fix: fix a panic when deleting

hamt/hamt.go

@@ -489,10 +489,28 @@ func (ds *Shard) modifyValue(ctx context.Context, hv *hashBits, key string, val
 				//       structures, this will help to normalize them.
 				return ds.childer.rm(idx)
 			case 1:
-				nchild := child.childer.children[0]
-				if nchild.isValueNode() {
+				// The single child _should_ be a value by
+				// induction. However, we allow for it to be a
+				// shard in case an implementation is broken.
+
+				// Have we loaded the child? Prefer that.
+				schild := child.childer.child(0)
+				if schild != nil {
+					if schild.isValueNode() {
+						ds.childer.set(schild, i)
+					}
+					return nil
+				}
+
+				// Otherwise, work with the link.
+				slnk := child.childer.link(0)
+				lnkType, err := child.childer.sd.childLinkType(slnk)
+				if err != nil {
+					return err
+				}
+				if lnkType == shardValueLink {
 					// sub-shard with a single value element, collapse it
-					ds.childer.set(nchild, i)
+					ds.childer.setLink(slnk, i)
 				}
 				return nil
 			}
@@ -517,6 +535,8 @@ type childer struct {
 	sd       *Shard
 	dserv    ipld.DAGService
 	bitfield bitfield.Bitfield
+
+	// Only one of links/children will be non-nil for every child/link.
 	links    []*ipld.Link
 	children []*Shard
 }
@@ -572,6 +592,12 @@ func (s *childer) insert(key string, lnk *ipld.Link, idx int) error {
 
 func (s *childer) set(sd *Shard, i int) {
 	s.children[i] = sd
+	s.links[i] = nil
+}
+
+func (s *childer) setLink(lnk *ipld.Link, i int) {
+	s.children[i] = nil
+	s.links[i] = lnk
 }
 
 func (s *childer) rm(childIndex int) error {

hamt/hamt_test.go

@@ -268,6 +268,45 @@ func TestRemoveElems(t *testing.T) {
 	}
 }
 
+func TestRemoveAfterMarshal(t *testing.T) {
+	ds := mdtest.Mock()
+	dirs, s, err := makeDir(ds, 500)
+	if err != nil {
+		t.Fatal(err)
+	}
+	nd, err := s.Node()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	s, err = NewHamtFromDag(ds, nd)
+
+	ctx := context.Background()
+
+	shuffle(time.Now().UnixNano(), dirs)
+
+	for i, d := range dirs {
+		err := s.Remove(ctx, d)
+		if err != nil {
+			t.Fatalf("%d/%d: %s", i, len(dirs), err)
+		}
+	}
+
+	nd, err = s.Node()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if len(nd.Links()) > 0 {
+		t.Fatal("shouldnt have any links here")
+	}
+
+	err = s.Remove(ctx, "doesnt exist")
+	if err != os.ErrNotExist {
+		t.Fatal("expected error does not exist")
+	}
+}
+
 func TestSetAfterMarshal(t *testing.T) {
 	ds := mdtest.Mock()
 	_, s, err := makeDir(ds, 300)