Add RefreshState to the eval context

Since plan uses the state as a scratch space for evaluation, we need an
entirely separate state to store the refreshed resources values during
planning. Add a RefreshState method to the EvalContext to retrieve a
state used only for refreshing resources.

terraform/eval_context.go

@@ -152,6 +152,11 @@ type EvalContext interface {
 	// the global state.
 	State() *states.SyncState
 
+	// RefreshState returns a wrapper object that provides safe concurrent
+	// access to the state used to store the most recently refreshed resource
+	// values.
+	RefreshState() *states.SyncState
+
 	// InstanceExpander returns a helper object for tracking the expansion of
 	// graph nodes during the plan phase in response to "count" and "for_each"
 	// arguments.

terraform/eval_context_builtin.go

@@ -71,6 +71,7 @@ type BuiltinEvalContext struct {
 	ProvisionerLock       *sync.Mutex
 	ChangesValue          *plans.ChangesSync
 	StateValue            *states.SyncState
+	RefreshStateValue     *states.SyncState
 	InstanceExpanderValue *instances.Expander
 }
 
@@ -350,6 +351,10 @@ func (ctx *BuiltinEvalContext) State() *states.SyncState {
 	return ctx.StateValue
 }
 
+func (ctx *BuiltinEvalContext) RefreshState() *states.SyncState {
+	return ctx.RefreshStateValue
+}
+
 func (ctx *BuiltinEvalContext) InstanceExpander() *instances.Expander {
 	return ctx.InstanceExpanderValue
 }

terraform/eval_context_mock.go

@@ -126,6 +126,9 @@ type MockEvalContext struct {
 	StateCalled bool
 	StateState  *states.SyncState
 
+	RefreshStateCalled bool
+	RefreshStateState  *states.SyncState
+
 	InstanceExpanderCalled   bool
 	InstanceExpanderExpander *instances.Expander
 }
@@ -338,6 +341,11 @@ func (c *MockEvalContext) State() *states.SyncState {
 	return c.StateState
 }
 
+func (c *MockEvalContext) RefreshState() *states.SyncState {
+	c.RefreshStateCalled = true
+	return c.RefreshStateState
+}
+
 func (c *MockEvalContext) InstanceExpander() *instances.Expander {
 	c.InstanceExpanderCalled = true
 	return c.InstanceExpanderExpander