Add PXE boot support to k3s_agent role (#409)

* Add conditional snapshotter for PXE-booted systems

**Added:**

- PXE Boot Check - Introduced tasks to check if the system is PXE-booted by
  analyzing `/proc/cmdline` in `roles/k3s_agent/tasks/main.yml`.
- Conditional Snapshotter in Template - Added logic in `k3s.service.j2` template
  to conditionally set `--snapshotter native` for PXE-booted systems.

**Changed:**

- `k3s.service.j2` Template Update - Modified the `ExecStart` line to include a
  conditional check for `is_pxe_booted` fact, dynamically setting the
  `--snapshotter` option for NFS-mounted systems.
- `main.yml` Task Modification - Added tasks to set `is_pxe_booted` fact based
  on the presence of `root=/dev/nfs` in the system's boot command line.

This update allows k3s agents on PXE-booted systems to use the native snapshotter
when running on NFS, addressing compatibility issues with OverlayFS.

* Introduce idiomatic practices for affected areas from previous commits

**Added:**

- Structured HTTP Proxy Configuration Block - Added a structured block in
  `http_proxy.yml` for managing HTTP proxy settings, aligning with Ansible's
  recommended practices. This includes creating directories and deploying
  configuration files in a clear, modular fashion.
- Conditional Execution for Proxy Setup - Implemented conditional execution
  for the proxy setup in `http_proxy.yml`, utilizing `proxy_env` to adhere
  to Ansible's best practices for conditional tasks.
- Improved PXE-Boot System Check Block - Introduced a more structured approach
  in `main.yml` for checking PXE-booted systems, enhancing readability and
  maintainability.

**Changed:**

- Adopted Ansible Builtin Modules - Transitioned existing tasks to use
  `ansible.builtin` modules, ensuring compatibility and future-proofing the
  role.
- Refined Task Grouping - Reorganized tasks into logical blocks, improving
  the overall structure and readability, and showcasing Ansible's capabilities
  for efficient task management.
- Updated K3s Service Configuration - Modified the K3s service configuration
  task in `main.yml` for a more streamlined approach using Ansible's template
  module, reflecting community-driven best practices.

**Removed:**

- Streamlined Task Definitions - Optimized task definitions to reduce
  redundancy, focusing on clarity and adherence to the evolving Ansible
  community standards.

* Added missing checks causing failures for agents

roles/k3s_agent/tasks/http_proxy.yml

@@ -1,13 +1,12 @@
 ---
-
 - name: Create k3s-node.service.d directory
   file:
     path: '{{ systemd_dir }}/k3s-node.service.d'
     state: directory
     owner: root
     group: root
     mode: '0755'
-
+  when: proxy_env is defined
 
 - name: Copy K3s http_proxy conf file
   template:
@@ -16,3 +15,4 @@
     owner: root
     group: root
     mode: '0755'
+  when: proxy_env is defined

roles/k3s_agent/tasks/main.yml

@@ -1,19 +1,35 @@
 ---
+- name: Check for PXE-booted system
+  block:
+    - name: Check if system is PXE-booted
+      ansible.builtin.command:
+        cmd: cat /proc/cmdline
+      register: boot_cmdline
+      changed_when: false
+      check_mode: false
+
+    - name: Set fact for PXE-booted system
+      ansible.builtin.set_fact:
+        is_pxe_booted: "{{ 'root=/dev/nfs' in boot_cmdline.stdout }}"
+      when: boot_cmdline.stdout is defined
+
+    - name: Include http_proxy configuration tasks
+      ansible.builtin.include_tasks: http_proxy.yml
 
 - name: Deploy K3s http_proxy conf
   include_tasks: http_proxy.yml
   when: proxy_env is defined
 
-- name: Copy K3s service file
-  template:
+- name: Configure the k3s service
+  ansible.builtin.template:
     src: "k3s.service.j2"
     dest: "{{ systemd_dir }}/k3s-node.service"
     owner: root
     group: root
-    mode: 0755
+    mode: '0755'
 
-- name: Enable and check K3s service
-  systemd:
+- name: Manage k3s service
+  ansible.builtin.systemd:
     name: k3s-node
     daemon_reload: true
     state: restarted

roles/k3s_agent/templates/k3s.service.j2

@@ -7,11 +7,14 @@ After=network-online.target
 Type=notify
 ExecStartPre=-/sbin/modprobe br_netfilter
 ExecStartPre=-/sbin/modprobe overlay
-ExecStart=/usr/local/bin/k3s agent --server https://{{ apiserver_endpoint | ansible.utils.ipwrap }}:6443 --token {{ hostvars[groups[group_name_master | default('master')][0]]['token'] | default(k3s_token) }} {{ extra_agent_args | default("") }}
+# Conditional snapshotter based on PXE boot status
+ExecStart=/usr/local/bin/k3s agent \
+  --server https://{{ apiserver_endpoint | ansible.utils.ipwrap }}:6443 \
+  {% if is_pxe_booted | default(false) %}--snapshotter native \
+  {% endif %}--token {{ hostvars[groups[group_name_master | default('master')][0]]['token'] | default(k3s_token) }} \
+  {{ extra_agent_args | default("") }}
 KillMode=process
 Delegate=yes
-# Having non-zero Limit*s causes performance problems due to accounting overhead
-# in the kernel. We recommend using cgroups to do container-local accounting.
 LimitNOFILE=1048576
 LimitNPROC=infinity
 LimitCORE=infinity