Do not cache the loop item label so that it will update with each item

[jctanner]

SUMMARY

In an item loop, the label is used to display context to the user. The value of the label can be templated with the item. After 2.4.x, the templating operation for the label was cached, and would not update for each item. This patch disabled the caching.

ISSUE TYPE

• Bugfix Pull Request

COMPONENT NAME

task_executor and loop labels

ANSIBLE VERSION

2.6

ADDITIONAL INFORMATION

- hosts: localhost
  connection: local
  gather_facts: False
  vars:
      loopthis:
          - name: foo
            label: foo_label
          - name: bar
            label: bar_label
  tasks:
    - name: Ensure the state of iBGP neighborship
      debug:
        msg: "{{ looped_var.name }}"
      with_items: "{{ loopthis }}"
      loop_control:
        loop_var: looped_var
        label: "looped_var {{ looped_var.label }}"

Example output before patch:

PLAY [localhost] ****************************************************************************

TASK [Ensure the state of iBGP neighborship] ************************************************
ok: [localhost] => (item=looped_var foo_label) => {
    "changed": false,
    "failed": false,
    "looped_var": {
        "label": "foo_label",
        "name": "foo"
    }
}

MSG:

foo

ok: [localhost] => (item=looped_var foo_label) => {
    "changed": false,
    "failed": false,
    "looped_var": {
        "label": "bar_label",
        "name": "bar"
    }
}

MSG:

bar


PLAY RECAP **********************************************************************************
localhost                  : ok=1    changed=0    unreachable=0    failed=0

Example after patch:

PLAY [localhost] ****************************************************************************

TASK [Ensure the state of iBGP neighborship] ************************************************
ok: [localhost] => (item=looped_var foo_label) => {
    "changed": false,
    "failed": false,
    "looped_var": {
        "label": "foo_label",
        "name": "foo"
    }
}

MSG:

foo

ok: [localhost] => (item=looped_var bar_label) => {
    "changed": false,
    "failed": false,
    "looped_var": {
        "label": "bar_label",
        "name": "bar"
    }
}

MSG:

bar


PLAY RECAP **********************************************************************************
localhost                  : ok=1    changed=0    unreachable=0    failed=0

[bcoca]

shouldn't the jinja2 cache invalidate when the variables involved change? aka 'item'.

[jctanner]

@bcoca jinja is not the "cacher" in this case.

https://github.com/ansible/ansible/blob/devel/lib/ansible/template/__init__.py#L470-L473

When templar.template is told to use a cache, it bypasses do_template and jinja completely. If the templated label is sent to do_template, jinja is called and returns the correct result.

With a little bit of python-q injection, here's what happens under the covers ...

# first item ...
 1.0s template: 'calling do_template'='calling do_template'
from __future__ import division
from jinja2.runtime import LoopContext, TemplateReference, Macro, Markup, TemplateRuntimeError, missing, concat, escape, markup_join, unicode_join, to_string, identity, TemplateNotFound, Namespace
name = None

def root(context, missing=missing, environment=environment):
    resolve = context.resolve_or_missing
    undefined = environment.undefined
    if 0: yield None
    l_0_looped_var = resolve('looped_var')
    pass
    yield u'looped_var '
    yield to_string(environment.finalize(environment.getattr((undefined(name='looped_var') if l_0_looped_var is missing else l_0_looped_var), 'label')))

blocks = {}
debug_info = '1=12' 1.0s compile:
      source="from __future__ import division\nfrom jinja2.runt...var), 'label')))\n\nblocks = {}\ndebug_info = '1=12'" (file:///tmp/q)
 1.0s template: result=u'looped_var foo_label'

# second item ...
  1.1s template: 'calling do_template'='calling do_template'
from __future__ import division
from jinja2.runtime import LoopContext, TemplateReference, Macro, Markup, TemplateRuntimeError, missing, concat, escape, markup_join, unicode_join, to_string, identity, TemplateNotFound, Namespace
name = None

def root(context, missing=missing, environment=environment):
    resolve = context.resolve_or_missing
    undefined = environment.undefined
    if 0: yield None
    l_0_looped_var = resolve('looped_var')
    pass
    yield u'looped_var '
    yield to_string(environment.finalize(environment.getattr((undefined(name='looped_var') if l_0_looped_var is missing else l_0_looped_var), 'label')))

blocks = {}
debug_info = '1=12' 1.1s compile:
      source="from __future__ import division\nfrom jinja2.runt...var), 'label')))\n\nblocks = {}\ndebug_info = '1=12'" (file:///tmp/q)
 1.1s template: result=u'looped_var bar_label

Without this patch, jinja's environment.compile is never invoked for bar_label. Based on what little I can find out jinja's "caching", there is no cache for results. It only caches template source.

https://stackoverflow.com/a/4214506
http://jinja.pocoo.org/docs/2.10/api/#loaders
http://jinja.pocoo.org/docs/2.10/api/#bytecode-cache