return car with enough data to prove path does not resolve

Author: hacdias

gateway/backend_car.go

@@ -1123,17 +1123,31 @@ func (api *CarBackend) GetCAR(ctx context.Context, p path.ImmutablePath, params
 			}
 			l := getLinksystem(teeBlock)
 
+			var isNotFound bool
+
 			// First resolve the path since we always need to.
 			md, terminalBlk, err := resolvePathWithRootsAndBlock(ctx, p, l)
 			if err != nil {
-				return err
+				if isErrNotFound(err) {
+					isNotFound = true
+				} else {
+					return err
+				}
 			}
+
 			if len(md.LastSegmentRemainder) > 0 {
 				return nil
 			}
 
 			if cw == nil {
-				cw, err = storage.NewWritable(w, []cid.Cid{md.LastSegment.RootCid()}, carv2.WriteAsCarV1(true), carv2.AllowDuplicatePuts(params.Duplicates.Bool()))
+				var roots []cid.Cid
+				if isNotFound {
+					roots = emptyRoot
+				} else {
+					roots = []cid.Cid{md.LastSegment.RootCid()}
+				}
+
+				cw, err = storage.NewWritable(w, roots, carv2.WriteAsCarV1(true), carv2.AllowDuplicatePuts(params.Duplicates.Bool()))
 				if err != nil {
 					// io.PipeWriter.CloseWithError always returns nil.
 					_ = w.CloseWithError(err)
@@ -1149,12 +1163,14 @@ func (api *CarBackend) GetCAR(ctx context.Context, p path.ImmutablePath, params
 				blockBuffer = nil
 			}
 
-			params.Duplicates = DuplicateBlocksIncluded
-			err = walkGatewaySimpleSelector(ctx, terminalBlk.Cid(), terminalBlk, []string{}, params, l)
-			// err = walkGatewaySimpleSelector2(ctx, terminalBlk, params.Scope, params.Range, l)
-			if err != nil {
-				return err
+			if !isNotFound {
+				params.Duplicates = DuplicateBlocksIncluded
+				err = walkGatewaySimpleSelector(ctx, terminalBlk.Cid(), terminalBlk, []string{}, params, l)
+				if err != nil {
+					return err
+				}
 			}
+
 			return nil
 		})
 

gateway/errors.go

@@ -13,6 +13,7 @@ import (
 	"github.com/ipfs/boxo/path/resolver"
 	"github.com/ipfs/go-cid"
 	"github.com/ipld/go-ipld-prime/datamodel"
+	"github.com/ipld/go-ipld-prime/schema"
 )
 
 var (
@@ -210,7 +211,7 @@ func webError(w http.ResponseWriter, r *http.Request, c *Config, err error, defa
 // isErrNotFound returns true for IPLD errors that should return 4xx errors (e.g. the path doesn't exist, the data is
 // the wrong type, etc.), rather than issues with just finding and retrieving the data.
 func isErrNotFound(err error) bool {
-	if errors.Is(err, &resolver.ErrNoLink{}) {
+	if errors.Is(err, &resolver.ErrNoLink{}) || errors.Is(err, schema.ErrNoSuchField{}) {
 		return true
 	}