Determine latest base image available

Change-Id: I616c7ea903114b084408b52dce28696b13dcbe5d
Closes-Bug: #1900884
Closes-Bug: #1900975

defaults/main.yml

@@ -201,7 +201,7 @@ lxc_image_cache_server_mirrors:
   - https://images.linuxcontainers.org
 
 # URL for the image to build our containers
-lxc_hosts_container_image_url: "{{ _lxc_hosts_container_image_url }}"
+lxc_hosts_container_image_url: "{{ _lxc_hosts_container_image_url | default('') }}"
 
 # If enabled the image will be fetched from the legacy [images.linuxcontainers.org].
 lxc_hosts_container_image_download_legacy: false

releasenotes/notes/lxc_image_url-ff6a50ff6c4e612b.yaml

@@ -0,0 +1,6 @@
+---
+fixes:
+  - |
+    Since Ubuntu has dropped older base images, which resulted in all previous
+    tags being broken, we've switched to downloading always latest base image
+    available. This should guarantee that we retrieve relevant images only.

tasks/lxc_cache_prestage.yml

@@ -41,6 +41,19 @@
   when:
     - lxc_hosts_container_image_download_legacy | bool
 
+- name: Determine lxc_hosts_container_image_url when it is not explicitly set
+  set_fact:
+    lxc_hosts_container_image_url: "{{ _lxc_hosts_container_image_url_base ~ '/' ~ lookup('url', _lxc_hosts_container_image_url_base ~ '/' ~ _lxc_hosts_container_image_checksum_file) | regex_findall(_lxc_hosts_container_image_checksum_regexp) | max }}"
+  register: image_url
+  retries: 3
+  delay: 1
+  until: image_url is success
+  when:
+    - not lxc_hosts_container_image_url | bool
+    - _lxc_hosts_container_image_url_base is defined
+    - _lxc_hosts_container_image_checksum_file is defined
+    - _lxc_hosts_container_image_checksum_regexp is defined
+
 - name: Set LXC cache basename
   set_fact:
     cache_basename: "{{ lxc_hosts_container_image_url | basename }}"

vars/ubuntu-16.04.yml

@@ -13,7 +13,9 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-_lxc_hosts_container_image_url: "http://cdimage.ubuntu.com/ubuntu-base/releases/16.04/release/ubuntu-base-16.04.4-base-{{ lxc_cache_map.arch }}.tar.gz"
+_lxc_hosts_container_image_url_base: "http://cdimage.ubuntu.com/ubuntu-base/releases/16.04/release"
+_lxc_hosts_container_image_checksum_file: SHA256SUMS
+_lxc_hosts_container_image_checksum_regexp: "ubuntu-base-16.04.[0-9]-base-{{ lxc_cache_map.arch }}.tar.gz"
 
 _lxc_cache_map:
   distro: ubuntu

vars/ubuntu-18.04.yml

@@ -13,7 +13,9 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-_lxc_hosts_container_image_url: "http://cdimage.ubuntu.com/ubuntu-base/releases/18.04/release/ubuntu-base-18.04.3-base-{{ lxc_cache_map.arch }}.tar.gz"
+_lxc_hosts_container_image_url_base: "http://cdimage.ubuntu.com/ubuntu-base/releases/18.04/release"
+_lxc_hosts_container_image_checksum_file: SHA256SUMS
+_lxc_hosts_container_image_checksum_regexp: "ubuntu-base-18.04.[0-9]-base-{{ lxc_cache_map.arch }}.tar.gz"
 
 _lxc_cache_map:
   distro: ubuntu