fix: avoid infinite recursion when accessing _copier_conf.answers_file via Jinja context hook

[sisp]

I've fixed a regression introduced by #2023 which causes infinite recursion when accessing _copier_conf.answers_file via Jinja context hook with copier-templates-extensions.

When rendering the answers file path in Worker.answers_relpath, which is accessible in the render context as _copier_conf.answers_file, the render context also includes _copier_conf.answers_file as a lazy dict value of _copier_conf. That's no problem when simply rendering the answers file path exposed as _copier_conf.answers_file because the answers file template never uses this variable itself. But when a context hook accesses _copier_conf.answers_file (e.g. in github.com/scientific-python/cookie:helpers/extensions.py#L17), then one invocation of the context hook intercepts the render context for rendering the template for that same value, which results in an infinite recursion. To avoid it, I've extended the LazyDict class to support setting entries and overwrote the answers_file entry with "". This is a bit of a hack, but I don't see another way to avoid this problem. I'll also submit a PR to copier-templates-extensions with a new test case for this scenario once we've agreed on the path forward here, @pawamoy.

Until v9.4.0, this problem never occurred because the render context of the answers file template consisted only of answers and didn't include additional variables such as _copier_conf. One would think that a context hook that accessed _copier_conf.answers_file would have failed because _copier_conf wasn't included in the render context of the answers file template, but no error occurred. That's because the context hook is invoked only when _copier_conf exists. Thus, while a context hook accessing, e.g., _copier_conf.* was working with Copier until v9.4.0 – in the sense that no error occurred –, the behavior wasn't entirely correct, as not all rendering contexts were updated because _copier_conf wasn't present in all of them until recent Copier versions.

Now, to the best of my knowledge, all rendering contexts include _copier_conf, so the context hook is invoked for all rendering contexts including those prior to answering any questions. I believe that this is correct behavior and previous behavior was unintended. (WDYT, @pawamoy?) This means unconditional access of answers variables or other variables that require answers variables for rendering does not work. For example, github.com/scientific-python/cookie:helpers/extensions.py#L18 raises a KeyError when accessing the url variable, as all rendering contexts prior to answering the url question don't contain it (@henryiii). In this specific example, I think the correct behavior is to inject the __ci variable into the context only when the url variable exists. This is consistent with using a computed value in copier.yaml:

url:
  type: str
  help: The url to your GitHub or GitLab repository
  # [[[end]]]
  default: "https://github.com/{{ org }}/{{ project_name }}"

ci: # Actually `__ci`, but we can't prefix questions (or computed values) with underscores
  type: str
  default: "{% if 'github.com' in url %}github{% else %}gitlab{% endif %}"
  when: false

I'm aware that this may be considered a breaking change. To restore previous behavior where context hooks are invoked only when rendering files/directories/paths, I believe we could add another condition in copier-templates-extensions to check for the value of _copier_phase, such that recent Copier versions that expose this variable don't cause a context hook invocation in other phases. However, I suggest to raise a deprecation warning and recommend to template authors to implement context updates with the different rendering phases in mind.

WDYT, @pawamoy @henryiii?

Fixes #2113.

[codecov]

Codecov Report

❌ Patch coverage is 81.81818% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 97.98%. Comparing base (be767f6) to head (33722d7).
⚠️ Report is 159 commits behind head on master.

Files with missing lines	Patch %	Lines
copier/_types.py	75.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2114      +/-   ##
==========================================
+ Coverage   97.86%   97.98%   +0.12%     
==========================================
  Files          55       55              
  Lines        5949     5957       +8     
==========================================
+ Hits         5822     5837      +15     
+ Misses        127      120       -7     

Flag	Coverage Δ
unittests	97.98% <81.81%> (+0.12%)	⬆️

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