casper: document that most libcasper and cap_net functions are not thread-safe

[asomers]

Summary

Because internally they all use cap_xfer_nvlist. cap_xfer_nvlist sends
and then receives data over a unix domain socket and associated with the
cap_channel_t argument. So absent synchronization, two threads may not
use the same cap_channel_t argument or they risk receiving the other's
reply.

MFC after: 2 weeks
Sponsored by: Axcient

[asomers]

@oshogbo do you agree with this wording? If so I'll add it to the other casper services' man pages and update the revision.

[oshogbo]

LGTM, thank you for working on this.

[asomers]

• Add reentrancy notices to the other casper service man pages, too.

[asomers]

@oshogbo I've updated all of them now. But do you think that this is too pedantic? Maybe I'm just a cossetted Rust programmer who expects that everything is thread-safe by default.

[oshogbo]

I've updated all of them now. But do you think that this is too pedantic

No, I think that this is a fair statement. I have a couple of questions about this and the libnv.

Maybe I'm just a cossetted Rust programmer who expects that everything is thread-safe by default.

Everything has pros and cons. Recently, I saw somebody implementing a double-linked list in Rust. It seems like a nightmare.