End an unassigned key with ended and remove no key of the chain (issue 1093)

ADR 0408 section 4 says no key of the chain is removed, a delete marker included. The Backend loses Remove and the Chain RemoveAssignment. An unassignment, and the end of a value only pulled in by another, write the value again with ended set, by compare-and-set; assigning it again writes a fresh standing value over it. A value with ended or a state is no assignment (Assignment.Stands) to the assignment lists, the joins, forgetting a module, seat holdings, the given values, the move and the assignment-changed event.

internal/chain/bus.go

@@ -41,7 +41,7 @@ func OnTheBus(ctx context.Context, conn *nats.Conn, holds Holds) (*Chain, error)
 	return New(builds, rollouts, assignments, holds), nil
 }
 
-// busBucket is a Backend on a key-value bucket. It calls no delete and no purge: the type has none to call.
+// busBucket is a Backend on a key-value bucket. It calls no delete and no purge: the type has none to call (ADR 0408 §4).
 type busBucket struct{ kv jetstream.KeyValue }
 
 func entryOf(e jetstream.KeyValueEntry) Entry {
@@ -76,16 +76,6 @@ func (b busBucket) Update(ctx context.Context, key string, value []byte, revisio
 	return rev, err
 }
 
-func (b busBucket) Remove(ctx context.Context, key string, revision uint64) error {
-	// A delete marker, never a purge: the key's earlier values stay in the bucket's history.
-	err := b.kv.Delete(ctx, key, jetstream.LastRevision(revision))
-	var apiErr *jetstream.APIError
-	if errors.As(err, &apiErr) && apiErr.ErrorCode == jetstream.JSErrCodeStreamWrongLastSequence {
-		return ErrMoved
-	}
-	return err
-}
-
 func (b busBucket) Keys(ctx context.Context) ([]string, error) {
 	keys, err := b.kv.Keys(ctx)
 	if errors.Is(err, jetstream.ErrNoKeysFound) {
@@ -137,7 +127,7 @@ func (b busBucket) History(ctx context.Context, key string) ([]Entry, error) {
 	out := make([]Entry, 0, len(all))
 	for _, e := range all {
 		if e.Operation() != jetstream.KeyValuePut {
-			continue
+			continue // a marker an earlier mirror left; nothing of the chain makes one
 		}
 		out = append(out, entryOf(e))
 	}