Checks for some thread-unsafe functions against a black list of known-to-be-unsafe functions. Usually they access static variables without synchronization (e.g. gmtime(3)) or utilize signals in a racy way (e.g. sleep(3)).
The patch adds a check instead of auto-fix as thread-safe alternatives usually have API with an additional argument (e.g. gmtime(3) v.s. gmtime_r(3)) or have a different semantics (e.g. exit(3) v.s. __exit(3)), so it is a rather tricky or non-expected fix.
An option specifies which functions in libc should be considered thread-safe, possible values are posix, glibc, or any (the most strict check). It defaults to 'any' as it is unknown what target libc type is - clang-tidy may be run on linux but check sources compiled for other *NIX.
The check is used in Yandex Taxi backend and has caught many unpleasant bugs. A similar patch for coroutine-unsafe API is coming next.