[musl] Is errno signal-safe?

[Markus Wichmann <nullplan@gmx.net>]

Hi all,

today I had a flash of inspiration while staring at some code: errno is
a global variable, right? OK, it is thread-local, but still a global
variable in the context of one thread. And looking at a global variable
while it may (or may not) be modified in a signal handler is not safe to
do.

So now I have to wonder. There are a bunch of functions that set errno,
that are on the ostensibly async-signal-safe list, like for example
write(). And to my knowledge, changes to errno are not turned back by
sigreturn(). So, are changes to errno made in a signal handler
propagated to the main program? If so, how do I inspect errno correctly
in the main program? I could block signals, but for one thing, doing so
every time errno might be relevant is going to be overkill, and for two,
if the system call I want the errno from is also blocking and I want to
allow signals while the call is blocking, there is no way to do that
without race condition.

But then again, now that I thought of it, this is so obvious that surely
someone else must have stumbled across it before, right? A solution must
exist, right?

Ciao,
Markus