UNIX Structured Concurrency
A short note on a particular structured concurrency pattern for UNIX systems programming.
That is, in the child process (which you control), do a blocking read on
stdin, and exit promptly
if the read returned zero bytes.
Example of the pattern from one of the side hacks:
Two bits of background reading here:
novel by Leo Tolstoy blog post by njs:
A less famous, but no less classic, gotchas.md from duct.py:
It is often desirable to spawn a process, and make sure that, when the parent process exits, the child process is also killed. This can not be achieved using a pattern equivalent to
The parent process itself might be abruptly killed, and the finally blocks / destructors / atexit hooks are not run in this case.
The natural habitat for this pattern are integration tests, where you often spawn external processes in large amounts, and expect occasional abrupt crashes.
Sadly, as far as I know, UNIX doesn’t provide an easy mechanism to bind the lifetimes of two
processes thusly. There’s process group mechanism, but it is one-level deep and is mostly reserved
for the shell. There’s
docker cgroups, but that’s a Linux-specific mechanism which isn’t usually
exposed by cross-platform standard libraries of various languages.
The trick is using closed stdin as the signal for exit, as that is evenly supported by all platforms, doesn’t require much code, and will do nearly the right thing most of the time.
The drawbacks of this pattern:
- It’s cooperative in the child (you must control the code of the child process to inject the exit logic)
It’s somewhat cooperative in the parent: while exiting on standard input EOF will do the right
thing most of the time, there are exceptions. For example, reading from
/dev/nullreturns 0 (as opposed to blocking), and daemon processes often have their stdin set to
/dev/null. Sadly, there’s no
- It is not actually structured. Ideally, parent’s exit should block on all descendants exiting, but that’s not the case in this pattern. Still, it’s good enough for cleaning up in tests!