
+			// Gone is the outcome wanted, already true. Recorded so the next sweep does not ask
+			// again for ever.
+			done = append(done, reference)
+		default:
+			refused++
+			if refused == 1 {
+				// Once per sweep. A store that refuses one refuses all of them, and a hundred
+				// identical lines would bury the reason.
+				fmt.Fprintf(os.Stderr, "the artifact store kept %s: %v\n", reference, err)
+			}
+		}
+	}
+	if len(done) > 0 {
+		if err := inv.MarkCollected(ctx, done); err != nil {
+			// Said, and that is all: the artifacts are gone either way, and the only cost of an
+			// unrecorded collection is that the next sweep asks about them again.
+			fmt.Fprintf(os.Stderr, "the store let go of %d artifact(s) and the record of it did not keep: %v\n",
+				len(done), err)
+			return
+		}
+		fmt.Fprintf(os.Stderr, "the artifact store let go of %d artifact(s) the mesh no longer keeps\n",
+			len(done))
+	}
+	if refused > 0 {
+		fmt.Fprintf(os.Stderr, "%d artifact(s) were not collected; the next build asks again\n", refused)
+	}
+}