fix: prevent context pollution

[dnlzrgz]

Description

I replaced context.update() with context.push() in order to avoid the template context being polluted after rendering the snippet. I also added a simple test case to check that the bug has been fixed and avoid regressions.

Related resources

• template tags pollutes global context with 'object' #132

Checklist

• I have opened this pull request against master
• I have added or modified the tests when changing logic
• I have followed the conventional commits guidelines to add meaningful information into the changelog
• I have read the contribution guidelines and I have joined #pr-review on
  Discord to find a “pr review buddy” who is
  going to review my pull request.

Summary by Sourcery

Prevent snippet template rendering from leaking variables into the outer template context and add regression coverage.

Bug Fixes:

• Isolate snippet rendering by pushing a temporary context layer for the snippet object to avoid polluting the caller's context.

Tests:

• Add a regression test ensuring snippet rendering does not modify existing context variables or leak additional context stack frames.

[sourcery-ai]

Reviewer's Guide

Reworks snippet rendering to use a context stack push instead of direct updates so that the calling template context is not mutated, and adds a regression test verifying that snippet rendering does not pollute the template context or leak stack frames.

Sequence diagram for snippet rendering with context.push to prevent pollution

sequenceDiagram
    participant TemplateEngine
    participant SnippetRenderer
    participant Context
    participant DjangoTemplate as DjangoTemplateSystem

    TemplateEngine->>SnippetRenderer: get_content_render(context, instance)
    SnippetRenderer->>Context: push({object: instance})
    activate Context

    alt instance has template
        SnippetRenderer->>Context: update({html: mark_safe(instance.html)})
        SnippetRenderer->>DjangoTemplate: render_to_string(instance.template, context.flatten())
        DjangoTemplate-->>SnippetRenderer: content
    else instance has no template
        SnippetRenderer->>DjangoTemplate: Template(instance.html)
        DjangoTemplate-->>SnippetRenderer: compiled_template
        SnippetRenderer->>compiled_template: render(context)
        compiled_template-->>SnippetRenderer: content
    end

    SnippetRenderer-->>TemplateEngine: return content
    Context-->>Context: automatic pop on with block exit
    deactivate Context

    TemplateEngine->>Context: read outer keys
    Context-->>TemplateEngine: unchanged outer context (no pollution)

Loading

File-Level Changes

Change	Details	Files
Isolate snippet rendering variables using Context.push() to avoid polluting the caller's template context.	Wrap snippet rendering logic in a context.push({"object": instance}) context manager so that the "object" binding is scoped to the snippet rendering only. Keep existing rendering branches for template-based and raw-HTML snippets, including TemplateDoesNotExist and generic exception handling paths. Ensure the function still returns the rendered content from within the new context manager block.	src/djangocms_snippet/templatetags/snippet_tags.py
Add regression test to ensure snippet rendering does not modify the outer context or leak context stack frames.	Create a snippet via SnippetWithVersionFactory, publish it, and render it through the snippet_fragment tag inside a pre-populated Context with an initial "object" value. Capture the initial context stack depth and assert it is unchanged after rendering the snippet. Assert that the original "object" value in the outer context remains unchanged after rendering.	tests/test_templatetags.py

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[sourcery-ai]

Hey - I've left some high level feedback:

• Inside the with context.push({"object": instance}): block you still call context.update({"html": ...}); consider passing "html" into the same push call for consistency and to keep all temporary keys clearly scoped to the pushed context.
• The new test relies on len(context.dicts) to detect leaks; adding a short comment there explaining why checking the stack length is important will make the intent clearer to future maintainers.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- Inside the `with context.push({"object": instance}):` block you still call `context.update({"html": ...})`; consider passing `"html"` into the same `push` call for consistency and to keep all temporary keys clearly scoped to the pushed context.
- The new test relies on `len(context.dicts)` to detect leaks; adding a short comment there explaining why checking the stack length is important will make the intent clearer to future maintainers.

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[fsbraun]

Suggested change

	template_to_render = Template('{% load snippet_tags %}{% snippet_fragment "pollution_test" %}')
	template_to_render = Template('{% load snippet_tags %}{% snippet_fragment "pollution_test" %} "{{ object }}"')

If you render out {{ object }} you'll directly see what the template engine puts there. Should be This shouldn't change.

This way you do not have to rely on the render method to not push context itself.

[fsbraun]

I just have a tiny suggestion to improve test clarity

[fsbraun]

Nice work, @dnlzrgz ! I appreciate the tests and that you use a context manager!