fix(archive): preserve special permission bits

[stephenlclarke]

Summary

Preserve sticky, set-user-ID, and set-group-ID permission bits when extracting archive entries.

Motivation and Context

During testing of my container-compose plugin, I found and fixed the following issue: archive extraction masked entry permissions with 0o777, which discarded all special POSIX permission bits.

This regressed in #816. A directory archived with mode 01777 was extracted as 0777; set-user-ID and set-group-ID bits were affected by the same mask.

Fixes #818.

Reproduction

1. Write a directory archive entry with mode 01777.

2. Extract it with ArchiveReader.

3. Inspect the extracted directory's POSIX permissions.

4. Run:

   swift test --disable-automatic-resolution -Xswiftc -warnings-as-errors --filter preserveSpecialPermissionBits

On stock main at 50f77222964ed3d01dcb53f803ea2d511656a9dc, the regression failed with:

Expectation failed: ((perms & permMask) -> 511) == (0o1777 -> 1023)
Special permission bits should be preserved

Changes

• Apply entry.permissions & 0o7777 instead of entry.permissions & 0o777.
• Add a sticky-directory archive extraction regression.

Validation

Validated on macOS 26.5.1 (25F80), Xcode 26.6 (17F113), and Swift 6.3.3.

• Regression before fix: failed with 0777 instead of 01777
• Regression after fix: passed
• make fmt: passed
• make check: passed
• swift test --enable-code-coverage --disable-automatic-resolution -Xswiftc -warnings-as-errors: 574 tests in 80 suites passed
• git diff --check: passed
• Commit 6e32963617b3ed8f4b63432dbcf834f94807342b is cryptographically signed and verified

Compatibility and Risk

The change preserves the complete POSIX permission field while still excluding file-type bits. It does not change archive formats or public APIs. The behavioural change is limited to restoring permissions that are already present in the archive entry.

Related

• Mask high mode bits in archive reader #816 introduced the narrower mask.
• [Bug]: archive ownership changes clear set-ID permission bits #819 and fix(archive): restore permissions after ownership #821 track the separate fchown ordering issue that can clear set-ID bits after this mask is corrected.

Release Note Highlight

Archive extraction now preserves sticky, set-user-ID, and set-group-ID permission bits.