fix(extensions): ensure trusted/safe templates extensions are seen during update or copy with --vcs-ref

[noirbizarre]

Hello 👋🏼

This PR fix a regression on template extensions.
When a template provides some extensions, they are not on sys.path anymore when:

• using copy with --vcs-ref
• using update

It fails with the following error (for a extensions package in the template directory)

Copier could not load some Jinja extensions:
No module named 'extensions'
Make sure to install these extensions alongside copier itself.

I can't say precisely in what version it broke, but it was working on Copier 9.3, 9.4 (I think it worked in 9.10) and not anymore since 9.12.

This PR fix this by explicitely adding the template path to sys.path when the template is trusted.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.27%. Comparing base (ba15e57) to head (296faa6).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2557      +/-   ##
==========================================
+ Coverage   97.25%   97.27%   +0.01%     
==========================================
  Files          58       58              
  Lines        6751     6782      +31     
==========================================
+ Hits         6566     6597      +31     
  Misses        185      185              

Flag	Coverage Δ
unittests	97.27% <100.00%> (+0.01%)	⬆️

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[sisp]

Are you sure this ever worked without copier-template-extensions? I've checked out the v9.4.1 tag, added your fixture and tests, and ran those tests – they're failing for me:

FAILED tests/test_jinja2_extensions.py::test_extension_on_update - copier.errors.ExtensionNotFoundError: Copier could not load some Jinja extensions:
FAILED tests/test_jinja2_extensions.py::test_extension_from_copy_with_vcs_ref - copier.errors.ExtensionNotFoundError: Copier could not load some Jinja extensions:

This is an excerpt of the stack trace:

    @cached_property
    def jinja_env(self) -> SandboxedEnvironment:
        """Return a pre-configured Jinja environment.
    
        Respects template settings.
        """
        paths = [str(self.template.local_abspath)]
        loader = FileSystemLoader(paths)
        default_extensions = [
            "jinja2_ansible_filters.AnsibleCoreFiltersExtension",
        ]
        extensions = default_extensions + list(self.template.jinja_extensions)
        # We want to minimize the risk of hidden malware in the templates
        # so we use the SandboxedEnvironment instead of the regular one.
        # Of course we still have the post-copy tasks to worry about, but at least
        # they are more visible to the final user.
        try:
            env = SandboxedEnvironment(
                loader=loader, extensions=extensions, **self.template.envops
            )
        except ModuleNotFoundError as error:
>           raise ExtensionNotFoundError(
                f"Copier could not load some Jinja extensions:\n{error}\n"
                "Make sure to install these extensions alongside Copier itself.\n"
                "See the docs at https://copier.readthedocs.io/en/latest/configuring/#jinja_extensions"
            )
E           copier.errors.ExtensionNotFoundError: Copier could not load some Jinja extensions:
E           No module named 'extensions'
E           Make sure to install these extensions alongside Copier itself.
E           See the docs at https://copier.readthedocs.io/en/latest/configuring/#jinja_extensions

[noirbizarre]

You're right, it only works with copier-template-extensions and in my case, it doesn't support imports (which is what triggered my error, that I solved by duplicating into the loaded extension module).

I'll close the PR unless you are interested in this feature (I would be truly interested into supporting template-local imports, it would simplify a lot of things)

[sisp]

I think that template-local extension loading using file paths is preferable over module names in the search path for better clarity. This is what copier-template-extensions is doing. I agree that loading template-local extensions is a rather common use case which may deserve first-class support by Copier. This has been discussed before (#1756), a few more points to consider were raised there. I'm curious what you think.