Collapse when we completely delete

amt_test.go

@@ -73,6 +73,23 @@ func TestBasicSetGet(t *testing.T) {
 
 }
 
+func TestRoundTrip(t *testing.T) {
+	bs := cbor.NewCborStore(newMockBlocks())
+	ctx := context.Background()
+	a := NewAMT(bs)
+	emptyCid, err := a.Flush(ctx)
+	require.NoError(t, err)
+
+	k := uint64(100000)
+	assertSet(t, a, k, "foo")
+	assertDelete(t, a, k)
+
+	c, err := a.Flush(ctx)
+	require.NoError(t, err)
+
+	require.Equal(t, emptyCid, c)
+}
+
 func TestOutOfRange(t *testing.T) {
 	ctx := context.Background()
 	bs := cbor.NewCborStore(newMockBlocks())

go.sum

@@ -4,7 +4,6 @@ github.com/davecgh/go-spew v1.1.0 h1:ZDRjVQ15GmhC3fiQ8ni8+OwkZQO4DARzQgrnXU1Liz8
 github.com/davecgh/go-spew v1.1.0/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
 github.com/davecgh/go-spew v1.1.1 h1:vj9j/u1bqnvCEfJOwUhtlOARqs3+rkHYY13jYWTU97c=
 github.com/davecgh/go-spew v1.1.1/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
-github.com/filecoin-project/go-amt-ipld v1.0.0 h1:jJteqdKcwFYoN9Wgs7MzDWVWvRWUfhhFo4bltJ7wrus=
 github.com/gogo/protobuf v1.3.1 h1:DqDEcV5aeaTmdFBePNpYsp3FlcVH/2ISVVM9Qf8PSls=
 github.com/gogo/protobuf v1.3.1/go.mod h1:SlYgWuQ5SjCEi6WLHjHCa1yvBfUnHcTbrrZtXPKa29o=
 github.com/google/go-cmp v0.4.0/go.mod h1:v8dTdLbMG2kIc/vJvl+f65V22dbkXbowE6jgT/gNBxE=

node.go

@@ -83,15 +83,19 @@ func newNode(nd internal.Node, allowEmpty, expectLeaf bool) (*node, error) {
 }
 
 func (nd *node) collapse(ctx context.Context, bs cbor.IpldStore, height int) (int, error) {
-	if nd.links[0] == nil {
-		return height, nil
-	}
+	// If we have any links going "to the right", we can't collapse any
+	// more.
 	for _, l := range nd.links[1:] {
 		if l != nil {
 			return height, nil
 		}
 	}
 
+	// If we have _no_ links, we've collapsed everything.
+	if nd.links[0] == nil {
+		return 0, nil
+	}
+
 	// only one child, collapse it.
 
 	subn, err := nd.links[0].load(ctx, bs, height-1)