fix(supervisor): manage received file descriptor ownership

[Park-Jiyeonn]

Overview

Close superseded state file descriptors and release descriptors received on invalid SCM_RIGHTS messages. Validate control-message truncation and require exactly one descriptor so malformed state transfers cannot leak resources or leave ambiguous ownership.

Change Type

Please select the type of change your pull request relates to:

• Bug Fix
• Feature Addition
• Documentation Update
• Code Refactoring
• Performance Improvement
• Other (please describe)

[codecov]

Codecov Report

❌ Patch coverage is 70.27027% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 25.84%. Comparing base (18e7e6a) to head (6d5836d).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/supervisor/supervisor.go	70.27%	6 Missing and 5 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #783      +/-   ##
==========================================
- Coverage   26.01%   25.84%   -0.17%     
==========================================
  Files         133      134       +1     
  Lines       12711    12833     +122     
==========================================
+ Hits         3307     3317      +10     
- Misses       8998     9110     +112     
  Partials      406      406              

Files with missing lines	Coverage Δ
pkg/supervisor/supervisor.go	70.44% <70.27%> (+1.53%)	⬆️

... and 4 files with indirect coverage changes

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[Zephyrcf]

A small suggestion: unify the return value of fd for error paths in recv to fd (default value -1).

[Zephyrcf]

LGTM, thanks!

[Park-Jiyeonn]

@Zephyrcf If there are no further concerns, could this be merged when convenient?

[imeoer]

Thanks for the PR! Maybe there is a race between load() and save()/closeFD(), after load() releases the lock, the returned FD may be closed or reused before send() uses it, causing EBADF or sending an unrelated FD, we can consider that dup the FD while holding the lock and close the duplicate after sending.