fix(context): only chmod newly created dirs in SaveUploadedFile (fixes #4622)

[nghiack7]

🐛 Bug Fix

Fixes #4622 - SaveUploadedFile no longer attempts to modify permissions on existing directories.

Problem

After upgrading from v1.10.1 to v1.12.0, SaveUploadedFile started failing when saving files directly into existing system directories like /tmp. The failure happens due to an attempt to call os.Chmod on the target directory, even if it already exists and is not owned by the process.

Solution

Only apply os.Chmod when the directory is newly created:

1. Check if directory exists before os.MkdirAll
2. Only call os.Chmod if directory was newly created
3. Preserves backward compatibility for newly created dirs

Impact

• ✅ Allows saving files to existing system directories (/tmp, etc.)
• ✅ Preserves chmod behavior for newly created directories
• ✅ Restores v1.10.1 behavior (fixes regression)
• ✅ No breaking changes for existing usage

Testing

Tested manually:

• Saving to /tmp/test.txt (existing dir) ✅
• Saving to new dir with custom perms ✅
• Build passes: go build ./... ✅

Code Changes

// Before: Always chmod (breaks /tmp)
os.MkdirAll(dir, mode)
os.Chmod(dir, mode)

// After: Only chmod if newly created
dirExists := !os.IsNotExist(os.Stat(dir))
os.MkdirAll(dir, mode)
if !dirExists {
    os.Chmod(dir, mode)
}

---

Co-authored-by: Nghia Nguyen nghiack7@users.noreply.github.com

[codecov]

Codecov Report

❌ Patch coverage is 60.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 98.37%. Comparing base (3dc1cd6) to head (ec746b4).
⚠️ Report is 276 commits behind head on master.

Files with missing lines	Patch %	Lines
context.go	60.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4650      +/-   ##
==========================================
- Coverage   99.21%   98.37%   -0.84%     
==========================================
  Files          42       48       +6     
  Lines        3182     3141      -41     
==========================================
- Hits         3157     3090      -67     
- Misses         17       42      +25     
- Partials        8        9       +1     

Flag	Coverage Δ
	?
--ldflags="-checklinkname=0" -tags sonic	98.36% <60.00%> (?)
-tags go_json	98.29% <60.00%> (?)
-tags nomsgpack	98.35% <60.00%> (?)
go-1.18	?
go-1.19	?
go-1.20	?
go-1.21	?
go-1.25	98.37% <60.00%> (?)
go-1.26	98.37% <60.00%> (?)
macos-latest	98.37% <60.00%> (-0.84%)	⬇️
ubuntu-latest	98.37% <60.00%> (-0.84%)	⬇️

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