Comment by nasretdinov

1 month ago

So many ways and no-one mentioned threads..?

Edit: by threads I mean creating a new thread to wait for the process, and then kill the process after a certain timeout if the process hasn't terminated. I guess I'm spoiled by Go...

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nasretdinov

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zbentley  1 month ago

The threading approach is roughly:

1. Start a thread

2. That thread starts a child process and signals "started" by storing its PID somewhere globally-visible (and hopefully atomic/lock-protected).

3. The thread then blocks in wait(2), taking advantage of its non-main-thread-ness to avoid some signals and optionally masking/ignoring some more.

4. When the process exits, the thread can write exitstatus/"completed" to the globally-visible state next to PID. The thread then exits.

3. External observers wait for the process with a timeout by attempting to join the thread with a timeout. If the timeout occurs, they can access the globally-visible PID and send a signal to it.

This is missing from the article (EDIT: it has since been added, thanks!). That doesn't mean it's a good solution on many platforms. It's more costly in resources (thread stack), more code than most of the listed options, vulnerable to PID-reuse problems that can cause a killsignal to go to the wrong process, likely plays poorly with spawning methods that request a SIGCHLD be sent to the parent on exit (and plays poorly with signals in general if any customization is needed there), and is probably often slower than most of TFA's alternatives as well, both due to syscall count and pessimal thread/scheduler switching conditions. Additionally, it multiplexes/composes to large numbers of processes poorly and with a high resource cost.

EDIT: Golang's version of this is less bad than described above, but not perfect. Go's spawning infrastructure mitigates resource cost (goroutines/segmented stacks are not as heavy as threads), is vulnerable to PID-reuse (as are most platforms' operations in this area), addresses the SIGCHLD risk through the runtime and signal channels, and mitigates slowness with a very good scheduler. For multiplexing, I would assume (but I have not verified) that the Go runtime is internally using pidfds/kqueue where supported. Where not supported, I would assume Go is internally tracking spawn requests through its stdlib, handling SIGCHLD, and has a single global routine calling wait(2) without a specific PID, waking goroutines waiting on a watched PID when it comes out of the call to wait(2).

  • nasretdinov  1 month ago

    Thanks. I believe that Go indeed _could_ use those APIs to wait for the child more efficiently if they chose to, but the current implementation suggests that they're just calling wait4() in a separate thread: https://cs.opensource.google/go/go/+/refs/tags/go1.23.3:src/...

    To be fair, in Go process spawning is very inefficient to begin with, since it requires lots of runtime coordination to not mess with the threads/goroutines state during fork, so running wait4() in a separate thread (although the thread can be re-used afterwards) is not the biggest concern here.