local_offset_at on Linux multi-threaded

[keepsimple1]

Currently local_offset_at does not work on Linux multi-threaded programs, as explained here:

time/time/src/sys/local_offset_at/unix.rs

Lines 31 to 33 in fadac54

	/// This method will remain `unsafe` until `std::env::set_var` is deprecated or has its behavior
	/// altered. This method is, on its own, safe. It is the presence of a safe, unsound way to set
	/// environment variables that makes it unsafe.

It seems that std::env::set_var is the problem, but which code in timestamp_to_tm calls std::env::set_var? Or is it referring to the C function of set_var?

Also: The POSIX doc seems to say localtime_r can provide the transformation and is thread-safe. But we still call tzset(). In what cases will localtime_r return incorrect info if we don't call tzset() explicitly?

[jhpratt]

The key part of the comment is this:

This method is, on its own, safe. It is the presence of a safe, unsound way to set environment variables that makes it unsafe.

Because std::env::set_var is unsound (it should be unsafe but isn't), code written entirely in Rust can cause a segfault when using time if it weren't for the soundness check. The same is true for C's set_var, but that's a separate issue isn't particularly relevant here.

With regard to localtime_r and tzset, tzset is required because localtime_r is not required to call it. There's probably something more subtle that I'm forgetting, but there was definitely a reason.

[keepsimple1]

Because std::env::set_var is unsound (it should be unsafe but isn't), code written entirely in Rust can cause a segfault when using time if it weren't for the soundness check.

Where does time use std::env::set_var? (I must have missed something, if time does not use std::env::set_var, then its problem is not relevant to time, no? ).

[jhpratt]

The problem is absolutely relevant to time, as we're the one dereferencing the pointer that is now a dangling pointer. time does not need to use std::env::set_var itself — anyone, anywhere setting an environment variable in any language is enough to cause the issue.