initial implementation of the dagstore.

accessor.go

@@ -0,0 +1,60 @@
+package dagstore
+
+import (
+	"context"
+	"fmt"
+
+	"github.com/filecoin-project/dagstore/mount"
+	"github.com/filecoin-project/dagstore/shard"
+	blocks "github.com/ipfs/go-block-format"
+	"github.com/ipfs/go-cid"
+	"github.com/ipld/go-car/v2"
+	"github.com/ipld/go-car/v2/blockstore"
+	"github.com/ipld/go-car/v2/index"
+)
+
+// ReadBlockstore is a read-only view of Blockstores. This will be implemented
+// by the CARv2 indexed blockstore.
+type ReadBlockstore interface {
+	Has(cid.Cid) (bool, error)
+	Get(cid.Cid) (blocks.Block, error)
+	GetSize(cid.Cid) (int, error)
+	AllKeysChan(ctx context.Context) (<-chan cid.Cid, error)
+	HashOnRead(enabled bool)
+}
+
+// ShardAccessor provides various means to access the data contained
+// in a shard.
+type ShardAccessor struct {
+	key  shard.Key
+	data *car.Reader
+	idx  index.Index
+}
+
+func NewShardAccessor(key shard.Key, data mount.Reader, idx index.Index) (*ShardAccessor, error) {
+	reader, err := car.NewReader(data)
+	if err != nil {
+		return nil, fmt.Errorf("failed to determine car version when opening shard accessor: %w", err)
+	}
+
+	return &ShardAccessor{
+		key:  key,
+		data: reader,
+		idx:  idx,
+	}, nil
+}
+
+func (sa *ShardAccessor) Shard() shard.Key {
+	return sa.key
+}
+
+func (sa *ShardAccessor) Blockstore() (ReadBlockstore, error) {
+	bs := blockstore.ReadOnlyOf(sa.data.CarV1Reader(), sa.idx)
+	return bs, nil
+}
+
+// Close terminates this shard accessor, releasing any resources associated
+// with it, and decrementing internal refcounts.
+func (sa *ShardAccessor) Close() error {
+	return sa.data.Close()
+}