TLS accesses are not documented to be async-signal-safe, but they mostly are – except for the first access to a variable. We rely on that: we install signal handlers that access thread-local variables, but we ensure they have been accessed before setting up the signal handler. This would be a good thing to discuss with the runtimes we depend on, since hopefully they can provide this as an actual guarantee, or else we'll learn that we can't rely on it.

@joboet @the8472 in terms of interaction with POSIX, what exactly are we relying on here? Or are the kind of TLS variables we are using not part of POSIX? Is it a libc thing, a linker thing, or something else?