fix(archive): restore permissions after ownership

[stephenlclarke]

Summary

Apply archived permissions after ownership so Darwin does not clear set-user-ID and set-group-ID bits during extraction.

Motivation and Context

During testing of my container-compose plugin, I found and fixed the following issue: ArchiveReader.setFileAttributes called fchmod before fchown. On Darwin, changing ownership clears set-user-ID and set-group-ID bits, so the final extracted mode does not match the archive entry.

Fixes #819.

Dependency and Merge Order

This PR targets the stock apple/containerization:main branch and depends on #820, which fixes #818. Please merge #820 first.

The head currently contains the signed #818 fix as its first prerequisite commit because both PRs target stock Apple main. After the prerequisite merges, the branch can merge current Apple main so this PR's remaining diff is only the ownership/permission ordering change and set-ID regression.

Reproduction

The syscall behaviour is directly reproducible:

1. Create and open a regular file.
2. Call fchmod(fd, 0o6755).
3. Call fchown(fd, getuid(), getgid()).
4. Inspect fstat(fd).st_mode & 0o7777.

Darwin reports 0755.

With the #818 mask fix present, the archive regression writes a regular entry with mode 06755, extracts it, and runs:

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

Before reordering the calls, it failed with:

Expectation failed: ((setIDPerms & permMask) -> 493) == (0o6755 -> 3565)
Set-ID permission bits should be preserved after ownership

Changes

• Call fchown before the final fchmod.
• Extend the archive regression with a regular 06755 entry whose owner and group are explicitly restored.

Validation

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

• Regression before fix: failed with 0755 instead of 06755
• 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
• Commits 6e32963617b3ed8f4b63432dbcf834f94807342b and 726e1ffdceada5cc62d32c8fc939aef30220e6ff are cryptographically signed and verified

Compatibility and Risk

The change only alters the order of two existing file-descriptor operations. Ownership is still applied when both owner and group are present, and the archived mode becomes the final mode. There are no archive-format or public-API changes.

Related

• fix(archive): preserve special permission bits #820 is the required permission-mask PR for [Bug]: archive extraction strips special permission bits #818.
• Mask high mode bits in archive reader #816 introduced the mask that currently hides this independent ordering defect.

Release Note Highlight

Archive extraction now preserves set-user-ID and set-group-ID bits when restoring file ownership.