Comment by zbentley

I think it's not quite that bad (and I know that this has been litigated to death all over the programmer internet).

If you are forking from a language/ecosystem that is extremely thread-friendly, (e.g. Go, Java, Erlang) fork is more risky. This is because such runtimes mean a high likelihood of there being threads doing fork-unsafe things at the moment of fork().

If you are forking from a language/ecosystem that is thread-unfriendly, fork is less risky. That isn't to say "it's always safe/low risk to run fork() in e.g. Python, Ruby, Perl", but in those contexts it's easier to prove/test invariants like "there are no threads running/so-and-so lock is not held at the point in my program when I fork", at which point the risks of fork(2) are much reduced.

To be clear, "reduced" is not the same as "gone"! You still have to reason about explicitly taken locks in the forking thread, file descriptors, signal handlers, and unexpected memory growth due to CoW/GC interactions. But that's a lot more tractable than the Java situation of "it's tricky to predict how many Java threads are active when I want to fork, and even trickier to know if there are any JNI/FFI-library-created raw pthreads running, the GC might be threaded, and checking for each of those things is still racy with my call to fork(2)".

You still have to make sure that that fork-safety invariants are true. But the effort to do that is extremely different depending on language platform.

Rust/C/C++ don't cleanly fit into either of those two (already mushy/subjective) categorizations, though. Whether forking is feasible in a given Rust/C/C++ codebase depends on what the code does and requires a tricky set of judgement calls and at-a-distance knowledge going forward to make sure that the codebase doesn't become fork-unsafe in harmful ways.