Add Synapse Maintenance docs and synapse-janitor integration

roles/matrix-postgres/tasks/run_vacuum.yml

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+---
+
+# Pre-checks
+
+- name: Fail if Postgres not enabled
+  fail:
+    msg: "Postgres via the matrix-postgres role is not enabled (`matrix_postgres_enabled`). Cannot run vacuum."
+  when: "not matrix_postgres_enabled|bool"
+
+
+# Defaults
+
+- name: Set postgres_start_wait_time, if not provided
+  set_fact:
+    postgres_start_wait_time: 15
+  when: "postgres_start_wait_time|default('') == ''"
+
+- name: Set postgres_vacuum_wait_time, if not provided
+  set_fact:
+    postgres_vacuum_wait_time: "{{ 7 * 86400 }}"
+  when: "postgres_vacuum_wait_time|default('') == ''"
+
+
+# Actual vacuuming work
+
+- name: Ensure matrix-postgres is started
+  service:
+    name: matrix-postgres
+    state: started
+    daemon_reload: yes
+
+- name: Wait a bit, so that Postgres can start
+  wait_for:
+    timeout: "{{ postgres_start_wait_time }}"
+  delegate_to: 127.0.0.1
+  become: false
+
+- import_tasks: tasks/util/detect_existing_postgres_version.yml
+
+- name: Abort, if no existing Postgres version detected
+  fail:
+    msg: "Could not find existing Postgres installation"
+  when: "not matrix_postgres_detected_existing|bool"
+
+- name: Generate Postgres database vacuum command
+  set_fact:
+    matrix_postgres_vacuum_command: >-
+      /usr/bin/docker run --rm --name matrix-postgres-synapse-vacuum
+      --user={{ matrix_user_uid }}:{{ matrix_user_gid }}
+      --cap-drop=ALL
+      --network={{ matrix_docker_network }}
+      --env-file={{ matrix_postgres_base_path }}/env-postgres-psql
+      {{ matrix_postgres_docker_image_latest }}
+      psql -v ON_ERROR_STOP=1 -h matrix-postgres {{ matrix_synapse_database_database }} -c 'VACUUM FULL VERBOSE'
+
+- name: Note about Postgres vacuum alternative
+  debug:
+    msg: >-
+      Running vacuum with the following Postgres command: `{{ matrix_postgres_vacuum_command }}`.
+      If this crashes, you can stop all processes (`systemctl stop matrix-*`),
+      start Postgres only (`systemctl start matrix-postgres`)
+      and manually run the above command directly on the server.
+
+- name: Populate service facts
+  service_facts:
+
+- set_fact:
+    matrix_postgres_synapse_was_running: "{{ ansible_facts.services['matrix-synapse.service']|default(none) is not none and ansible_facts.services['matrix-synapse.service'].state == 'running' }}"
+
+- name: Ensure matrix-synapse is stopped
+  service:
+    name: matrix-synapse
+    state: stopped
+    daemon_reload: yes
+
+- name: Run Postgres vacuum command
+  command: "{{ matrix_postgres_vacuum_command }}"
+  async: "{{ postgres_vacuum_wait_time }}"
+  poll: 10
+  register: matrix_postgres_synapse_vacuum_result
+
+# Intentionally show the results
+- debug: var="matrix_postgres_synapse_vacuum_result"
+
+- name: Ensure matrix-synapse is started, if it previously was
+  service:
+    name: matrix-synapse
+    state: started
+    daemon_reload: yes
+  when: "matrix_postgres_synapse_was_running|bool"