Dynamic targets and cross-task vars

[umputun]

Discussed in #206

^{Originally posted by odyfey May 25, 2024}
Hi!

I'm having a few issues while trying to run this playbook:

targets:
  default:
    hosts: [{host: "pghost1"}, {host: "pghost2"}, {host: "pghost3"}]

tasks:
  - name: check-recovery
    targets: ["default"]
    commands:
      - name: check recovery mode and set leader
        script: |
          is_recovery=$(psql -U ${pguser} -p ${pgport} -A -t -c "select pg_is_in_recovery();")
          if [ "$is_recovery" = "f" ]; then
            leader=$(hostname -I)
          fi
        register: [leader]

  - name: run on host
    targets: ["$leader"]
    commands: 
      - name: do something on leader
        script: |
          echo "doing something on $leader"

The playbook performs a check-recovery task on each host to discover the leader. Then, in the next task, some queries are executed on the leader host.

1. Is it correct to use a variable from the previous task as a dynamic target?
2. The playbook is successful only if the last element in the default targets (pghost3) is the leader, and as a result, the res.Vars variable has a value of map[leader:pghost3]. If you change the order in default targets, the last assigned value in the res.Vars map will be [leader:] and spot will error out on the last task:

[]  ! failed to dial: dial tcp :22: connect: connection refused
2024/05/24 17:36:29.424 [ERROR] can't run task "run on host" for target "": 1 error(s) occurred: [0] {failed to dial: dial tcp :22: connect: connection refused}

3. Based on this comment, I suggest that the scope of res.Vars should be limited to one task, but when using register, this variable is used to set targets in another task. Is this correct?
4. If the previous statements are correct, is it possible to change the behavior of spot so that once a non-empty value is written to map, it is not overwritten by the next empty value? I'm talking about code like this:

// changes in this line: https://github.com/umputun/spot/blob/master/pkg/runner/runner.go#L113
if rv, ok := allVars[k]; ok && rv != "" {
	continue
}

allVars[k] = v
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