feat: handle retrieval queries for unindexed identity payload CIDs

There are valid cases where a CAR may have an identity CID as its root that is
not represented as a 'block' within the CAR body and therefore isn't indexed
by the dagstore. In this case, we inspect the identity CID content and treat
the query as a query for the intersection of all of the links within the block.

Ref: filecoin-project/boost#715

retrievalmarket/impl/provider.go

@@ -10,6 +10,10 @@ import (
 	"github.com/ipfs/go-cid"
 	"github.com/ipfs/go-datastore"
 	"github.com/ipfs/go-datastore/namespace"
+	"github.com/ipld/go-ipld-prime"
+	cidlink "github.com/ipld/go-ipld-prime/linking/cid"
+	"github.com/ipld/go-ipld-prime/traversal"
+	"github.com/multiformats/go-multihash"
 	"golang.org/x/xerrors"
 
 	"github.com/filecoin-project/go-address"
@@ -341,7 +345,7 @@ func (p *Provider) HandleQueryStream(stream rmnet.RetrievalQueryStream) {
 	pieceInfo, isUnsealed, err := p.getPieceInfoFromCid(ctx, query.PayloadCID, pieceCID)
 	if err != nil {
 		log.Errorf("Retrieval query: getPieceInfoFromCid: %s", err)
-		if !xerrors.Is(err, retrievalmarket.ErrNotFound) {
+		if !errors.Is(err, retrievalmarket.ErrNotFound) {
 			answer.Status = retrievalmarket.QueryResponseError
 			answer.Message = fmt.Sprintf("failed to fetch piece to retrieve from: %s", err)
 		} else {
@@ -397,7 +401,15 @@ func (p *Provider) getPieceInfoFromCid(ctx context.Context, payloadCID, clientPi
 	// Get all pieces that contain the target block
 	piecesWithTargetBlock, err := p.dagStore.GetPiecesContainingBlock(payloadCID)
 	if err != nil {
-		return piecestore.PieceInfoUndefined, false, xerrors.Errorf("getting pieces for cid %s: %w", payloadCID, err)
+		// this payloadCID may be an identity CID that's in the root of a CAR but
+		// not recorded in the index
+		piecesWithTargetBlock, err := p.getCommonPiecesFromIdentityCidLinks(ctx, payloadCID)
+		if err != nil {
+			return piecestore.PieceInfoUndefined, false, err
+		}
+		if len(piecesWithTargetBlock) == 0 {
+			return piecestore.PieceInfoUndefined, false, fmt.Errorf("getting pieces for cid %s: %w", payloadCID, err)
+		}
 	}
 
 	// For each piece that contains the target block
@@ -448,6 +460,60 @@ func (p *Provider) getPieceInfoFromCid(ctx context.Context, payloadCID, clientPi
 	return piecestore.PieceInfoUndefined, false, xerrors.Errorf("could not locate piece: %w", lastErr)
 }
 
+// getCommonPiecesFromIdentityCidLinks will inspect a payloadCID and if it has an identity multihash,
+// will determine which pieces contain all of the links within the decoded identity multihash block
+func (p *Provider) getCommonPiecesFromIdentityCidLinks(ctx context.Context, payloadCID cid.Cid) ([]cid.Cid, error) {
+	if payloadCID.Prefix().MhType != multihash.IDENTITY {
+		return nil, nil
+	}
+
+	// decode the identity multihash, if possible (i.e. it's valid and we have the right codec loaded)
+	decoder, err := cidlink.DefaultLinkSystem().DecoderChooser(cidlink.Link{Cid: payloadCID})
+	if err != nil {
+		return nil, fmt.Errorf("choosing decoder for identity CID %s: %w", payloadCID, err)
+	}
+	mh, err := multihash.Decode(payloadCID.Hash())
+	if err != nil {
+		return nil, fmt.Errorf("decoding identity CID multihash %s: %w", payloadCID, err)
+	}
+	node, err := ipld.Decode(mh.Digest, decoder)
+	if err != nil {
+		return nil, fmt.Errorf("decoding identity CID %s: %w", payloadCID, err)
+	}
+	links, err := traversal.SelectLinks(node)
+	if err != nil {
+		return nil, fmt.Errorf("collecting links from identity CID %s: %w", payloadCID, err)
+	}
+
+	pieces := make([]cid.Cid, 0)
+	// for each link, query the dagstore for pieces that contain it
+	for i, link_ := range links {
+		link := link_.(cidlink.Link).Cid
+		piecesWithThisCid, err := p.dagStore.GetPiecesContainingBlock(link)
+		if len(piecesWithThisCid) == 0 {
+			return nil, fmt.Errorf("getting pieces for identity CID sub-link %s: %w", link, err)
+		}
+		if i == 0 {
+			pieces = append(pieces, piecesWithThisCid...)
+		} else {
+			// after the first, find the intersection between these pieces and the previous ones
+			intersection := make([]cid.Cid, 0)
+			for _, cj := range piecesWithThisCid {
+				for _, ck := range pieces {
+					if cj.Equals(ck) {
+						intersection = append(intersection, cj)
+						break
+					}
+				}
+			}
+			pieces = intersection
+		}
+		if len(pieces) == 0 {
+			break
+		}
+	}
+	return pieces, nil
+}
 func (p *Provider) pieceInUnsealedSector(ctx context.Context, pieceInfo piecestore.PieceInfo) bool {
 	for _, di := range pieceInfo.Deals {
 		isUnsealed, err := p.sa.IsUnsealed(ctx, di.SectorID, di.Offset.Unpadded(), di.Length.Unpadded())