continue on output errors during Input

Module outputs may not have complete information during Input, because
it happens before refresh. Continue process on output interpolation
errors during the Input walk.

terraform/eval_output.go

@@ -41,15 +41,16 @@ type EvalWriteOutput struct {
 	Name      string
 	Sensitive bool
 	Value     *config.RawConfig
+	// ContinueOnErr allows interpolation to fail during Input
+	ContinueOnErr bool
 }
 
 // TODO: test
 func (n *EvalWriteOutput) Eval(ctx EvalContext) (interface{}, error) {
+	// This has to run before we have a state lock, since interpolation also
+	// reads the state
 	cfg, err := ctx.Interpolate(n.Value, nil)
-	if err != nil {
-		// Log error but continue anyway
-		log.Printf("[WARN] Output interpolation %q failed: %s", n.Name, err)
-	}
+	// handle the error after we have the module from the state
 
 	state, lock := ctx.State()
 	if state == nil {
@@ -59,13 +60,28 @@ func (n *EvalWriteOutput) Eval(ctx EvalContext) (interface{}, error) {
 	// Get a write lock so we can access this instance
 	lock.Lock()
 	defer lock.Unlock()
-
 	// Look for the module state. If we don't have one, create it.
 	mod := state.ModuleByPath(ctx.Path())
 	if mod == nil {
 		mod = state.AddModule(ctx.Path())
 	}
 
+	// handling the interpolation error
+	if err != nil {
+		if n.ContinueOnErr {
+			log.Printf("[ERROR] Output interpolation %q failed: %s", n.Name, err)
+			// if we're continueing, make sure the output is included, and
+			// marked as unknown
+			mod.Outputs[n.Name] = &OutputState{
+				Type:  "string",
+				Value: config.UnknownVariableValue,
+			}
+			return nil, EvalEarlyExitError{}
+		}
+
+		return nil, err
+	}
+
 	// Get the value from the config
 	var valueRaw interface{} = config.UnknownVariableValue
 	if cfg != nil {

terraform/node_output.go

@@ -72,8 +72,18 @@ func (n *NodeApplyableOutput) EvalTree() EvalNode {
 	return &EvalSequence{
 		Nodes: []EvalNode{
 			&EvalOpFilter{
-				Ops: []walkOperation{walkRefresh, walkPlan, walkApply,
-					walkInput, walkValidate},
+				// Don't let interpolation errors stop Input, since it happens
+				// before Refresh.
+				Ops: []walkOperation{walkInput},
+				Node: &EvalWriteOutput{
+					Name:          n.Config.Name,
+					Sensitive:     n.Config.Sensitive,
+					Value:         n.Config.RawConfig,
+					ContinueOnErr: true,
+				},
+			},
+			&EvalOpFilter{
+				Ops: []walkOperation{walkRefresh, walkPlan, walkApply, walkValidate},
 				Node: &EvalWriteOutput{
 					Name:      n.Config.Name,
 					Sensitive: n.Config.Sensitive,