starlarkstruct Freeze() can cause a stack overflow if circular references are involved

[oprypin]

If starlarkstruct.Struct is exposed to the starlark environment, and one creates a struct with a circular reference, and causes it to be frozen, then there will be a stack overflow.

A module with this Starlark content should reproduce the issue, but I haven't double-checked that it actually does:

def make_struct():
    items = []
    foo = struct(items=items)
    foo.items.append(foo)
    return foo

toplevel = make_struct()

A fix for this issue could be done as follows:

--- a/starlarkstruct/struct.go
+++ b/starlarkstruct/struct.go
@@ -99,6 +99,7 @@ func FromStringDict(constructor starlark
 type Struct struct {
        constructor starlark.Value
        entries     entries // sorted by name
+       frozen      bool
 }
 
 // Default is the default constructor for structs.
@@ -172,8 +173,11 @@ func (s *Struct) Hash() (uint32, error) 
        return x, nil
 }
 func (s *Struct) Freeze() {
-       for _, e := range s.entries {
-               e.value.Freeze()
+       if !s.frozen {
+               s.frozen = true
+               for _, e := range s.entries {
+                       e.value.Freeze()
+               }
        }
 }

[adonovan]

Thanks, that fix is correct. The Freeze function should give better guidance about setting the flag before descending for precisely this reason. Do you want to send a PR?

[oprypin]

I'll try to make a PR, hopefully creating a test will not be too hard :)