godwarf: assert children are not ignored (#2388)

The godwarf package provides two ways to turn a dwarf.Entry into a
godwarf.Tree: LoadTree and EntryToTree. The former doesn't handle
children - it doesn't advance a Reader past them (in fact, it doesn't
even know about a Reader). EntryToTree is only used for variables and
formal param DIEs, which don't have children, and it would very likely
be incorrect to use it for DIEs with children. This patch makes the
function panic if the entry can have children.

pkg/dwarf/godwarf/tree.go

@@ -75,7 +75,7 @@ func LoadTree(off dwarf.Offset, dw *dwarf.Data, staticBase uint64) (*Tree, error
 	if err != nil {
 		return nil, err
 	}
-	r := EntryToTree(e)
+	r := entryToTreeInternal(e)
 	r.Children, err = loadTreeChildren(e, rdr)
 	if err != nil {
 		return nil, err
@@ -90,8 +90,16 @@ func LoadTree(off dwarf.Offset, dw *dwarf.Data, staticBase uint64) (*Tree, error
 	return r, nil
 }
 
-// EntryToTree converts a single entry, without children to a *Tree object
+// EntryToTree converts a single entry, without children, to a *Tree object.
 func EntryToTree(entry *dwarf.Entry) *Tree {
+	if entry.Children {
+		panic(fmt.Sprintf("EntryToTree called on entry with children; " +
+			"LoadTree should have been used instead. entry: %+v", entry))
+	}
+	return entryToTreeInternal(entry)
+}
+
+func entryToTreeInternal(entry *dwarf.Entry) *Tree {
 	return &Tree{Entry: entry, Offset: entry.Offset, Tag: entry.Tag}
 }
 
@@ -108,7 +116,7 @@ func loadTreeChildren(e *dwarf.Entry, rdr *dwarf.Reader) ([]*Tree, error) {
 		if e.Tag == 0 {
 			break
 		}
-		child := EntryToTree(e)
+		child := entryToTreeInternal(e)
 		child.Children, err = loadTreeChildren(e, rdr)
 		if err != nil {
 			return nil, err