add custom diff for LBListener forward options

config/elbv2/config.go

@@ -4,10 +4,17 @@
 
 package elbv2
 
-import "github.com/crossplane/upjet/pkg/config"
+import (
+	"regexp"
+	"strings"
+
+	"github.com/hashicorp/terraform-plugin-sdk/v2/terraform"
+
+	"github.com/crossplane/upjet/pkg/config"
+)
 
 // Configure adds configurations for the elbv2 group.
-func Configure(p *config.Provider) {
+func Configure(p *config.Provider) { //nolint:gocyclo
 	p.AddResourceConfigurator("aws_lb", func(r *config.Resource) {
 		r.ExternalName.OmittedFields = append(r.ExternalName.OmittedFields, "name_prefix")
 		r.References = config.References{
@@ -44,6 +51,64 @@ func Configure(p *config.Provider) {
 				TerraformName: "aws_lb_target_group",
 			},
 		}
+
+		// lb_listener schema allows to configure "default_action" with type
+		// "forward", in 2 different ways.
+		// 1. you can specify, default_action.0.forward, which allows configuring
+		//    multiple target groups.
+		// 2. you can specify default_action.0.target_group_arn if you want
+		//    to configure only one target group.
+		// Former is a more general way, and latter is more of a shortcut for
+		// a specific case, which can already be expressed with #1.
+		// TF implementation instructs to specify either #1 or #2, not both.
+		// However, they both end up in the state redundantly and cause
+		// unnecessary diff.
+		r.TerraformCustomDiff = func(diff *terraform.InstanceDiff, _ *terraform.InstanceState, _ *terraform.ResourceConfig) (*terraform.InstanceDiff, error) { //nolint:gocyclo
+			// skip no diff or destroy diffs
+			if diff == nil || diff.Empty() || diff.Destroy || diff.Attributes == nil {
+				return diff, nil
+			}
+			duplicatedAction := ""
+			for k, attrDiff := range diff.Attributes {
+				// user specified the "default_action.0.target_group_arn" and
+				// "default_action.0.forward" is not specified in config.
+				// In that case, default_action.0.forward is populated
+				// by the AWS API, which can cause an unnecessary diff,
+				// trying to remove that "auto-populated" element.
+				if regexp.MustCompile(`^default_action\.\d+\.forward\.#$`).MatchString(k) &&
+					attrDiff.New == "0" && attrDiff.Old == "1" {
+					delete(diff.Attributes, k)
+					// save this attribute path to remove remaining diffs
+					// of its nested fields if any in a second pass
+					duplicatedAction = strings.TrimSuffix(k, ".#")
+				}
+				// this is the same case as above
+				if regexp.MustCompile(`^default_action\.\d+\.forward\.\d+\.target_group\.#$`).MatchString(k) &&
+					attrDiff.New == "0" && attrDiff.Old == "1" {
+					delete(diff.Attributes, k)
+				}
+				// this is the same case but vice versa. user specified
+				// forward target via default_action.0.forward, and
+				// default_action.0.target_group_arn is omitted.
+				// In that case, default_action.0.target_group_arn is populated
+				// by the AWS API and ends up in the state, causing
+				// an unnecessary diff, which we omit here.
+				if regexp.MustCompile(`^default_action\.\d+\.target_group_arn$`).MatchString(k) &&
+					attrDiff.New == "" && attrDiff.Old != "" && attrDiff.NewRemoved {
+					delete(diff.Attributes, k)
+				}
+			}
+			// if we have caught an unnecessary diff for default_action.0.forward, remove
+			// any sub-element diffs of it.
+			if duplicatedAction != "" {
+				for k := range diff.Attributes {
+					if strings.HasPrefix(k, duplicatedAction) {
+						delete(diff.Attributes, k)
+					}
+				}
+			}
+			return diff, nil
+		}
 	})
 
 	p.AddResourceConfigurator("aws_lb_target_group", func(r *config.Resource) {