add relevant_attributes to the json plan format

Add the resource instances and individual attributes which may have
contributed to the planned changes to the json format of the plan. We
use the existing path encoding for individual attributes, which is
already used in the replace_paths change field.

internal/command/jsonplan/plan.go

@@ -38,6 +38,7 @@ type plan struct {
 	OutputChanges      map[string]change `json:"output_changes,omitempty"`
 	PriorState         json.RawMessage   `json:"prior_state,omitempty"`
 	Config             json.RawMessage   `json:"configuration,omitempty"`
+	RelevantAttributes []resourceAttr    `json:"relevant_attributes,omitempty"`
 }
 
 func newPlan() *plan {
@@ -46,6 +47,13 @@ func newPlan() *plan {
 	}
 }
 
+// resourceAttr contains the address and attribute of an external for the
+// RelevantAttributes in the plan.
+type resourceAttr struct {
+	Resource string          `json:"resource"`
+	Attr     json.RawMessage `json:"attribute"`
+}
+
 // Change is the representation of a proposed change for an object.
 type change struct {
 	// Actions are the actions that will be taken on the object selected by the
@@ -151,6 +159,10 @@ func Marshal(
 		}
 	}
 
+	if err := output.marshalRelevantAttrs(p); err != nil {
+		return nil, fmt.Errorf("error marshaling relevant attributes for external changes: %s", err)
+	}
+
 	// output.ResourceChanges
 	if p.Changes != nil {
 		output.ResourceChanges, err = output.marshalResourceChanges(p.Changes.Resources, schemas)
@@ -482,6 +494,19 @@ func (p *plan) marshalPlannedValues(changes *plans.Changes, schemas *terraform.S
 	return nil
 }
 
+func (p *plan) marshalRelevantAttrs(plan *plans.Plan) error {
+	for _, ra := range plan.RelevantAttributes {
+		addr := ra.Resource.String()
+		path, err := encodePath(ra.Attr)
+		if err != nil {
+			return err
+		}
+
+		p.RelevantAttributes = append(p.RelevantAttributes, resourceAttr{addr, path})
+	}
+	return nil
+}
+
 // omitUnknowns recursively walks the src cty.Value and returns a new cty.Value,
 // omitting any unknowns.
 //
@@ -655,6 +680,17 @@ func encodePaths(pathSet cty.PathSet) (json.RawMessage, error) {
 	jsonPaths := make([]json.RawMessage, 0, len(pathList))
 
 	for _, path := range pathList {
+		jsonPath, err := encodePath(path)
+		if err != nil {
+			return nil, err
+		}
+		jsonPaths = append(jsonPaths, jsonPath)
+	}
+
+	return json.Marshal(jsonPaths)
+}
+
+func encodePath(path cty.Path) (json.RawMessage, error) {
 	steps := make([]json.RawMessage, 0, len(path))
 	for _, step := range path {
 		switch s := step.(type) {
@@ -674,12 +710,5 @@ func encodePaths(pathSet cty.PathSet) (json.RawMessage, error) {
 			return nil, fmt.Errorf("Unsupported path step %#v (%t)", step, step)
 		}
 	}
-		jsonPath, err := json.Marshal(steps)
-		if err != nil {
-			return nil, err
-		}
-		jsonPaths = append(jsonPaths, jsonPath)
-	}
-
-	return json.Marshal(jsonPaths)
+	return json.Marshal(steps)
 }