proof+tapcfg: add and use new MultiArchiveNotifier

This commit creates a new notifier that can relay registrations for new
proofs to multiple notifier/archiver backends.
We then use that to pass in both the local assets store as well as the
multiverse store to the custodian to detect incoming proofs.

proof/archive.go

@@ -140,6 +140,89 @@ type NotifyArchiver interface {
 	fn.EventPublisher[Blob, []*Locator]
 }
 
+// MultiArchiveNotifier is a NotifyArchiver that wraps several other archives
+// and notifies subscribers about new proofs that are added to any of the
+// archives.
+type MultiArchiveNotifier struct {
+	archives []NotifyArchiver
+}
+
+// NewMultiArchiveNotifier creates a new MultiArchiveNotifier based on the set
+// of specified backends.
+func NewMultiArchiveNotifier(archives ...NotifyArchiver) *MultiArchiveNotifier {
+	return &MultiArchiveNotifier{
+		archives: archives,
+	}
+}
+
+// FetchProof fetches a proof for an asset uniquely identified by the passed
+// Identifier. The returned proof can either be a full proof file or just a
+// single proof.
+//
+// If a proof cannot be found, then ErrProofNotFound should be returned.
+//
+// NOTE: This is part of the NotifyArchiver interface.
+func (m *MultiArchiveNotifier) FetchProof(ctx context.Context,
+	id Locator) (Blob, error) {
+
+	for idx := range m.archives {
+		a := m.archives[idx]
+
+		proofBlob, err := a.FetchProof(ctx, id)
+		if errors.Is(err, ErrProofNotFound) {
+			// Try the next archive.
+			continue
+		} else if err != nil {
+			return nil, fmt.Errorf("error fetching proof "+
+				"from archive: %w", err)
+		}
+
+		return proofBlob, nil
+	}
+
+	return nil, ErrProofNotFound
+}
+
+// RegisterSubscriber adds a new subscriber for receiving events. The
+// registration request is forwarded to all registered archives.
+func (m *MultiArchiveNotifier) RegisterSubscriber(
+	receiver *fn.EventReceiver[Blob], deliverExisting bool,
+	deliverFrom []*Locator) error {
+
+	for idx := range m.archives {
+		a := m.archives[idx]
+
+		err := a.RegisterSubscriber(
+			receiver, deliverExisting, deliverFrom,
+		)
+		if err != nil {
+			return fmt.Errorf("error registering subscriber: %w",
+				err)
+		}
+	}
+
+	return nil
+}
+
+// RemoveSubscriber removes the given subscriber and also stops it from
+// processing events. The removal request is forwarded to all registered
+// archives.
+func (m *MultiArchiveNotifier) RemoveSubscriber(
+	subscriber *fn.EventReceiver[Blob]) error {
+
+	for idx := range m.archives {
+		a := m.archives[idx]
+
+		err := a.RemoveSubscriber(subscriber)
+		if err != nil {
+			return fmt.Errorf("error removing subscriber: "+
+				"%w", err)
+		}
+	}
+
+	return nil
+}
+
 // FileArchiver implements proof Archiver backed by an on-disk file system. The
 // archiver takes a single root directory then creates the following overlap
 // mapping:

tapcfg/server.go

@@ -330,6 +330,8 @@ func genServerConfig(cfg *Config, cfgLogger btclog.Logger,
 		ChainParams:  &tapChainParams,
 	})
 
+	multiNotifier := proof.NewMultiArchiveNotifier(assetStore, multiverse)
+
 	return &tap.Config{
 		DebugLevel:   cfg.DebugLevel,
 		RuntimeID:    runtimeID,
@@ -364,7 +366,7 @@ func genServerConfig(cfg *Config, cfgLogger btclog.Logger,
 				),
 				AddrBook:            addrBook,
 				ProofArchive:        proofArchive,
-				ProofNotifier:       assetStore,
+				ProofNotifier:       multiNotifier,
 				ErrChan:             mainErrChan,
 				ProofCourierCfg:     proofCourierCfg,
 				ProofRetrievalDelay: cfg.CustodianProofRetrievalDelay,