Display tags diff during diff and apply commands

.rubocop_todo.yml

@@ -257,6 +257,7 @@ Metrics/CyclomaticComplexity:
     - 'lib/stack_master/aws_driver/s3.rb'
     - 'lib/stack_master/cli.rb'
     - 'lib/stack_master/parameter_resolvers/latest_container.rb'
+    - 'lib/stack_master/stack.rb'
     - 'lib/stack_master/stack_definition.rb'
     - 'lib/stack_master/stack_events/streamer.rb'
 
@@ -276,8 +277,6 @@ Metrics/MethodLength:
     - 'lib/stack_master/config.rb'
     - 'lib/stack_master/paged_response_accumulator.rb'
     - 'lib/stack_master/parameter_resolvers/latest_container.rb'
-    - 'lib/stack_master/parameter_resolvers/stack_output.rb'
-    - 'lib/stack_master/parameter_validator.rb'
     - 'lib/stack_master/prompter.rb'
     - 'lib/stack_master/security_group_finder.rb'
     - 'lib/stack_master/sso_group_id_finder.rb'
@@ -306,6 +305,7 @@ Metrics/PerceivedComplexity:
   Exclude:
     - 'lib/stack_master/aws_driver/s3.rb'
     - 'lib/stack_master/cli.rb'
+    - 'lib/stack_master/stack.rb'
     - 'lib/stack_master/stack_definition.rb'
 
 Naming/AccessorMethodName:

CHANGELOG.md

@@ -12,10 +12,12 @@ The format is based on [Keep a Changelog], and this project adheres to
 
 ### Changed
 
-- Resolve style issues identified by RuboCop ([#396])
+- Display Tags diff (stack tags) in `stack_master diff` and `stack_master apply` commands. ([#397])
+- Resolve style issues identified by RuboCop. ([#396])
 
 [Unreleased]: https://github.com/envato/stack_master/compare/v2.17.1...HEAD
 [#396]: https://github.com/envato/stack_master/pull/396
+[#397]: https://github.com/envato/stack_master/pull/397
 
 ## [2.17.1] - 2025-12-19
 

features/diff.feature

@@ -177,3 +177,77 @@ Feature: Diff command
     | +        "GroupDescription": "Test SG 2",          |
     Then the exit status should be 0
 
+  Scenario: Run diff showing tags added when current stack has no tags
+    Given a file named "stack_master.yml" with:
+      """
+      stacks:
+        us_east_1:
+          myapp_vpc:
+            template: myapp_vpc.json
+            tags:
+              Application: myapp
+              Environment: staging
+      """
+    And a directory named "parameters"
+    And a file named "parameters/myapp_vpc.yml" with:
+      """
+      KeyName: my-key
+      """
+    And a directory named "templates"
+    And a file named "templates/myapp_vpc.json" with:
+      """
+      {
+        "Description": "Test template",
+        "AWSTemplateFormatVersion": "2010-09-09",
+        "Parameters": {
+          "KeyName": {
+            "Description": "Key Name",
+            "Type": "String"
+          }
+        },
+        "Resources": {
+          "TestSg": {
+            "Type": "AWS::EC2::SecurityGroup",
+            "Properties": {
+              "GroupDescription": "Test SG",
+              "VpcId": {
+                "Ref": "VpcId"
+              }
+            }
+          }
+        }
+      }
+      """
+    And I stub the following stacks:
+      | stack_id | stack_name | parameters          | region    |
+      | 1        | myapp-vpc  | KeyName=changed-key | us-east-1 |
+    And I stub a template for the stack "myapp-vpc":
+      """
+      {
+        "Description": "Test template",
+        "AWSTemplateFormatVersion": "2010-09-09",
+        "Parameters": {
+          "KeyName": {
+            "Description": "Key Name",
+            "Type": "String"
+          }
+        },
+        "Resources": {
+          "TestSg": {
+            "Type": "AWS::EC2::SecurityGroup",
+            "Properties": {
+              "GroupDescription": "Test SG",
+              "VpcId": {
+                "Ref": "VpcId"
+              }
+            }
+          }
+        }
+      }
+      """
+    When I run `stack_master diff us-east-1 myapp-vpc --trace`
+    Then the output should contain all of these lines:
+      | Tags diff: |
+      | +Application: myapp |
+      | +Environment: staging |
+    And the exit status should be 0

features/step_definitions/stack_steps.rb

@@ -31,6 +31,7 @@ def extract_hash_from_kv_string(string)
   table.hashes.each do |row|
     row.symbolize_keys!
     row[:parameters] = StackMaster::Utils.hash_to_aws_parameters(extract_hash_from_kv_string(row[:parameters]))
+    row[:tags] = StackMaster::Utils.hash_to_aws_tags(extract_hash_from_kv_string(row[:tags])) if row.key?(:tags)
     outputs = extract_hash_from_kv_string(row[:outputs]).each_with_object([]) do |(k, v), array|
       array << OpenStruct.new(output_key: k, output_value: v)
     end

lib/stack_master/stack.rb

@@ -41,11 +41,15 @@ def self.find(region, stack_name)
       template_format = TemplateUtils.identify_template_format(template_body)
       stack_policy_body ||= cf.get_stack_policy({ stack_name: stack_name }).stack_policy_body
       outputs = cf_stack.outputs
+      tags = cf_stack.tags&.each_with_object({}) do |tag_struct, tags_hash|
+        tags_hash[tag_struct.key] = tag_struct.value
+      end || {}
 
       new(region: region,
           stack_name: stack_name,
           stack_id: cf_stack.stack_id,
           parameters: parameters,
+          tags: tags,
           template_body: template_body,
           template_format: template_format,
           outputs: outputs,

lib/stack_master/stack_differ.rb

@@ -61,9 +61,34 @@ def parameters_diff
                                after: proposed_parameters)
     end
 
+    def tags_different?
+      tags_diff.different?
+    end
+
+    def tags_diff
+      @tags_diff ||= Diff.new(name: 'Tags',
+                              before: current_tags,
+                              after: proposed_tags)
+    end
+
+    def current_tags
+      tags_hash = @current_stack&.tags
+      return '' if tags_hash.nil? || tags_hash.empty?
+
+      YAML.dump(sort_params(tags_hash))
+    end
+
+    def proposed_tags
+      tags_hash = @proposed_stack.tags
+      return '' if tags_hash.nil? || tags_hash.empty?
+
+      YAML.dump(sort_params(tags_hash))
+    end
+
     def output_diff
       body_diff.display
       parameters_diff.display
+      tags_diff.display
 
       StackMaster.stdout.puts ' * can not tell if NoEcho parameters are different.' unless noecho_keys.empty?
       StackMaster.stdout.puts 'No stack found' if @current_stack.nil?

lib/stack_master/test_driver/cloud_formation.rb

@@ -28,6 +28,14 @@ def parameters
                          parameter_value: hash[:parameter_value])
         end
       end
+
+      def tags
+        return [] if @tags.nil?
+
+        @tags.map do |hash|
+          OpenStruct.new(key: hash[:key], value: hash[:value])
+        end
+      end
     end
 
     class StackEvent

spec/stack_master/stack_differ_spec.rb

@@ -56,6 +56,94 @@
         expect { differ.output_diff }.to_not output(/No stack found/).to_stdout
       end
     end
+
+    context 'tags diff' do
+      context 'when tags are added on a new proposal' do
+        let(:stack) do
+          StackMaster::Stack.new(
+            stack_name: stack_name,
+            region: region,
+            template_body: '{}',
+            template_format: :json,
+            parameters: {},
+            tags: {}
+          )
+        end
+        let(:proposed_stack) do
+          StackMaster::Stack.new(
+            stack_name: stack_name,
+            region: region,
+            template_body: '{}',
+            template_format: :json,
+            parameters: {},
+            tags: { 'Application' => 'myapp', 'Environment' => 'staging' }
+          )
+        end
+
+        it 'prints a tags diff header and one-line additions for each tag' do
+          expect { differ.output_diff }.to output(/Tags diff:/).to_stdout
+          expect { differ.output_diff }.to output(/\+Application: myapp/).to_stdout
+          expect { differ.output_diff }.to output(/\+Environment: staging/).to_stdout
+        end
+      end
+
+      context 'when tags are unchanged and empty' do
+        let(:stack) do
+          StackMaster::Stack.new(
+            stack_name: stack_name,
+            region: region,
+            template_body: '{}',
+            template_format: :json,
+            parameters: {},
+            tags: {}
+          )
+        end
+        let(:proposed_stack) do
+          StackMaster::Stack.new(
+            stack_name: stack_name,
+            region: region,
+            template_body: '{}',
+            template_format: :json,
+            parameters: {},
+            tags: {}
+          )
+        end
+
+        it 'prints Tags diff: No changes' do
+          expect { differ.output_diff }.to output(/Tags diff: No changes/).to_stdout
+        end
+      end
+
+      context 'when tags are modified with additions and removals' do
+        let(:stack) do
+          StackMaster::Stack.new(
+            stack_name: stack_name,
+            region: region,
+            template_body: '{}',
+            template_format: :json,
+            parameters: {},
+            tags: { 'Application' => 'old', 'Environment' => 'staging' }
+          )
+        end
+        let(:proposed_stack) do
+          StackMaster::Stack.new(
+            stack_name: stack_name,
+            region: region,
+            template_body: '{}',
+            template_format: :json,
+            parameters: {},
+            tags: { 'Application' => 'new', 'Owner' => 'team' }
+          )
+        end
+
+        it 'prints +/- lines for changed/added/removed tags, one per line' do
+          expect { differ.output_diff }.to output(/-Application: old/).to_stdout
+          expect { differ.output_diff }.to output(/\+Application: new/).to_stdout
+          expect { differ.output_diff }.to output(/-Environment: staging/).to_stdout
+          expect { differ.output_diff }.to output(/\+Owner: team/).to_stdout
+        end
+      end
+    end
   end
 
   describe '#single_param_update?' do

spec/stack_master/stack_spec.rb

@@ -29,6 +29,10 @@
                 creation_time: Time.now,
                 stack_status: 'UPDATE_COMPLETE',
                 parameters: parameters,
+                tags: [
+                  { key: 'Environment', value: 'staging' },
+                  { key: 'Application', value: 'myapp' }
+                ],
                 notification_arns: ['test_arn'],
                 role_arn: 'test_service_role_arn'
               }
@@ -62,6 +66,10 @@
       it 'sets the stack policy' do
         expect(stack.stack_policy_body).to eq stack_policy_body
       end
+
+      it 'sets tags' do
+        expect(stack.tags).to eq({ 'Environment' => 'staging', 'Application' => 'myapp' })
+      end
     end
 
     context 'when the stack does not exist in AWS' do