feat: add ability to disable dynamic variable caching

[pd93]

See comment on discord copied below:

I came across something strange.

test:
  cmds:
    - echo {{ .FILE }} == {{ .TEMPFILE }}
  vars:
    FILE:
      sh: mktemp
    TEMPFILE:
      sh: mktemp

> task test
task: [test] echo /var/folders/6s/2k2vd19s7kg20405_blbh7fm0000gn/T/tmp.705g66B7Ku == /var/folders/6s/2k2vd19s7kg20405_blbh7fm0000gn/T/tmp.705g66B7Ku
/var/folders/6s/2k2vd19s7kg20405_blbh7fm0000gn/T/tmp.705g66B7Ku == /var/folders/6s/2k2vd19s7kg20405_blbh7fm0000gn/T/tmp.705g66B7Ku

Why FILE and TEMPFILE has the same value?

This is happening because of Task's dynamic variable cache. We should add the ability to disable this on a per-variable basis:

  without-cache:
    silent: true
    vars:
      FOO:
        sh: mktemp
        cache: false
      BAR:
        sh: mktemp
        cache: false
    cmds:
      - echo {{ .FOO }}
      - echo {{ .BAR }}

/tmp/tmp.vGKqfv8fm7
/tmp/tmp.4Jtfmihw58

[andreynering]

Hey @pd93,

I think this changes needs a bit of thought:

1. I'm not sure, but I think it may happen, with cache: false, that a given variable shell will be run multiple times, which will be unexpected (and perhaps even dangerous) to the user. That should at least be checked before merging.
2. I have an idea (Idea: Consider having functions to access variables and envs #1065) to have "lazy variables" at some point. That would be a overhaul on how vars and envs are handled, in which a variable would only be computed once really requested by the template engine. Depending on how we implement that, that may make cache: false obsolete (because vars would not be cached by default to begin with). So this makes me question: should we introduce an option that may soon become obsolete?

[pd93]

@andreynering Yeah, you make some very good points. I'll admit that I cobbled this together quite quickly without giving it too much thought in response to the comments on Discord.

Lazy variables are definitely the way forwards long term. I have given some thought about how this could be done. However, I'm not a massive fan of having to call a function inside of templates to get variable values. I actually think there might be a way to do it without the need for functions which would also keep compatibility with v3. Perhaps this is a discussion for another time/place though.

Let's close this for now 👍