use recorded changes for outputs

We record output changes in the plan, but don't currently use them for
anything other than display. If we have a wholly known output value
stored in the plan, we should prefer that for apply in order to ensure
consistency with the planned values. This also avoids cases where
evaluation during apply cannot happen correctly, like when all resources
are being removed or we are executing a destroy.

We also need to record output Delete changes when the plan is for
destroy operation. Otherwise without a change, the apply step will
attempt to evaluate the outputs, causing errors, or leaving them in the
state with stale values.

terraform/context_apply_test.go

@@ -9142,10 +9142,13 @@ func TestContext2Apply_plannedDestroyInterpolatedCount(t *testing.T) {
 	}
 
 	// Applying the plan should now succeed
-	_, diags = ctx.Apply()
+	state, diags = ctx.Apply()
 	if diags.HasErrors() {
 		t.Fatalf("apply failed: %s", diags.Err())
 	}
+	if !state.Empty() {
+		t.Fatalf("state not empty: %s\n", state)
+	}
 }
 
 func TestContext2Apply_scaleInMultivarRef(t *testing.T) {

terraform/graph_builder_apply.go

@@ -91,7 +91,7 @@ func (b *ApplyGraphBuilder) Steps() []GraphTransformer {
 		&RootVariableTransformer{Config: b.Config},
 		&ModuleVariableTransformer{Config: b.Config},
 		&LocalTransformer{Config: b.Config},
-		&OutputTransformer{Config: b.Config},
+		&OutputTransformer{Config: b.Config, Changes: b.Changes},
 
 		// Creates all the resource instances represented in the diff, along
 		// with dependency edges against the whole-resource nodes added by

terraform/graph_builder_destroy_plan.go

@@ -72,6 +72,11 @@ func (b *DestroyPlanGraphBuilder) Steps() []GraphTransformer {
 			State:           b.State,
 		},
 
+		&OutputTransformer{
+			Config:  b.Config,
+			Destroy: true,
+		},
+
 		// Attach the state
 		&AttachStateTransformer{State: b.State},
 

terraform/node_output.go

@@ -11,6 +11,7 @@ import (
 	"github.com/hashicorp/terraform/lang"
 	"github.com/hashicorp/terraform/plans"
 	"github.com/hashicorp/terraform/states"
+	"github.com/hashicorp/terraform/tfdiags"
 	"github.com/zclconf/go-cty/cty"
 )
 
@@ -20,6 +21,8 @@ type nodeExpandOutput struct {
 	Addr    addrs.OutputValue
 	Module  addrs.Module
 	Config  *configs.Output
+	Changes []*plans.OutputChangeSrc
+	Destroy bool
 }
 
 var (
@@ -39,16 +42,62 @@ func (n *nodeExpandOutput) temporaryValue() bool {
 }
 
 func (n *nodeExpandOutput) DynamicExpand(ctx EvalContext) (*Graph, error) {
+	if n.Destroy {
+		return n.planDestroyOutputs(ctx)
+	}
+
 	var g Graph
 	expander := ctx.InstanceExpander()
 	for _, module := range expander.ExpandModule(n.Module) {
+		absAddr := n.Addr.Absolute(module)
+
+		// Find any recorded change for this output
+		var change *plans.OutputChangeSrc
+		for _, c := range n.Changes {
+			if c.Addr.String() == absAddr.String() {
+				change = c
+				break
+			}
+		}
+
 		o := &NodeApplyableOutput{
-			Addr:   n.Addr.Absolute(module),
+			Addr:   absAddr,
+			Config: n.Config,
+			Change: change,
+		}
+		log.Printf("[TRACE] Expanding output: adding %s as %T", o.Addr.String(), o)
+		g.Add(o)
+	}
+	return &g, nil
+}
+
+// if we're planing a destroy operation, add destroy nodes for all root outputs
+// in the state.
+func (n *nodeExpandOutput) planDestroyOutputs(ctx EvalContext) (*Graph, error) {
+	// we only need to plan destroying root outputs
+	// Other module outputs may be used during destruction by providers that
+	// need to interpolate values.
+	if !n.Module.IsRoot() {
+		return nil, nil
+	}
+
+	state := ctx.State()
+	if state == nil {
+		return nil, nil
+	}
+
+	ms := state.Module(addrs.RootModuleInstance)
+
+	var g Graph
+	for _, output := range ms.OutputValues {
+		o := &NodeDestroyableOutput{
+			Addr:   output.Addr,
 			Config: n.Config,
 		}
 		log.Printf("[TRACE] Expanding output: adding %s as %T", o.Addr.String(), o)
 		g.Add(o)
 	}
+
 	return &g, nil
 }
 
@@ -108,6 +157,8 @@ func (n *nodeExpandOutput) References() []*addrs.Reference {
 type NodeApplyableOutput struct {
 	Addr   addrs.AbsOutputValue
 	Config *configs.Output // Config is the output in the config
+	// If this is being evaluated during apply, we may have a change recorded already
+	Change *plans.OutputChangeSrc
 }
 
 var (
@@ -199,11 +250,31 @@ func (n *NodeApplyableOutput) References() []*addrs.Reference {
 
 // GraphNodeExecutable
 func (n *NodeApplyableOutput) Execute(ctx EvalContext, op walkOperation) error {
+	var diags tfdiags.Diagnostics
+	state := ctx.State()
+	if state == nil {
+		return nil
+	}
+
+	changes := ctx.Changes() // may be nil, if we're not working on a changeset
+
+	val := cty.UnknownVal(cty.DynamicPseudoType)
+	changeRecorded := n.Change != nil
+	// we we have a change recorded, we don't need to re-evaluate if the value
+	// was known
+	if changeRecorded {
+		var err error
+		val, err = n.Change.After.Decode(cty.DynamicPseudoType)
+		diags = diags.Append(err)
+	}
+
+	// If there was no change recorded, or the recorded change was not wholly
+	// known, then we need to re-evaluate the output
+	if !changeRecorded || !val.IsWhollyKnown() {
 		// This has to run before we have a state lock, since evaluation also
 		// reads the state
-	val, diags := ctx.EvaluateExpr(n.Config.Expr, cty.DynamicPseudoType, nil)
+		val, diags = ctx.EvaluateExpr(n.Config.Expr, cty.DynamicPseudoType, nil)
 		// We'll handle errors below, after we have loaded the module.
-
 		// Outputs don't have a separate mode for validation, so validate
 		// depends_on expressions here too
 		diags = diags.Append(validateDependsOn(ctx, n.Config.DependsOn))
@@ -219,14 +290,8 @@ func (n *NodeApplyableOutput) Execute(ctx EvalContext, op walkOperation) error {
 				Subject:  n.Config.DeclRange.Ptr(),
 			})
 		}
-
-	state := ctx.State()
-	if state == nil {
-		return nil
 	}
 
-	changes := ctx.Changes() // may be nil, if we're not working on a changeset
-
 	// handling the interpolation error
 	if diags.HasErrors() {
 		if flagWarnOutputErrors {
@@ -261,7 +326,7 @@ func (n *NodeApplyableOutput) DotNode(name string, opts *dag.DotOpts) *dag.DotNo
 	}
 }
 
-// NodeDestroyableOutput represents an output that is "destroybale":
+// NodeDestroyableOutput represents an output that is "destroyable":
 // its application will remove the output from the state.
 type NodeDestroyableOutput struct {
 	Addr   addrs.AbsOutputValue
@@ -293,6 +358,32 @@ func (n *NodeDestroyableOutput) Execute(ctx EvalContext, op walkOperation) error
 	if state == nil {
 		return nil
 	}
+
+	changes := ctx.Changes()
+	if changes != nil {
+		change := &plans.OutputChange{
+			Addr: n.Addr,
+			Change: plans.Change{
+				// This is just a weird placeholder delete action since
+				// we don't have an actual prior value to indicate.
+				// FIXME: Generate real planned changes for output values
+				// that include the old values.
+				Action: plans.Delete,
+				Before: cty.NullVal(cty.DynamicPseudoType),
+				After:  cty.NullVal(cty.DynamicPseudoType),
+			},
+		}
+
+		cs, err := change.Encode()
+		if err != nil {
+			// Should never happen, since we just constructed this right above
+			panic(fmt.Sprintf("planned change for %s could not be encoded: %s", n.Addr, err))
+		}
+		log.Printf("[TRACE] planDestroyOutput: Saving %s change for %s in changeset", change.Action, n.Addr)
+		changes.RemoveOutputChange(n.Addr) // remove any existing planned change, if present
+		changes.AppendOutputChange(cs)     // add the new planned change
+	}
+
 	state.RemoveOutputValue(n.Addr)
 	return nil
 }

terraform/transform_output.go

@@ -6,6 +6,7 @@ import (
 	"github.com/hashicorp/terraform/addrs"
 	"github.com/hashicorp/terraform/configs"
 	"github.com/hashicorp/terraform/dag"
+	"github.com/hashicorp/terraform/plans"
 )
 
 // OutputTransformer is a GraphTransformer that adds all the outputs
@@ -16,6 +17,11 @@ import (
 // even if the dependent items aren't changing.
 type OutputTransformer struct {
 	Config  *configs.Config
+	Changes *plans.Changes
+
+	// if this is a planed destroy, root outputs are still in the configuration
+	// so we need to record that we wish to remove them
+	Destroy bool
 }
 
 func (t *OutputTransformer) Transform(g *Graph) error {
@@ -40,11 +46,19 @@ func (t *OutputTransformer) transform(g *Graph, c *configs.Config) error {
 	// Add plannable outputs to the graph, which will be dynamically expanded
 	// into NodeApplyableOutputs to reflect possible expansion
 	// through the presence of "count" or "for_each" on the modules.
+
+	var changes []*plans.OutputChangeSrc
+	if t.Changes != nil {
+		changes = t.Changes.Outputs
+	}
+
 	for _, o := range c.Module.Outputs {
 		node := &nodeExpandOutput{
 			Addr:    addrs.OutputValue{Name: o.Name},
 			Module:  c.Path,
 			Config:  o,
+			Changes: changes,
+			Destroy: t.Destroy,
 		}
 		log.Printf("[TRACE] OutputTransformer: adding %s as %T", o.Name, node)
 		g.Add(node)