[Detail Bug] Django templates: `get_files` does not deduplicate common chunks when `skip_common_chunks=True`

[detail-app]

Summary

• Context: The django-webpack-loader template tags allow users to include assets from Webpack bundles in their Django templates, with an option to skip common chunks to avoid duplication.
• Bug: The get_files template tag fails to track and skip already rendered common chunks when skip_common_chunks=True is used, because it does not update the _webpack_loader_used_urls attribute on the request object.
• Actual vs. expected: get_files correctly filters against existing URLs in _webpack_loader_used_urls but fails to add its own returned URLs to this set, leading to subsequent tags (like render_bundle or another get_files) rendering the same assets again.
• Impact: Duplicate assets (especially common vendor chunks) are loaded multiple times in the browser, increasing page size and potentially causing side effects in JavaScript execution.

Code with bug

@register.simple_tag(takes_context=True)
def get_files(
        context, bundle_name, extension=None, config='DEFAULT',
        skip_common_chunks=None):
    # ... (omitted setup)
    used_urls = getattr(request, '_webpack_loader_used_urls', None)
    if not used_urls:
        used_urls = set()
    if skip_common_chunks:
        result = [chunk for chunk in result if chunk['url'] not in used_urls]
    return result # <-- BUG 🔴 [The used_urls set is never updated or attached to the request if it was None]

Evidence

Reproduction Script

A reproduction script shows that calling get_files twice with skip_common_chunks=True returns the same files both times, and a subsequent render_bundle also includes them.

# First call
files1 = get_files(context, 'main', skip_common_chunks=True)
# Result: ['/static/bundles/common.js', '/static/bundles/main.js']

# Second call
files2 = get_files(context, 'main', skip_common_chunks=True)
# Result: ['/static/bundles/common.js', '/static/bundles/main.js'] 
# EXPECTED: []

Failing Test Case

A new test case in the project's test suite confirms the failure:

def test_get_files_deduplication(self):
    request = self.factory.get('/')
    template = Template(template_string=(
        '{% load render_bundle get_files from webpack_loader %}'
        '{% get_files "app1" skip_common_chunks=True as app1_files %}'
        '{% for f in app1_files %}<script src="{{ f.url }}"></script>{% endfor %}'
        '{% get_files "app2" skip_common_chunks=True as app2_files %}'
        '{% for f in app2_files %}<script src="{{ f.url }}"></script>{% endfor %}'
    ))
    output = template.render(context=Context({'request': request}))
    asset_vendor = '/static/django_webpack_loader_bundles/vendors.js'
    self.assertEqual(output.count(asset_vendor), 1) # Fails: returns 2

Why has this bug gone undetected?

get_files is less commonly used than render_bundle, and it's often used for prefetching or in scenarios where duplication might not be immediately obvious (e.g., hidden in a list of link tags). Developers using get_files might expect it to only return the files, and may not realize it needs to participate in the global deduplication state managed by render_bundle.

Recommended fix

Update get_files to match the behavior of render_bundle by ensuring used_urls is attached to the request and updated with the returned URLs.

    used_urls = getattr(request, '_webpack_loader_used_urls', None)
    if used_urls is None: # <-- FIX 🟢
        used_urls = set()
        setattr(request, '_webpack_loader_used_urls', used_urls) # <-- FIX 🟢
    if skip_common_chunks:
        result = [chunk for chunk in result if chunk['url'] not in used_urls]
    used_urls.update(chunk['url'] for chunk in result) # <-- FIX 🟢
    return result

History

This bug was introduced in commit 1792e80 (@karolyi, 2023-12-29, PR #386). This commit added the skip_common_chunks option to the get_files template tag but failed to include an update to the shared used_urls set, meaning files returned by this tag were never marked as rendered for subsequent tags.

[karolyi]

@usedetail

I would highly appreciate if you wouldn't spam up repos with useless [bugreports] (slop).

get_files was never the intended target, render_bundle was.