Make userspace libs more threadsafe

[rkojedzinszky]

Motivation and Context

Improves the userspace lib to be threadsafe.

Description

Replaced non-threadsafe functions with their threadsafe counterparts in lib/.

How Has This Been Tested?

Compiled the code on linux/freebsd.

Compiled on Debian bookworm, FreeBSD 13.2.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Breaking change (fix or feature that would cause existing functionality to change)
• Library ABI change (libzfs, libzfs_core, libnvpair, libuutil and libzfsbootenv)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the OpenZFS code style requirements.
• I have updated the documentation accordingly.
• I have read the contributing document.
• I have added tests to cover my changes.
• I have run the ZFS Test Suite with this change applied.
• All commit messages are properly formatted and contain Signed-off-by.

[rkojedzinszky]

Thanks for the reviews! Do anyone has an idea on how to prevent future modifications to lib/ which would break thread safety? How to detect calls to libc functions that are not threadsafe, or how to detect non thread-local global variable usage?

[behlendorf]

One idea would be to add a codeql checker for any libc functions which are not threadsafe. This way any PRs attempting to use these functions will be caught by the CI. We already do something similar for deprecated functions we want to keep out of the code base. That of course won't handle everything, but it's a start.

https://github.com/openzfs/zfs/blob/master/.github/codeql/custom-queries/cpp/deprecatedFunctionUsage.ql