Add support for running synapse-admin (on Traefik) at the root path

Previously, we had to run it at a subpath, like `/synapse-admin`.

We can now dedicate a whole domain and the `/` path to it, should we
wish to do so.

roles/custom/matrix-synapse-admin/defaults/main.yml

@@ -37,9 +37,9 @@ matrix_synapse_admin_container_extra_arguments: []
 matrix_synapse_admin_container_labels_traefik_enabled: true
 matrix_synapse_admin_container_labels_traefik_docker_network: "{{ matrix_synapse_admin_container_network }}"
 matrix_synapse_admin_container_labels_traefik_hostname: "{{ matrix_server_fqn_matrix }}"
-# The path prefix must not end with a slash
+# The path prefix must either be `/` or not end with a slash (e.g. `/synapse-admin`).
 matrix_synapse_admin_container_labels_traefik_path_prefix: /synapse-admin
-matrix_synapse_admin_container_labels_traefik_rule: "Host(`{{ matrix_synapse_admin_container_labels_traefik_hostname }}`) && PathPrefix(`{{ matrix_synapse_admin_container_labels_traefik_path_prefix | quote }}`)"
+matrix_synapse_admin_container_labels_traefik_rule: "Host(`{{ matrix_synapse_admin_container_labels_traefik_hostname }}`){% if matrix_synapse_admin_container_labels_traefik_path_prefix != '/' %} && PathPrefix(`{{ matrix_synapse_admin_container_labels_traefik_path_prefix | quote }}`){% endif %}"
 matrix_synapse_admin_container_labels_traefik_entrypoints: web-secure
 matrix_synapse_admin_container_labels_traefik_tls: "{{ matrix_synapse_admin_container_labels_traefik_entrypoints != 'web' }}"
 matrix_synapse_admin_container_labels_traefik_tls_certResolver: default  # noqa var-naming

roles/custom/matrix-synapse-admin/tasks/validate_config.yml

@@ -28,6 +28,5 @@
     - name: Fail if matrix_synapse_admin_container_labels_traefik_path_prefix ends with a slash
       ansible.builtin.fail:
         msg: >-
-          matrix_synapse_admin_container_labels_traefik_path_prefix (`{{ matrix_synapse_admin_container_labels_traefik_path_prefix }}`) must not end with a slash.
-          Example: `/synapse-admin`.
-      when: "matrix_synapse_admin_container_labels_traefik_path_prefix[-1] == '/'"
+          matrix_synapse_admin_container_labels_traefik_path_prefix (`{{ matrix_synapse_admin_container_labels_traefik_path_prefix }}`) must either be `/` or not end with a slash (e.g. `/synapse-admin`).
+      when: "matrix_synapse_admin_container_labels_traefik_path_prefix != '/' and matrix_synapse_admin_container_labels_traefik_path_prefix[-1] == '/'"

roles/custom/matrix-synapse-admin/templates/labels.j2

@@ -5,13 +5,23 @@ traefik.enable=true
 traefik.docker.network={{ matrix_synapse_admin_container_labels_traefik_docker_network }}
 {% endif %}
 
+{% set middlewares = [] %}
+
+{% if matrix_synapse_admin_container_labels_traefik_path_prefix != '/' %}
 traefik.http.middlewares.matrix-synapse-admin-slashless-redirect.redirectregex.regex=({{ matrix_synapse_admin_container_labels_traefik_path_prefix | quote }})$
 traefik.http.middlewares.matrix-synapse-admin-slashless-redirect.redirectregex.replacement=${1}/
+{% set middlewares = middlewares + ['matrix-synapse-admin-slashless-redirect'] %}
+{% endif %}
 
-traefik.http.middlewares.matrix-synapse-admin-replacepath.replacepath.path=/
+{% if matrix_synapse_admin_container_labels_traefik_path_prefix != '/' %}
+traefik.http.middlewares.matrix-synapse-admin-strip-prefix.stripprefix.prefixes={{ matrix_synapse_admin_container_labels_traefik_path_prefix }}
+{% set middlewares = middlewares + ['matrix-synapse-admin-strip-prefix'] %}
+{% endif %}
 
 traefik.http.routers.matrix-synapse-admin.rule={{ matrix_synapse_admin_container_labels_traefik_rule }}
-traefik.http.routers.matrix-synapse-admin.middlewares=matrix-synapse-admin-slashless-redirect,matrix-synapse-admin-replacepath
+{% if middlewares | length > 0 %}
+traefik.http.routers.matrix-synapse-admin.middlewares={{ middlewares | join(',') }}
+{% endif %}
 traefik.http.routers.matrix-synapse-admin.service=matrix-synapse-admin
 traefik.http.routers.matrix-synapse-admin.entrypoints={{ matrix_synapse_admin_container_labels_traefik_entrypoints }}
 traefik.http.routers.matrix-synapse-admin.tls={{ matrix_synapse_admin_container_labels_traefik_tls | to_json }}