Django 'static' template tag fails to generate URLs with SCRIPT_NAME prefix

[Stewart Adam]

According to the docs, when MEDIA_URL and STATIC_URL are relative (now the default), SCRIPT_NAME should automatically be prepended to them when it is set to ensure that paths are generated correctly. This appears to work everywhere *except* when calling {% static %} in HTML templates.

Best I can tell, when this functionality was first added, it appears that SCRIPT_NAME was evaluated every request: ​https://github.com/django/django/commit/c574bec0929cd2527268c96a492d25223a9fd576

Looking at this same file today, it appears that setting values are now cached in a dict. When the Django application initializes, it reads SCRIPT_NAME as / and so all subsequent uses of {% static %} are output without the current value of SCRIPT_NAME. This happens even if using a hardcoded FORCE_SCRIPT_NAME.

url() appears to be unaffected by the same issue, so the net result is that links and redirects honor SCRIPT_NAME as expected, and static files are served using the SCRIPT_NAME prefix, but all output from HTML templates simply use /static/... for their links which causes all css/js/favicon/related media to fail to load with a 404.

[Stewart Adam]

Related issues elsewhere:

• ​https://stackoverflow.com/questions/45557034/django-static-url-when-script-name-set
• ​https://github.com/evansd/whitenoise/issues/88
• ​https://github.com/TandoorRecipes/recipes/issues/1878

[Stewart Adam]

The code path leading to the issue appears to be:

1. static() handler for the template tag calls StaticNode.handle_simple() in ​templatetags/static.py L174
2. StaticNode.handle_simple() calls staticfiles_storage.url() at ​templatetags/static.py L127
3. staticfiles_storage.url() is backed by a FileSystemStorage: ​core/files/storage.py L392

None of these calls attempt to verify SCRIPT_NAME or inject STATIC_URL. An ​alternate code path during step 2 *does* appear to inject STATIC_URL. however to due application settings now being cached into a dict/module the value does not reflect the current value of SCRIPT_NAME and uses the value at application initialization (empty string) instead, so STATIC_URL is always just /static/.

[Florian Apolloner]

Hi,

I cannot reproduce this. It works as it should:

from django.template import engines
django_engine = engines['django']
from django.conf import settings
print(settings.FORCE_SCRIPT_NAME)
print(settings.STATIC_URL) # Note: in settings.py it is 'static/' but the output here will include FORCE_SCRIPT_NAME
print(django_engine.from_string("{% load static %}{% static 'test_file' %}").render())

/forced_script_name
/forced_script_name/static/
/forced_script_name/static/test_file

None of these calls attempt to verify SCRIPT_NAME or inject STATIC_URL.

This isn't true: {% static %} uses staticfiles_storage which in turn uses StaticFilesStorage which sets the prefix to STATIC_URL which includes FORCE_SCRIPT_NAME. Feel free to reopen with a reproducer if you are still running into issues.

[Florian Apolloner]

Setting a custom script name also works:

from django.urls.base import *
set_script_prefix('/lala')
from django.template import engines
django_engine = engines['django']
from django.conf import settings
print(settings.FORCE_SCRIPT_NAME)
print(settings.STATIC_URL) # Note: in settings.py it is 'static/' but the output here will include FORCE_SCRIPT_NAME
print(django_engine.from_string("{% load static %}{% static 'test_file' %}").render())

None
/lala/static/
/lala/static/test_file

That said it is true that the value of SCRIPT_NAME is assumed to be static and not change over the runtime of Django. If Django is "loaded" outside of a webcontext first, then you will see the behavior you are describing. I'll accept the ticket for now but I am not sure if this is fixable easily or at all.

[Stewart Adam]

Replying to Florian Apolloner:

None
/lala/static/
/lala/static/test_file

Admittedly I'm not a Django export, I'm just trying to troubleshoot another OSS app I use that's based on Django. Can you share how you're running this?

That said, when I put this code into view on a sample Django app (django-admin startproject repro), I can confirm the behavior that the first request made to the server sets the prefix for the lifetime of the app. Code at ​https://github.com/stewartadam/django-static-repro

./run.sh &

# try using SCRIPT_NAME to set the prefix on first request
curl -H 'SCRIPT_NAME: /prefix' http://localhost:8000/prefix/foo/
   FORCE_SCRIPT_NAME: None
   STATIC_URL: /prefix/static/
   call static for test_file: /prefix/static/test_file

# second request paths correctly with /another, but uses first request's /prefix for all variable values/output
curl -H 'SCRIPT_NAME: /another' localhost:8000/another/foo/
   FORCE_SCRIPT_NAME: None
   STATIC_URL: /prefix/static/
   call static for test_file: /prefix/static/test_file

FORCE_SCRIPT_NAME also doesn't behave how I'd expect (overriding SCRIPT_NAME header):

FORCE_SCRIPT_NAME=prefix ./run.sh &

# FORCE_SCRIPT_NAME is supposed to override SCRIPT_NAME header, but isn't here in the pathing
curl -H 'SCRIPT_NAME: /another' http://localhost:8000/another/foo/
   FORCE_SCRIPT_NAME: prefix
   STATIC_URL: /prefix/static/
   call static for test_file: /prefix/static/test_file

# if supplied SCRIPT_NAME doesn't match FORCE_SCRIPT_NAME, serve fails to serve content
curl -I -H 'SCRIPT_NAME: /another' http://localhost:8000/prefix/foo/
  HTTP/1.1 500 Internal Server Error
  Connection: close
  Content-Type: text/html
  Content-Length: 141

That said it is true that the value of SCRIPT_NAME is assumed to be static and not change over the runtime of Django. If Django is "loaded" outside of a webcontext first, then you will see the behavior you are describing. I'll accept the ticket for now but I am not sure if this is fixable easily or at all.

I agree at face value this might be an unusual thing to support, however I can see a few cases where this might happen:

1. middleware initializes and accesses settings before a request arrives (e.g. whitenoise appears to do this) - SCRIPT_NAME won't be set from the HTTP request at that point in time
2. on many servers healthchecks or other internal requests might result in a request coming through without SCIRPT_NAME set as it would be for end-users via reverse proxy.

It seems like for configuring the Django prefix when hosted in a sub-paths, it should either (a) move to a hardcoded config-based value so the prefix is the same in all contexts or (b) reset the thread locals based on the request headers - but right now behavior appears to be a mix of both which is what causes issues.

[Stewart Adam]

Had a chance to debug a bit more and caching of SCRIPT_NAME aside, even if users use FORCE_SCRIPT_NAME to ensure the prefix is hardcoded and correctly set at all times it looks like there's still something awry:

Whitenoise middleware sees the prefix as '/' during initialization - set_script_prefix() only appears to be called during the first HTTP request, but whitenoise needs to verify the user-provided values for STATIC_URL during middleware initialization. During that middleware initialization, the prefix remains '/' despite FORCE_SCRIPT_NAME being set.

I've posted more details here: ​https://github.com/evansd/whitenoise/issues/271#issuecomment-1257389536

[David Sanders]

I've been investigating SCRIPT_NAME / FORCE_SCRIPT_NAME and found some interesting quirks when used with runserver which may be affecting testing these issues (at least it was affecting me when investigating #34185

I'm not entirely sure why yet but I'll post the following reproducible issue (at least for me) to show that prefixing STATIC_URL works intermittently due to a race condition with runserver and the multiple processes setup with autoreload:

In django/conf/__init__.py I added a line to print the result of prefixing STATIC_URL:

    @staticmethod
    def _add_script_prefix(value):
        """
        Add SCRIPT_NAME prefix to relative paths.

        Useful when the app is being served at a subpath and manually prefixing
        subpath to STATIC_URL and MEDIA_URL in settings is inconvenient.
        """
        # Don't apply prefix to absolute paths and URLs.
        if value.startswith(("http://", "https://", "/")):
            return value
        from django.urls import get_script_prefix

        val = "%s%s" % (get_script_prefix(), value)
        print("*********** PREFIXING ***********: " + val)
        return val

The result when runserver is run with autoreload on then off:

sample-project % dj runserver
Watching for file changes with StatReloader
Performing system checks...

*********** PREFIXING ***********: /static/
System check identified no issues (0 silenced).
November 29, 2022 - 13:22:27
Django version 4.2.dev20221129074011, using settings 'django_sample.settings'
Starting development server at http://127.0.0.1:8000/
Quit the server with CONTROL-C.
^C
django-sample % dj runserver --noreload
Performing system checks...

*********** PREFIXING ***********: /django/static/
System check identified no issues (0 silenced).
November 29, 2022 - 13:22:40
Django version 4.2.dev20221129074011, using settings 'django_sample.settings'
Starting development server at http://127.0.0.1:8000/
Quit the server with CONTROL-C.
^C
sample-project %

Now the interesting thing is that I can get the same results without specifying --noreload by just adding a line to print() in setup() ! :)

[Sarah Boyce]

I was able to replicate the issue that if someone tries to access the script prefix in a middleware init (as they do in whitenoise), the result is unexpected.
I have raised a PR which has a regression test and a potential fix 👍

I think this relates to this ticket a bit: #16734

[Mariusz Felisiak]

This can be fixed by #34461.

[Mariusz Felisiak]

Let's document this caveat (see ​PR) and close this as "invalid". Discussion about constructing URLs outside of the request-response cycle (new feature) was moved to the #34461.

[GitHub <noreply@…>]

In bdf59bf:

Refs #34028 -- Doc'd that get_script_prefix() cannot be used outside of the request-response cycle.

[Mariusz Felisiak <felisiak.mariusz@…>]

In e34a54a3:

[4.2.x] Refs #34028 -- Doc'd that get_script_prefix() cannot be used outside of the request-response cycle.

Backport of bdf59bff657975e577b86b194b39ec2f77983d2b from main