Add migration validation system to catch breaking changes early

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

roles/custom/matrix_playbook_migration/defaults/main.yml

@@ -1,9 +1,27 @@
-# SPDX-FileCopyrightText: 2023 - 2025 Slavi Pantaleev
+# SPDX-FileCopyrightText: 2023 - 2026 Slavi Pantaleev
 # SPDX-FileCopyrightText: 2024 Suguru Hirahara
 #
 # SPDX-License-Identifier: AGPL-3.0-or-later
 
 ---
+
+# The version that the user has validated their setup against.
+# When empty, the user will be prompted to set this variable.
+# New users should set this to the current expected version (see below).
+# See `examples/vars.yml` and `matrix_playbook_migration_expected_version` for the recommended value.
+matrix_playbook_migration_validated_version: ''
+
+# The version that the playbook expects the user to have validated against.
+# This is bumped whenever a breaking change is introduced.
+# The value configured here needs to exist in `matrix_playbook_migration_breaking_changes` as well.
+matrix_playbook_migration_expected_version: "v2026.03.23.0"
+
+# A list of breaking changes, used to inform users what changed between their validated version and the expected version.
+matrix_playbook_migration_breaking_changes:
+  - version: "v2026.03.23.0"
+    summary: "Initial migration validation system"
+    changelog_url: "https://github.com/spantaleev/matrix-docker-ansible-deploy/blob/master/CHANGELOG.md#2026-03-22"
+
 # Controls if (`matrix_prometheus_nginxlog_exporter` -> `prometheus_nginxlog_exporter`) validation will run.
 matrix_playbook_migration_matrix_prometheus_nginxlog_exporter_migration_validation_enabled: true
 

roles/custom/matrix_playbook_migration/tasks/main.yml

@@ -1,9 +1,14 @@
-# SPDX-FileCopyrightText: 2022 - 2024 Slavi Pantaleev
+# SPDX-FileCopyrightText: 2022 - 2026 Slavi Pantaleev
 #
 # SPDX-License-Identifier: AGPL-3.0-or-later
 
 ---
 
+- tags:
+    - always
+  block:
+    - ansible.builtin.include_tasks: "{{ role_path }}/tasks/validate_migration_version.yml"
+
 - tags:
     - setup-all
     - install-all

roles/custom/matrix_playbook_migration/tasks/validate_migration_version.yml

@@ -0,0 +1,34 @@
+# SPDX-FileCopyrightText: 2026 Slavi Pantaleev
+#
+# SPDX-License-Identifier: AGPL-3.0-or-later
+
+---
+
+- name: Fail if migration version is not validated (first-time onboarding)
+  ansible.builtin.fail:
+    msg: >-
+      This playbook now uses a migration validation system to help you stay aware of breaking changes.
+
+      It appears that you haven't configured the `matrix_playbook_migration_validated_version` variable yet.
+
+      Please review the changelog (https://github.com/spantaleev/matrix-docker-ansible-deploy/blob/master/CHANGELOG.md)
+      and then add the following to your vars.yml file:
+
+      matrix_playbook_migration_validated_version: {{ matrix_playbook_migration_expected_version }}
+  when: "matrix_playbook_migration_validated_version == ''"
+
+- name: Fail if migration version is outdated
+  ansible.builtin.fail:
+    msg: |-
+      Your validated migration version ({{ matrix_playbook_migration_validated_version }}) is behind the expected version ({{ matrix_playbook_migration_expected_version }}).
+
+      The following breaking changes have been introduced since your last validation:
+
+      {% for item in matrix_playbook_migration_breaking_changes | selectattr('version', '>', matrix_playbook_migration_validated_version) | sort(attribute='version') %}
+      - {{ item.version }}: {{ item.summary }} ({{ item.changelog_url }})
+      {% endfor %}
+
+      After reviewing the above changes and adapting your setup, update your vars.yml:
+
+      matrix_playbook_migration_validated_version: "{{ matrix_playbook_migration_expected_version }}"
+  when: "matrix_playbook_migration_validated_version != '' and matrix_playbook_migration_validated_version < matrix_playbook_migration_expected_version"