Add support for hosting Element (on Traefik) at a subpath

roles/custom/matrix-client-element/defaults/main.yml

@@ -38,7 +38,9 @@ matrix_client_element_container_http_host_bind_port: ''
 matrix_client_element_container_labels_traefik_enabled: true
 matrix_client_element_container_labels_traefik_docker_network: "{{ matrix_client_element_container_network }}"
 matrix_client_element_container_labels_traefik_hostname: "{{ matrix_server_fqn_element }}"
-matrix_client_element_container_labels_traefik_rule: "Host(`{{ matrix_client_element_container_labels_traefik_hostname }}`)"
+# The path prefix must either be `/` or not end with a slash (e.g. `/element`).
+matrix_client_element_container_labels_traefik_path_prefix: /
+matrix_client_element_container_labels_traefik_rule: "Host(`{{ matrix_client_element_container_labels_traefik_hostname }}`){% if matrix_client_element_container_labels_traefik_path_prefix != '/' %} && PathPrefix(`{{ matrix_client_element_container_labels_traefik_path_prefix }}`){% endif %}"
 matrix_client_element_container_labels_traefik_entrypoints: web-secure
 matrix_client_element_container_labels_traefik_tls: "{{ matrix_client_element_container_labels_traefik_entrypoints != 'web' }}"
 matrix_client_element_container_labels_traefik_tls_certResolver: default  # noqa var-naming

roles/custom/matrix-client-element/tasks/validate_config.yml

@@ -26,3 +26,23 @@
   when: "vars | dict2items | selectattr('key', 'match', item.old) | list | items2dict"
   with_items:
     - {'old': 'matrix_riot_web_.*', 'new': 'matrix_client_element_.*'}
+
+- when: matrix_client_element_container_labels_traefik_enabled | bool
+  block:
+    - name: Fail if required matrix-client-element Traefik settings not defined
+      ansible.builtin.fail:
+        msg: >-
+          You need to define a required configuration setting (`{{ item }}`).
+      when: "vars[item] == ''"
+      with_items:
+        - matrix_synapse_admin_container_labels_traefik_hostname
+        - matrix_client_element_container_labels_traefik_path_prefix
+
+    # We ensure it doesn't end with a slash, because we handle both (slash and no-slash).
+    # Knowing that `matrix_client_element_container_labels_traefik_path_prefix` does not end with a slash
+    # ensures we know how to set these routes up without having to do "does it end with a slash" checks elsewhere.
+    - name: Fail if matrix_client_element_container_labels_traefik_path_prefix ends with a slash
+      ansible.builtin.fail:
+        msg: >-
+          matrix_client_element_container_labels_traefik_path_prefix (`{{ matrix_client_element_container_labels_traefik_path_prefix }}`) must either be `/` or not end with a slash (e.g. `/element`).
+      when: "matrix_client_element_container_labels_traefik_path_prefix != '/' and matrix_client_element_container_labels_traefik_path_prefix[-1] == '/'"

roles/custom/matrix-client-element/templates/labels.j2

@@ -7,6 +7,17 @@ traefik.docker.network={{ matrix_client_element_container_labels_traefik_docker_
 
 {% set middlewares = [] %}
 
+{% if matrix_client_element_container_labels_traefik_path_prefix != '/' %}
+traefik.http.middlewares.matrix-client-element-slashless-redirect.redirectregex.regex=({{ matrix_client_element_container_labels_traefik_path_prefix | quote }})$
+traefik.http.middlewares.matrix-client-element-slashless-redirect.redirectregex.replacement=${1}/
+{% set middlewares = middlewares + ['matrix-client-element-slashless-redirect'] %}
+{% endif %}
+
+{% if matrix_client_element_container_labels_traefik_path_prefix != '/' %}
+traefik.http.middlewares.matrix-client-element-strip-prefix.stripprefix.prefixes={{ matrix_client_element_container_labels_traefik_path_prefix }}
+{% set middlewares = middlewares + ['matrix-client-element-strip-prefix'] %}
+{% endif %}
+
 {% if matrix_client_element_container_labels_traefik_additional_response_headers.keys() | length > 0 %}
 {% for name, value in matrix_client_element_container_labels_traefik_additional_response_headers.items() %}
 traefik.http.middlewares.matrix-client-element-add-headers.headers.customresponseheaders.{{ name }}={{ value }}