Ansible跨主机访问已定义变量：如何过滤未定义循环项？

我有一个运行在集群组节点上的Ansible Playbook，会根据条件设置变量（例如当节点是集群主节点时，设置变量为"Master"）。相关Play代码如下：

- name: Determine master node in the cluster
  hosts: cluster
  gather_facts: no
  tasks:
   - name: Get the Master node hostname
     shell: <command> 
     register: host_info

   - set_fact: 
       master_node: "{{ host_info.stdout }}"
     when: host_info.stdout is search("master")  # 示例条件，仅作理解用

我需要在其他主机上访问这个master_node变量，于是编写了如下Play：

- name: Add async standby node
  hosts: new_node
  become: yes
  tasks:
   - debug:
       var: hostvars[item]['master_node']
     loop: "{{ groups['cluster'] }}"

Inventory配置为：

[cluster]
cluster_01
cluster_02
[new_node]
new_node_01

执行后debug任务输出如下：

TASK [debug] *********************************************************************************************************************************************************************************
ok: [new_node_01] => (item=cluster_01) => {
    "ansible_loop_var": "item",
    "hostvars[item]['primary_node']": "cluster_01",
    "item": "cluster_01"
}
ok: [new_node_01] => (item=cluster_02) => {
    "ansible_loop_var": "item",
    "hostvars[item]['primary_node']": "VARIABLE IS NOT DEFINED!",
    "item": "cluster_02"
}

请问如何只访问循环中已定义变量的项？

方法1：筛选已定义变量的主机列表 #

通过selectattr过滤器结合defined测试，提前筛选出集群组中存在master_node变量的主机，再循环该列表：

- name: Add async standby node
  hosts: new_node
  become: yes
  tasks:
   - debug:
       var: hostvars[item]['master_node']
     loop: "{{ groups['cluster'] | selectattr('master_node', 'defined') | list }}"

方法2：在循环任务中添加条件判断 #

如果不需要修改循环列表，可直接给debug任务添加when条件，仅当变量存在时执行输出：

- name: Add async standby node
  hosts: new_node
  become: yes
  tasks:
   - debug:
       var: hostvars[item]['master_node']
     loop: "{{ groups['cluster'] }}"
     when: hostvars[item]['master_node'] is defined

方法3：将主节点设为全局变量（更高效） #

在第一个Play中，把主节点信息存储为全局可用变量，后续Play无需遍历集群组，直接调用即可：

- name: Determine master node in the cluster
  hosts: cluster
  gather_facts: no
  tasks:
   - name: Get the Master node hostname
     shell: <command> 
     register: host_info

   - set_fact: 
       master_node: "{{ host_info.stdout }}"
     when: host_info.stdout is search("master")
     delegate_to: localhost
     run_once: yes  # 仅执行一次，避免重复设置

在new_node的Play中直接使用变量：

- name: Add async standby node
  hosts: new_node
  become: yes
  tasks:
   - debug:
       var: master_node

内容的提问来源于stack exchange，提问作者Mukesh Tanuku