feat: use last segment cid as root

gateway/blocks_backend.go

@@ -233,19 +233,24 @@ func (bb *BlocksBackend) Head(ctx context.Context, path ImmutablePath) (ContentP
 }
 
 func (bb *BlocksBackend) GetCAR(ctx context.Context, p ImmutablePath, params CarParams) (io.ReadCloser, error) {
+	// Try resolving the path and set the last segment as the root CID. If the path
+	// is not resolvable, there's likely an error in the path. Then, we send an empty
+	// root. This allows the response to succeed and the client can verify by themselves
+	// that the request path is non existent.
+	var roots []cid.Cid
+	pathMetadata, err := bb.ResolvePath(ctx, p)
+	if err == nil {
+		roots = append(roots, pathMetadata.LastSegment.Cid())
+	}
+
 	contentPathStr := p.String()
 	if !strings.HasPrefix(contentPathStr, "/ipfs/") {
 		return nil, fmt.Errorf("path does not have /ipfs/ prefix")
 	}
-	firstSegment, _, _ := strings.Cut(contentPathStr[6:], "/")
-	rootCid, err := cid.Decode(firstSegment)
-	if err != nil {
-		return nil, err
-	}
 
 	r, w := io.Pipe()
 	go func() {
-		cw, err := storage.NewWritable(w, []cid.Cid{rootCid}, car.WriteAsCarV1(true))
+		cw, err := storage.NewWritable(w, roots, car.WriteAsCarV1(true))
 		if err != nil {
 			// io.PipeWriter.CloseWithError always returns nil.
 			_ = w.CloseWithError(err)