Add MirrorWriter skeleton in GCP storage

storage/gcp/gcp.go

@@ -35,6 +35,7 @@ import (
 	"errors"
 	"fmt"
 	"io"
+	"iter"
 	"math"
 	"net/http"
 	"os"
@@ -1455,6 +1456,75 @@ func (s *gcsStorage) deleteObjectsWithPrefix(ctx context.Context, objPrefix stri
 	})
 }
 
+// MirrorWriter creates a new GCP storage for the MirrorTarget lifecycle mode.
+func (s *Storage) MirrorWriter(ctx context.Context, opts *tessera.MirrorOptions) (tessera.MirrorWriter, tessera.LogReader, error) {
+	var err error
+	if s.cfg.GCSClient == nil {
+		s.cfg.GCSClient, err = gcs.NewClient(ctx, gcs.WithJSONReads())
+		if err != nil {
+			return nil, nil, fmt.Errorf("failed to create GCS client: %v", err)
+		}
+	}
+	if s.cfg.SpannerClient == nil {
+		s.cfg.SpannerClient, err = spanner.NewClient(ctx, s.cfg.Spanner)
+		if err != nil {
+			return nil, nil, fmt.Errorf("failed to connect to Spanner: %v", err)
+		}
+	}
+	table := func(t string) string {
+		return s.cfg.SpannerTablePrefix + t
+	}
+	if err := initDB(ctx, s.cfg.Spanner, table); err != nil {
+		return nil, nil, fmt.Errorf("failed to verify/init Spanner schema: %v", err)
+	}
+	seq, err := newSpannerCoordinator(ctx, s.cfg.SpannerClient, table, 0)
+	if err != nil {
+		return nil, nil, fmt.Errorf("failed to create Spanner coordinator: %v", err)
+	}
+	gs := &gcsStorage{
+		gcsClient:    s.cfg.GCSClient,
+		bucket:       s.cfg.Bucket,
+		bucketPrefix: s.cfg.BucketPrefix,
+	}
+	m := &mirrorWriter{
+		seq: seq,
+		logStore: &logResourceStore{
+			objStore:    gs,
+			entriesPath: opts.EntriesPath(),
+		},
+	}
+	lr := &LogReader{
+		lrs:            *m.logStore,
+		integratedSize: m.IntegratedSize,
+		nextIndex:      seq.nextIndex,
+	}
+	return m, lr, nil
+}
+
+// mirrorWriter implements the tessera.MirrorWriter lifecycle contract.
+type mirrorWriter struct {
+	seq      *spannerCoordinator
+	logStore *logResourceStore
+}
+
+var _ tessera.MirrorWriter = &mirrorWriter{}
+
+func (w *mirrorWriter) IntegrateBundles(ctx context.Context, from uint64, bundles iter.Seq2[*api.EntryBundle, error]) (uint64, []byte, error) {
+	return 0, nil, errors.New("not implemented")
+}
+
+func (w *mirrorWriter) IntegratedSize(ctx context.Context) (uint64, error) {
+	size, _, err := w.seq.currentTree(ctx)
+	if err != nil {
+		return 0, fmt.Errorf("failed to read integrated size: %w", err)
+	}
+	return size, nil
+}
+
+func (w *mirrorWriter) UpdateCheckpoint(ctx context.Context, g func(oldCP []byte) (newCP []byte, err error)) error {
+	return errors.New("not implemented")
+}
+
 // MigrationWriter creates a new GCP storage for the MigrationTarget lifecycle mode.
 func (s *Storage) MigrationWriter(ctx context.Context, opts *tessera.MigrationOptions) (migrate.MigrationWriter, tessera.LogReader, error) {
 	var err error