Avoid leaking implementation details of the exporter (#206)

The current implementation for forcibly exporting fields relies on
the reflect.Value.Addr method for the parent struct value,
where it copies a non-addressable struct onto to the heap so
that it is addressable. However, this action leaks a subtle detail
of how the internal implementation works since the accessed field
for within a struct is only addressable if and only if the
parent struct is also addressable.

Modify the implementation to avoid leaking this implementation detail
by shallow copying the accessed unexported field to remove any
notion of addressability if the parent struct is also unaddressable.

Fixes #181

1 files changed, 3 insertions, 0 deletions

diff --git a/cmp/compare.go b/cmp/compare.go
index a58ada5..c82c062 100644
--- a/cmp/compare.go
+++ b/cmp/compare.go
@@ -386,6 +386,7 @@ func sanitizeValue(v reflect.Value, t reflect.Type) reflect.Value {
 }
 
 func (s *state) compareStruct(t reflect.Type, vx, vy reflect.Value) {
+	var addr bool
 	var vax, vay reflect.Value // Addressable versions of vx and vy
 
 	var mayForce, mayForceInit bool
@@ -407,6 +408,7 @@ func (s *state) compareStruct(t reflect.Type, vx, vy reflect.Value) {
 				// For retrieveUnexportedField to work, the parent struct must
 				// be addressable. Create a new copy of the values if
 				// necessary to make them addressable.
+				addr = vx.CanAddr() || vy.CanAddr()
 				vax = makeAddressable(vx)
 				vay = makeAddressable(vy)
 			}
@@ -417,6 +419,7 @@ func (s *state) compareStruct(t reflect.Type, vx, vy reflect.Value) {
 				mayForceInit = true
 			}
 			step.mayForce = mayForce
+			step.paddr = addr
 			step.pvx = vax
 			step.pvy = vay
 			step.field = t.Field(i)