Idempotency for export no longer works.

changelogs/fragments/1199-docker_image-push.yml

@@ -1,2 +1,5 @@
 bugfixes:
   - "docker_image, docker_image_push - adjust image push detection to Docker 29 (https://github.com/ansible-collections/community.docker/pull/1199)."
+known_issues:
+  - "docker_image, docker_image_export - idempotency for archiving images depends on whether the image IDs used by the image storage backend correspond to the IDs used in the tarball's ``manifest.json`` files.
+     The new default backend in Docker 29 apparently uses image IDs that no longer correspond, whence idempotency no longer works (https://github.com/ansible-collections/community.docker/pull/1199)."

plugins/modules/docker_image.py

@@ -21,6 +21,8 @@ description:
 notes:
   - Building images is done using Docker daemon's API. It is not possible to use BuildKit / buildx this way. Use M(community.docker.docker_image_build)
     to build images with BuildKit.
+  - Exporting images is generally not idempotent. It depends on whether the image ID equals the IDs found in the generated tarball's C(manifest.json).
+    This was the case with the default storage backend up to Docker 28, but seems to have changed in Docker 29.
 extends_documentation_fragment:
   - community.docker._docker.api_documentation
   - community.docker._attributes

plugins/modules/docker_image_export.py

@@ -28,7 +28,13 @@ attributes:
   diff_mode:
     support: none
   idempotent:
-    support: full
+    support: partial
+    details:
+      - Whether the module is idempotent depends on the storage API used for images,
+        which determines how the image ID is computed. The idempotency check needs
+        that the image ID equals the ID stored in archive's C(manifest.json).
+        This seemed to have worked fine with the default storage backend up to Docker 28,
+        but seems to have changed in Docker 29.
 
 options:
   names:

tests/integration/targets/docker_image/tasks/tests/options.yml

@@ -256,6 +256,10 @@
 - ansible.builtin.assert:
     that:
       - archive_image_2 is not changed
+  when: docker_cli_version is version("29.0.0", "<")
+  # Apparently idempotency no longer works with the default storage backend
+  # in Docker 29.0.0.
+  # https://github.com/ansible-collections/community.docker/pull/1199
 
 - name: Archive image 3rd time, should overwrite due to different id
   community.docker.docker_image:

tests/integration/targets/docker_image_export/tasks/tests/basic.yml

@@ -67,3 +67,7 @@
         manifests_json: "{{ manifests.results | map(attribute='stdout') | map('from_json') }}"
         manifest_json_images: "{{ item.2 | map(attribute='Config') | map('regex_replace', '.json$', '') | map('regex_replace', '^blobs/sha256/', '') | sort }}"
         export_image_ids: "{{ item.1 | map('regex_replace', '^sha256:', '') | unique | sort }}"
+      when: docker_cli_version is version("29.0.0", "<")
+      # Apparently idempotency no longer works with the default storage backend
+      # in Docker 29.0.0.
+      # https://github.com/ansible-collections/community.docker/pull/1199