fix: make tags accessible to callers of signerootreader (#18)

A quirk of the interface for lazily reading the root seal blobs
prevented the blob tags from being available to the caller of
ReadLogicalContext.

This also now works consistently for all methods on the signed root
reader which use this method internaly.

Rather than egregiously break an existing public interface, and also
because it seems cleaner, we just save the last context read and provide
a method to retrieve it if wanted.

The method to retrieve will error if the context hasn't been read at
all.

AB#9570

Co-authored-by: Robin Bryce <robin.bryce@datatrails.ai>

massifs/signedrootreader.go

@@ -2,19 +2,27 @@ package massifs
 
 import (
 	"context"
+	"errors"
 
 	"github.com/datatrails/go-datatrails-common/azblob"
 	"github.com/datatrails/go-datatrails-common/cbor"
 	dtcose "github.com/datatrails/go-datatrails-common/cose"
 	"github.com/datatrails/go-datatrails-common/logger"
 )
 
+var (
+	ErrLogContextNotRead = errors.New("attempted to use lastContext before it was read")
+)
+
 // SignedRootReader provides a context for reading the signed tree head associated with a massif
 // Note: the acronym is due to RFC 9162
 type SignedRootReader struct {
 	log   logger.Logger
 	store logBlobReader
 	codec cbor.CBORCodec
+	// lastContext saves the last context read from blob store, this includes
+	// Tags if they were requested
+	lastContext LogBlobContext
 }
 
 func NewSignedRootReader(log logger.Logger, store logBlobReader, codec cbor.CBORCodec) SignedRootReader {
@@ -26,6 +34,16 @@ func NewSignedRootReader(log logger.Logger, store logBlobReader, codec cbor.CBOR
 	return r
 }
 
+// GetLastReadContext returns a copy of the most recently read context. Use this
+// to get access to the tags when using WithGetTags.  If the context hasn't been
+// read (directly or indirectly) an error is returned.
+func (s *SignedRootReader) GetLastReadContext() (LogBlobContext, error) {
+	if s.lastContext.BlobPath == "" {
+		return LogBlobContext{}, ErrLogContextNotRead
+	}
+	return s.lastContext, nil
+}
+
 func (s *SignedRootReader) GetLazyContext(
 	ctx context.Context, tenantIdentity string, which LogicalBlob,
 	opts ...azblob.Option,
@@ -48,6 +66,7 @@ func (s *SignedRootReader) GetLazyContext(
 	if err != nil {
 		return LogBlobContext{}, 0, err
 	}
+	s.lastContext = logBlobContext
 	return logBlobContext, count, nil
 }
 
@@ -60,6 +79,7 @@ func (s *SignedRootReader) ReadLogicalContext(
 	if err != nil {
 		return nil, MMRState{}, err
 	}
+	s.lastContext = logContext
 
 	signed, unverifiedState, err := DecodeSignedRoot(s.codec, logContext.Data)
 	if err != nil {
@@ -130,14 +150,15 @@ func (s *SignedRootReader) GetLatestMassifSignedRoot(
 	opts ...azblob.Option,
 ) (*dtcose.CoseSign1Message, MMRState, error) {
 
-	lc := LogBlobContext{
+	logContext := LogBlobContext{
 		BlobPath: TenantMassifSignedRootPath(tenantIdentity, massifIndex),
 	}
-	err := lc.ReadData(ctx, s.store, opts...)
+	err := logContext.ReadData(ctx, s.store, opts...)
 	if err != nil {
 		return nil, MMRState{}, err
 	}
-	signed, unverifiedState, err := DecodeSignedRoot(s.codec, lc.Data)
+	s.lastContext = logContext
+	signed, unverifiedState, err := DecodeSignedRoot(s.codec, logContext.Data)
 	if err != nil {
 		return nil, MMRState{}, err
 	}