Parse the passwd and group files as UTF-8 instead of ASCII

[i098]

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation update

Motivation and Context

Closes apple/container#2376.

User.parse reads /etc/passwd and /etc/group with String(contentsOf:encoding: .ascii). One non-ASCII byte in either file, for example an accented GECOS name from useradd -c 'José Müller', makes the read fail. Every later container exec then fails with NSCocoaErrorDomain Code=259, for every user.

This change decodes the files with String(decoding:as: UTF8.self). That fixes UTF-8 names. It also keeps a legacy non-UTF-8 byte (for example Latin-1) from failing the whole file. The parser only splits on : and newlines, so a replacement character in a GECOS field has no effect.

Repro, from the issue:

container run --detach --name passwdtest registry.fedoraproject.org/fedora:43 sleep infinity
container exec passwdtest useradd -c 'José Müller' passwdtest
container exec passwdtest true

Before: container exec fails with NSCocoaErrorDomain Code=259 "The file isn’t in the correct format.".
After: User.parse reads the file, so the exec no longer fails at this step.

Testing

• Tested locally
• Added/updated tests
• Added/updated docs

I ran the unit tests on macOS 26.2 with Swift 6.3.3. I did not run container exec end to end.

swift test --filter ContainerizationOSTests.UsersTests with the old User.swift:

✘ Test testExecUserNonASCIIPasswdGroup() recorded an issue at UserTests.swift:154:6: Caught error: Error Domain=NSCocoaErrorDomain Code=259 "The file couldn’t be opened because it isn’t in the correct format."
✘ Test testExecUserInvalidUTF8Passwd() recorded an issue at UserTests.swift:183:6: Caught error: Error Domain=NSCocoaErrorDomain Code=259 "The file couldn’t be opened because it isn’t in the correct format."
✘ Test run with 5 tests in 1 suite failed after 0.005 seconds with 2 issues.

swift test --filter ContainerizationOSTests with this change:

✔ Test run with 70 tests in 7 suites passed after 1.065 seconds.

swift format leaves both changed files unchanged.

I am new to open source, and this pull request was made with AI assistance.