Add an InstanceState.Set method to set all fields

We con no longer copy an InstanceState via a simple
dereference+assignment because of the mutex which can't be copied. This
adds a set method to properly set all field from another InstanceState,
and take the appropriate locks while doing so.

terraform/state.go

@@ -1332,6 +1332,21 @@ func (i *InstanceState) init() {
 	i.Ephemeral.init()
 }
 
+// Copy all the Fields from another InstanceState
+func (i *InstanceState) Set(from *InstanceState) {
+	i.Lock()
+	defer i.Unlock()
+
+	from.Lock()
+	defer from.Unlock()
+
+	i.ID = from.ID
+	i.Attributes = from.Attributes
+	i.Ephemeral = from.Ephemeral
+	i.Meta = from.Meta
+	i.Tainted = from.Tainted
+}
+
 func (i *InstanceState) DeepCopy() *InstanceState {
 	copy, err := copystructure.LockedCopy(i)
 	if err != nil {

terraform/state_add.go

@@ -219,7 +219,7 @@ func stateAddFunc_Instance_Instance(s *State, fromAddr, addr *ResourceAddress, r
 	instance := instanceRaw.(*InstanceState)
 
 	// Set it
-	*instance = *src
+	instance.Set(src)
 
 	return nil
 }