🐛 Fix static middleware subdirectory browsing redirects losing URL prefix

[Copilot]

The static middleware with directory browsing enabled was incorrectly handling redirects for subdirectories when URL prefixes were used. When clicking on subdirectory links in the directory listing, the redirect would lose the URL prefix, breaking navigation.

Problem

Consider a Fiber app serving static files with browsing:

app.Get("/static*", static.New("./public", static.Config{Browse: true}))

Before this fix:

1. Visit /static → shows directory listing with "js" folder
2. Click "js" folder → redirects to /js/ (loses /static prefix) → 404 error

Root cause: When fasthttp.FS generates redirects for directory access (to add trailing slashes), it uses the path-rewritten version without the original URL prefix context.

Solution

This fix intercepts redirects generated by fasthttp.FS and corrects the Location header to preserve the URL prefix:

• /static/js → redirects to /static/js/ (preserves prefix)
• /static/subfolder → redirects to /static/subfolder/ (preserves prefix)

The fix handles both absolute and relative redirect URLs and only applies the correction when:

1. Browse mode is enabled
2. A redirect is generated
3. The redirect location lacks the expected URL prefix
4. The redirect matches the expected pattern for directory access

Testing

Added comprehensive tests covering:

• Directory listing shows correct prefixed links
• Subdirectory access redirects preserve URL prefix
• Both trailing slash and non-trailing slash cases
• No regression in existing functionality

All existing tests continue to pass, ensuring backward compatibility.

Fixes #3667.

Warning

Firewall rules blocked me from connecting to one or more addresses

I tried to connect to the following addresses, but was blocked by firewall rules:

• exampleretry.com
  • Triggering command: /tmp/go-build4097136303/b274/client.test -test.paniconexit0 -test.v=test2json -test.timeout=10m0s -test.count=1 -test.shuffle=on (dns block)
  • Triggering command: /tmp/go-build4063505421/b274/client.test -test.paniconexit0 -test.v=test2json -test.timeout=10m0s -test.count=1 -test.shuffle=on (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)

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[ReneWerner87]

@copilot Please check the code again and make it smart so that as many cases as possible are handled with as little code as possible and few bugs occur.
This will also prevent the code from becoming too complex.

[ReneWerner87]

@efectn WDYT is this an edge case ?

[Copilot]

@copilot Please check the code again and make it smart so that as many cases as possible are handled with as little code as possible and few bugs occur.
This will also prevent the code from becoming ...

I've refactored the code to be much smarter and simpler as requested. The changes in commit 17f8746:

Made it smarter:

• Extracted calculatePrefix() and fixRedirectLocation() helper functions that handle edge cases systematically
• Centralized logic to avoid duplication

Reduced complexity:

• Simplified the main redirect fixing logic from 68 complex lines to just 4 clean lines
• Eliminated nested conditions and redundant string manipulations
• Better separation of concerns

Fewer bugs:

• Helper functions can be unit tested independently
• Cleaner logic reduces places where bugs can hide
• DRY principle applied - no more duplicated prefix calculations

All existing tests continue to pass, ensuring the functionality remains intact while achieving significant code simplification.

[codecov]

Codecov Report

❌ Patch coverage is 90.56604% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.81%. Comparing base (1569a61) to head (17f8746).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
middleware/static/static.go	90.56%	4 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3668      +/-   ##
==========================================
- Coverage   91.84%   91.81%   -0.04%     
==========================================
  Files         113      113              
  Lines       11447    11490      +43     
==========================================
+ Hits        10514    10549      +35     
- Misses        669      675       +6     
- Partials      264      266       +2     

Flag	Coverage Δ
unittests	91.81% <90.56%> (-0.04%)	⬇️

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[ReneWerner87]

fixed by #3673