borg!: support multiple backups to same target

This brings support for multiple backups pointing to the same borg
server, different repositories, from the same client. We support this by
parsing the SSH authorized key file first and appending allowed
repositories to allow independent definitions of the borg targets.

So that the decryption_keys do not clash we include the repository name
as well as the host in the key. This will lead to new keys being created
for existing hosts in the new format.

    Old Format: {{ combine({inventory_hostname: borg_keys.stdout}) }}
    New Format: {{ combine({(inventory_hostname ~ '_' ~ borg_repo_name): borg_keys.stdout}) }}

If upgrading from an older version that used just the hostname as the
key, your existing `decryption_keys.yml` can be manually removed once
the new format is also added.

2 files changed, 61 insertions, 21 deletions

diff --git a/tasks/client_setup.yml b/tasks/client_setup.yml
index 1eae105..8769852 100644
--- a/tasks/client_setup.yml
+++ b/tasks/client_setup.yml
@@ -26,15 +26,67 @@
   become: true
   register: ssh_key
 
-- name: Deploy Keys to Borg server
+- name: Ensure authorized_keys file exists on borg server
+  ansible.builtin.file:
+    path: "{{ borg_server_user_home }}/.ssh/authorized_keys"
+    state: touch
+    owner: borg
+    group: borg
+    mode: "0600"
+    access_time: preserve
+    modification_time: preserve
+  become: true
+  delegate_to: "{{ borg_server_host }}"
+
+- name: Read existing authorized_keys content
+  ansible.builtin.slurp:
+    src: "{{ borg_server_user_home }}/.ssh/authorized_keys"
+  become: true
+  delegate_to: "{{ borg_server_host }}"
+  register: auth_keys_content
+
+- name: Find existing authorized_keys line for this host
+  ansible.builtin.set_fact:
+    existing_line: >-
+      {{
+        (auth_keys_content.content | b64decode).splitlines()
+        | select("search", ssh_key.public_key | trim | regex_escape)
+        | first
+        | default("")
+      }}
+
+- name: Detect --append-only setting from existing entry
+  ansible.builtin.set_fact:
+    existing_append_only: "{{ existing_line is search('--append-only') }}"
+  when: existing_line | length > 0
+
+- name: Fail if --append-only setting differs from existing entry
+  ansible.builtin.fail:
+    msg: |
+      Inconsistent --append-only setting for host {{ inventory_hostname }}.
+      Existing entry has --append-only={{ existing_append_only }}, but current invocation uses --append-only={{ borg_mode_append_only }}.
+      All repositories for a host must have the same --append-only setting.
+  when:
+    - existing_line | length > 0
+    - existing_append_only != borg_mode_append_only
+
+- name: Compute all repos for this host
+  ansible.builtin.set_fact:
+    all_repos: >-
+      {{
+        (
+          (existing_line | regex_findall('--restrict-to-repository ([^\s"]+)'))
+          + [borg_server_user_home + '/' + borg_repo_name]
+        ) | unique | sort }}
+
+- name: Update authorized_keys entry for this host
   ansible.builtin.lineinfile:
     path: "{{ borg_server_user_home }}/.ssh/authorized_keys"
-    line: >
-      restrict,command="borg serve
-      {{ "--append-only" if borg_mode_append_only }}
-      --restrict-to-repository {{ borg_repo_name }}"
-      {{ ssh_key.public_key }} root@{{ inventory_hostname }}
-    search_string: --restrict-to-repository {{ borg_repo_name }}" {{ ssh_key.public_key }}
+    search_string: "{{ ssh_key.public_key | trim }}"
+    line: >-
+      restrict,command="borg serve{{ " --append-only" if borg_mode_append_only }}
+      {{ all_repos | map('regex_replace', '^', '--restrict-to-repository ') | join(' ') }}"
+      {{ ssh_key.public_key | trim }} root@{{ inventory_hostname }}
     state: present
   become: true
   delegate_to: "{{ borg_server_host }}"
@@ -68,7 +120,7 @@
   register: local
 
 - name: Add repository encryption keys to ansible repo
-  when: not inventory_hostname in local.ansible_facts
+  when: not (inventory_hostname ~ '_' ~ borg_repo_name) in local.ansible_facts
   throttle: 1
   block:
     - name: If host new read encryption keys
@@ -81,7 +133,7 @@
 
     - name: If host new add encryption keys to vars
       ansible.builtin.set_fact:
-        decryption_keys: "{{ local.ansible_facts | combine({inventory_hostname: borg_keys.stdout}) }}"
+        decryption_keys: "{{ local.ansible_facts | combine({(inventory_hostname ~ '_' ~ borg_repo_name): borg_keys.stdout}) }}"
 
 - name: Update encryption vars
   ansible.builtin.copy:
diff --git a/tasks/server_setup.yml b/tasks/server_setup.yml
index 603dd3d..e47d012 100644
--- a/tasks/server_setup.yml
+++ b/tasks/server_setup.yml
@@ -8,15 +8,3 @@
     generate_ssh_key: true
   become: true
   delegate_to: "{{ borg_server_host }}"
-
-- name: Make sure authorized keys exists
-  ansible.builtin.file:
-    path: "{{ borg_server_user_home }}/.ssh/authorized_keys"
-    state: touch
-    owner: borg
-    group: borg
-    mode: "644"
-    access_time: preserve
-    modification_time: preserve
-  become: true
-  delegate_to: "{{ borg_server_host }}"