Reject making LINENO, OLDPWD, OPTARG, OPTIND, and PWD readonly

[magicant]

POSIX.1-2024 XBD 8.1 clarifies the allowed behavior of the shell when the following variables are made read-only:

• LINENO
• OLDPWD
• OPTARG
• OPTIND
• PWD

The readonly built-in now refuses to make the above variables read-only in the POSIXly-correct mode because it is the most restrictive behavior allowed by the standard.

Summary by Copilot

This pull request introduces changes to prevent specific variables from being made read-only in POSIXly-correct mode. The updates include modifications to documentation, tests, and the codebase to enforce this restriction and provide appropriate error handling.

Changes to functionality:

• Updated the readonly built-in to disallow making the variables $LINENO, $OLDPWD, $OPTARG, $OPTIND, and $PWD read-only in POSIXly-correct mode. [1] [2]

Documentation updates:

• Updated doc/_readonly.txt and doc/posix.txt to reflect the restriction on making specific variables read-only in POSIXly-correct mode. [1] [2]
• Updated the corresponding Japanese documentation files, doc/ja/_readonly.txt and doc/ja/posix.txt, with the same information. [1] [2]

Test additions:

• Added new test cases in tests/readonly-y.tst to verify that attempting to make the restricted variables read-only results in appropriate error messages.

Codebase enhancements:

• Introduced a new helper function can_make_readonly in variable.c to determine if a variable can be made read-only based on the POSIXly-correct mode. [1] [2]

News updates:

• Updated the NEWS and NEWS.ja files to document the new behavior of the readonly built-in in POSIXly-correct mode. [1] [2]

[Copilot]

Pull Request Overview

This pull request updates the shell’s behavior in POSIXly-correct mode by preventing certain internal variables from being made read-only. Key changes include introducing a helper function (can_make_readonly) to check allowed variables, updating the typeset built-in error handling, and revising documentation and tests accordingly.

Files not reviewed (7)

• NEWS: Language not supported
• NEWS.ja: Language not supported
• doc/_readonly.txt: Language not supported
• doc/ja/_readonly.txt: Language not supported
• doc/ja/posix.txt: Language not supported
• doc/posix.txt: Language not supported
• tests/readonly-y.tst: Language not supported

Comments suppressed due to low confidence (1)

variable.c:1760

• [nitpick] Consider enhancing the error message with additional context (e.g. indicating that the restriction is due to POSIXly-correct mode) to improve user feedback.

xerror(0, Ngt("$%ls cannot be read-only"), arg);